operators take more than one argument, while unary operators can never
take more than one argument. Thus, a comma terminates the argument of
a unary operator, but merely separates the arguments of a list
-operator. A unary operator generally provides a scalar context to its
+operator. A unary operator generally provides scalar context to its
argument, while a list operator may provide either scalar or list
-contexts for its arguments. If it does both, the scalar arguments will
-be first, and the list argument will follow. (Note that there can ever
-be only one such list argument.) For instance, splice() has three scalar
+contexts for its arguments. If it does both, scalar arguments
+come first and list argument follow, and there can only ever
+be one such list argument. For instance, splice() has three scalar
arguments followed by a list, whereas gethostbyname() has four scalar
arguments.
In the syntax descriptions that follow, list operators that expect a
-list (and provide list context for the elements of the list) are shown
+list (and provide list context for elements of the list) are shown
with LIST as an argument. Such a list may consist of any combination
of scalar arguments or list values; the list values will be included
in the list as if each individual element were interpolated at that
point in the list, forming a longer single-dimensional list value.
-Commas should separate elements of the LIST.
+Commas should separate literal elements of the LIST.
Any function in the list below may be used either with or without
parentheses around its arguments. (The syntax descriptions omit the
-parentheses.) If you use the parentheses, the simple (but occasionally
-surprising) rule is this: It I<looks> like a function, therefore it I<is> a
+parentheses.) If you use parentheses, the simple but occasionally
+surprising rule is this: It I<looks> like a function, therefore it I<is> a
function, and precedence doesn't matter. Otherwise it's a list
-operator or unary operator, and precedence does matter. And whitespace
-between the function and left parenthesis doesn't count--so you need to
-be careful sometimes:
+operator or unary operator, and precedence does matter. Whitespace
+between the function and left parenthesis doesn't count, so sometimes
+you need to be careful:
print 1+2+4; # Prints 7.
print(1+2) + 4; # Prints 3.
C<time() + 86_400>.
For functions that can be used in either a scalar or list context,
-nonabortive failure is generally indicated in a scalar context by
-returning the undefined value, and in a list context by returning the
-null list.
+nonabortive failure is generally indicated in scalar context by
+returning the undefined value, and in list context by returning the
+empty list.
Remember the following important rule: There is B<no rule> that relates
the behavior of an expression in list context to its behavior in scalar
there, not the list construction version of the comma. That means it
was never a list to start with.
-In general, functions in Perl that serve as wrappers for system calls
-of the same name (like chown(2), fork(2), closedir(2), etc.) all return
+In general, functions in Perl that serve as wrappers for system calls ("syscalls")
+of the same name (like chown(2), fork(2), closedir(2), etc.) return
true when they succeed and C<undef> otherwise, as is usually mentioned
in the descriptions below. This is different from the C interfaces,
-which return C<-1> on failure. Exceptions to this rule are C<wait>,
+which return C<-1> on failure. Exceptions to this rule include C<wait>,
C<waitpid>, and C<syscall>. System calls also set the special C<$!>
variable on failure. Other functions do not, except accidentally.
=item Functions for real @ARRAYs
X<array>
-C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
+C<each>, C<keys>, C<pop>, C<push>, C<shift>, C<splice>, C<unshift>, C<values>
=item Functions for list data
X<list>
C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
C<warn>, C<write>
-=item Functions for fixed length data or records
+=item Functions for fixed-length data or records
C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
-=item Keywords related to switch
+=item Keywords related to the switch feature
-C<break>, C<continue>, C<given>, C<when>, C<default>
+C<break>, C<continue>, C<default, >C<given>, C<when>
-(These are only available if you enable the "switch" feature.
-See L<feature> and L<perlsyn/"Switch statements">.)
+These are available only if you enable the C<"switch"> feature.
+See L<feature> and L<perlsyn/"Switch statements">.
+Alternately, include a C<use v5.10> or later to the current scope.
=item Keywords related to scoping
-C<caller>, C<import>, C<local>, C<my>, C<our>, C<state>, C<package>,
-C<use>
+C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<state>, C<use>
-(C<state> is only available if the "state" feature is enabled. See
-L<feature>.)
+C<state> is available only if the C<"state"> feature is enabled. See
+L<feature>. Alternately, include a C<use v5.10> or later to the current scope.
=item Miscellaneous functions
C<pipe>, C<qx//>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
C<times>, C<wait>, C<waitpid>
-=item Keywords related to perl modules
+=item Keywords related to Perl modules
X<module>
C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>,
C<ucfirst>, C<untie>, C<use>, C<when>
-* - C<sub> was a keyword in perl4, but in perl5 it is an
+* C<sub> was a keyword in Perl 4, but in Perl 5 it is an
operator, which can be used in expressions.
=item Functions obsoleted in perl5
Perl was born in Unix and can therefore access all common Unix
system calls. In non-Unix environments, the functionality of some
-Unix system calls may not be available, or details of the available
+Unix system calls may not be available or details of the available
functionality may differ slightly. The Perl functions affected
by this are:
=head2 Alphabetical Listing of Perl Functions
-=over 8
+=over
=item -X FILEHANDLE
X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
#...
}
+Note that C<-s/a/b/> does not do a negated substitution. Saying
+C<-exp($foo)> still works as expected, however: only single letters
+following a minus are interpreted as file tests.
+
+These operators are exempt from the "looks like a function rule" described
+above. That is, an opening parenthesis after the operator does not affect
+how much of the following code constitutes the argument. Put the opening
+parentheses before the operator to separate it from code that follows (this
+applies only to operators with higher precedence than unary operators, of
+course):
+
+ -s($file) + 1024 # probably wrong; same as -s($file + 1024)
+ (-s $file) + 1024 # correct
+
The interpretation of the file permission operators C<-r>, C<-R>,
C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
of the file and the uids and gids of the user. There may be other
If you are using ACLs, there is a pragma called C<filetest> that may
produce more accurate results than the bare stat() mode bits.
-When under the C<use filetest 'access'> the above-mentioned filetests
-will test whether the permission can (not) be granted using the
-access() family of system calls. Also note that the C<-x> and C<-X> may
+When under C<use filetest 'access'> the above-mentioned filetests
+test whether the permission can(not) be granted using the
+access(2) family of system calls. Also note that the C<-x> and C<-X> may
under this pragma return true even if there are no execute permission
bits set (nor any extra execute permission ACLs). This strangeness is
due to the underlying system calls' definitions. Note also that, due to
in effect. Read the documentation for the C<filetest> pragma for more
information.
-Note that C<-s/a/b/> does not do a negated substitution. Saying
-C<-exp($foo)> still works as expected, however--only single letters
-following a minus are interpreted as file tests.
-
The C<-T> and C<-B> switches work as follows. The first block or so of the
file is examined for odd characters such as strange control codes or
characters with the high bit set. If too many strange characters (>30%)
are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
-containing null in the first block is considered a binary file. If C<-T>
+containing a zero byte in the first block is considered a binary file. If C<-T>
or C<-B> is used on a filehandle, the current IO buffer is examined
-rather than the first block. Both C<-T> and C<-B> return true on a null
+rather than the first block. Both C<-T> and C<-B> return true on an empty
file, or a file at EOF when testing a filehandle. Because you have to
read a file to do the C<-T> test, on most occasions you want to use a C<-f>
against the file first, as in C<next unless -f $file && -T $file>.
-If any of the file tests (or either the C<stat> or C<lstat> operators) are given
+If any of the file tests (or either the C<stat> or C<lstat> operator) is given
the special filehandle consisting of a solitary underline, then the stat
structure of the previous file test (or stat operator) is used, saving
a system call. (This doesn't work with C<-t>, and you need to remember
-that lstat() and C<-l> will leave values in the stat structure for the
+that lstat() and C<-l> leave values in the stat structure for the
symbolic link, not the real file.) (Also, if the stat buffer was filled by
an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
Example:
As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
test operators, in a way that C<-f -w -x $file> is equivalent to
-C<-x $file && -w _ && -f _>. (This is only syntax fancy: if you use
+C<-x $file && -w _ && -f _>. (This is only fancy fancy: if you use
the return value of C<-f $file> as an argument to another filetest
operator, no special magic will happen.)
+Portability issues: L<perlport/-X>.
+
=item abs VALUE
X<abs> X<absolute>
=item accept NEWSOCKET,GENERICSOCKET
X<accept>
-Accepts an incoming socket connect, just as the accept(2) system call
+Accepts an incoming socket connect, just as accept(2)
does. Returns the packed address if it succeeded, false otherwise.
See the example in L<perlipc/"Sockets: Client/Server Communication">.
For more information see L<perlipc>.
+Portability issues: L<perlport/alarm>.
+
=item atan2 Y,X
X<atan2> X<arctangent> X<tan> X<tangent>
The return value for C<atan2(0,0)> is implementation-defined; consult
your atan2(3) manpage for more information.
+Portability issues: L<perlport/atan2>.
+
=item bind SOCKET,NAME
X<bind>
-Binds a network address to a socket, just as the bind system call
+Binds a network address to a socket, just as bind(2)
does. Returns true if it succeeded, false otherwise. NAME should be a
packed address of the appropriate type for the socket. See the examples in
L<perlipc/"Sockets: Client/Server Communication">.
taken as the name of the filehandle. Returns true on success,
otherwise it returns C<undef> and sets C<$!> (errno).
-On some systems (in general, DOS and Windows-based systems) binmode()
+On some systems (in general, DOS- and Windows-based systems) binmode()
is necessary when you're not working with a text file. For the sake
-of portability it is a good idea to always use it when appropriate,
-and to never use it when it isn't appropriate. Also, people can
-set their I/O to be by default UTF-8 encoded Unicode, not bytes.
+of portability it is a good idea always to use it when appropriate,
+and never to use it when it isn't appropriate. Also, people can
+set their I/O to be by default UTF8-encoded Unicode, not bytes.
In other words: regardless of platform, use binmode() on binary data,
-like for example images.
+like images, for example.
If LAYER is present it is a single string, but may contain multiple
-directives. The directives alter the behaviour of the file handle.
-When LAYER is present using binmode on a text file makes sense.
+directives. The directives alter the behaviour of the filehandle.
+When LAYER is present, using binmode on a text file makes sense.
If LAYER is omitted or specified as C<:raw> the filehandle is made
suitable for passing binary data. This includes turning off possible CRLF
translation and marking it as bytes (as opposed to Unicode characters).
Note that, despite what may be implied in I<"Programming Perl"> (the
-Camel) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>
---other layers that would affect the binary nature of the stream are
-I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
+Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
+Other layers that would affect the binary nature of the stream are
+I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
PERLIO environment variable.
-The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
+The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
establish default I/O layers. See L<open>.
of this version of Perl therefore refers to "layers" rather than to
"disciplines". Now back to the regularly scheduled documentation...>
-To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(utf8)>.
+To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>.
C<:utf8> just marks the data as UTF-8 without further checking,
-while C<:encoding(utf8)> checks the data for actually being valid
+while C<:encoding(UTF-8)> checks the data for actually being valid
UTF-8. More details can be found in L<PerlIO::encoding>.
In general, binmode() should be called after open() but before any I/O
-is done on the filehandle. Calling binmode() will normally flush any
+is done on the filehandle. Calling binmode() normally flushes any
pending buffered output data (and perhaps pending input data) on the
handle. An exception to this is the C<:encoding> layer that
-changes the default character encoding of the handle, see L<open>.
+changes the default character encoding of the handle; see L</open>.
The C<:encoding> layer sometimes needs to be called in
mid-stream, and it doesn't flush the stream. The C<:encoding>
also implicitly pushes on top of itself the C<:utf8> layer because
-internally Perl will operate on UTF-8 encoded Unicode characters.
+internally Perl operates on UTF8-encoded Unicode characters.
The operating system, device drivers, C libraries, and Perl run-time
-system all work together to let the programmer treat a single
-character (C<\n>) as the line terminator, irrespective of the external
+system all conspire to let the programmer treat a single
+character (C<\n>) as the line terminator, irrespective of external
representation. On many operating systems, the native text file
representation matches the internal representation, but on some
platforms the external representation of C<\n> is made up of more than
one character.
-Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
-character to end each line in the external representation of text (even
-though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
-on Unix and most VMS files). In other systems like OS/2, DOS and the
-various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
-but what's stored in text files are the two characters C<\cM\cJ>. That
-means that, if you don't use binmode() on these systems, C<\cM\cJ>
-sequences on disk will be converted to C<\n> on input, and any C<\n> in
-your program will be converted back to C<\cM\cJ> on output. This is what
-you want for text files, but it can be disastrous for binary files.
+All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use
+a single character to end each line in the external representation of text
+(even though that single character is CARRIAGE RETURN on old, pre-Darwin
+flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other
+systems like OS/2, DOS, and the various flavors of MS-Windows, your program
+sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the
+two characters C<\cM\cJ>. That means that if you don't use binmode() on
+these systems, C<\cM\cJ> sequences on disk will be converted to C<\n> on
+input, and any C<\n> in your program will be converted back to C<\cM\cJ> on
+output. This is what you want for text files, but it can be disastrous for
+binary files.
Another consequence of using binmode() (on some systems) is that
special end-of-file markers will be seen as part of the data stream.
-For systems from the Microsoft family this means that if your binary
-data contains C<\cZ>, the I/O subsystem will regard it as the end of
+For systems from the Microsoft family this means that, if your binary
+data contain C<\cZ>, the I/O subsystem will regard it as the end of
the file, unless you use binmode().
-binmode() is not only important for readline() and print() operations,
+binmode() is important not only for readline() and print() operations,
but also when using read(), seek(), sysread(), syswrite() and tell()
(see L<perlport> for more details). See the C<$/> and C<$\> variables
in L<perlvar> for how to manually set your input and output
line-termination sequences.
+Portability issues: L<perlport/binmode>.
+
=item bless REF,CLASSNAME
X<bless>
Break out of a C<given()> block.
-This keyword is enabled by the "switch" feature: see L<feature>
-for more information.
+This keyword is enabled by the C<"switch"> feature: see
+L<feature> for more information. Alternately, include a C<use
+v5.10> or later to the current scope.
=item caller EXPR
X<caller> X<call stack> X<stack> X<stack trace>
C<< N > 1 >>. In particular, C<@DB::args> might have information from the
previous time C<caller> was called.
+Be aware that setting C<@DB::args> is I<best effort>, intended for
+debugging or generating backtraces, and should not be relied upon. In
+particular, as C<@_> contains aliases to the caller's arguments, Perl does
+not take a copy of C<@_>, so C<@DB::args> will contain modifications the
+subroutine makes to C<@_> or its contents, not the original values at call
+time. C<@DB::args>, like C<@_>, does not hold explicit references to its
+elements, so under certain cases its elements may have become freed and
+reallocated for other variables or temporary values. Finally, a side effect
+of the current implementation is that the effects of C<shift @_> can
+I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a
+reference to C<@_> has been taken, I<and> subject to the caveat about reallocated
+elements), so C<@DB::args> is actually a hybrid of the current state and
+initial state of C<@_>. Buyer beware.
+
=item chdir EXPR
X<chdir>
X<cd>
neither is set, C<chdir> does nothing. It returns true on success,
false otherwise. See the example under C<die>.
-On systems that support fchdir, you may pass a file handle or
-directory handle as argument. On systems that don't support fchdir,
-passing handles produces a fatal error at run time.
+On systems that support fchdir(2), you may pass a filehandle or
+directory handle as the argument. On systems that don't support fchdir(2),
+passing handles raises an exception.
=item chmod LIST
X<chmod> X<permission> X<mode>
Changes the permissions of a list of files. The first element of the
-list must be the numerical mode, which should probably be an octal
+list must be the numeric mode, which should probably be an octal
number, and which definitely should I<not> be a string of octal digits:
-C<0644> is okay, C<'0644'> is not. Returns the number of files
-successfully changed. See also L</oct>, if all you have is a string.
+C<0644> is okay, but C<"0644"> is not. Returns the number of files
+successfully changed. See also L</oct> if all you have is a string.
- $cnt = chmod 0755, 'foo', 'bar';
+ $cnt = chmod 0755, "foo", "bar";
chmod 0755, @executables;
- $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
+ $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
# --w----r-T
- $mode = '0644'; chmod oct($mode), 'foo'; # this is better
- $mode = 0644; chmod $mode, 'foo'; # this is best
+ $mode = "0644"; chmod oct($mode), "foo"; # this is better
+ $mode = 0644; chmod $mode, "foo"; # this is best
-On systems that support fchmod, you may pass file handles among the
-files. On systems that don't support fchmod, passing file handles
-produces a fatal error at run time. The file handles must be passed
-as globs or references to be recognized. Barewords are considered
-file names.
+On systems that support fchmod(2), you may pass filehandles among the
+files. On systems that don't support fchmod(2), passing filehandles raises
+an exception. Filehandles must be passed as globs or glob references to be
+recognized; barewords are considered filenames.
open(my $fh, "<", "foo");
my $perm = (stat $fh)[2] & 07777;
chmod($perm | 0600, $fh);
-You can also import the symbolic C<S_I*> constants from the Fcntl
+You can also import the symbolic C<S_I*> constants from the C<Fcntl>
module:
- use Fcntl ':mode';
-
+ use Fcntl qw( :mode );
chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
- # This is identical to the chmod 0755 of the above example.
+ # Identical to the chmod 0755 of the example above.
+
+Portability issues: L<perlport/chmod>.
=item chomp VARIABLE
X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
that the final record may be missing its newline. When in paragraph
mode (C<$/ = "">), it removes all trailing newlines from the string.
When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
-a reference to an integer or the like
, see L<perlvar>) chomp() won't
+a reference to an integer or the like
; see L<perlvar>) chomp() won't
remove anything.
If VARIABLE is omitted, it chomps C<$_>. Example:
$cnt = chown $uid, $gid, 'foo', 'bar';
chown $uid, $gid, @filenames;
-On systems that support fchown, you may pass file handles among the
-files. On systems that don't support fchown, passing file handles
-produces a fatal error at run time. The file handles must be passed
-as globs or references to be recognized. Barewords are considered
-file names.
+On systems that support fchown(2), you may pass filehandles among the
+files. On systems that don't support fchown(2), passing filehandles raises
+an exception. Filehandles must be passed as globs or glob references to be
+recognized; barewords are considered filenames.
Here's an example that looks up nonnumeric uids in the passwd file:
use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
$can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
+Portability issues: L<perlport/chmod>.
+
=item chr NUMBER
X<chr> X<character> X<ASCII> X<Unicode>
reasons, this call is restricted to the superuser. If FILENAME is
omitted, does a C<chroot> to C<$_>.
+Portability issues: L<perlport/chroot>.
+
=item close FILEHANDLE
X<close>
=item close
-Closes the file or pipe associated with the file handle, flushes the IO
+Closes the file or pipe associated with the filehandle, flushes the IO
buffers, and closes the system file descriptor. Returns true if those
-operations have succeeded and if no error was reported by any PerlIO
+operations succeed and if no error was reported by any PerlIO
layer. Closes the currently selected filehandle if the argument is
omitted.
You don't have to close FILEHANDLE if you are immediately going to do
-another C<open> on it, because C<open> will close it for you. (See
+another C<open> on it, because C<open> closes it for you. (See
C<open>.) However, an explicit C<close> on an input file resets the line
counter (C<$.>), while the implicit close done by C<open> does not.
-If the file handle came from a piped open, C<close> will additionally
-return false if one of the other system calls involved fails, or if the
-program exits with non-zero status. (If the only problem was that the
-program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
-also waits for the process executing on the pipe to exit, in case you
-wish to look at the output of the pipe afterwards, and
-implicitly puts the exit status value of that command into C<$?> and
-C<${^CHILD_ERROR_NATIVE}>.
+If the filehandle came from a piped open, C<close> returns false if one of
+the other syscalls involved fails or if its program exits with non-zero
+status. If the only problem was that the program exited non-zero, C<$!>
+will be set to C<0>. Closing a pipe also waits for the process executing
+on the pipe to exit--in case you wish to look at the output of the pipe
+afterwards--and implicitly puts the exit status value of that command into
+C<$?> and C<${^CHILD_ERROR_NATIVE}>.
+
+If there are multiple threads running, C<close> on a filehandle from a
+piped open returns true without waiting for the child process to terminate,
+if the filehandle is still open in another thread.
Closing the read end of a pipe before the process writing to it at the
other end is done writing results in the writer receiving a SIGPIPE. If
or die "Can't open 'foo' for input: $!";
FILEHANDLE may be an expression whose value can be used as an indirect
-filehandle, usually the real filehandle name.
+filehandle, usually the real filehandle name or an autovivified handle.
=item closedir DIRHANDLE
X<closedir>
statement).
C<last>, C<next>, or C<redo> may appear within a C<continue>
-block. C<last> and C<redo> will behave as if they had been executed within
+block; C<last> and C<redo> behave as if they had been executed within
the main block. So will C<next>, but since it will execute a C<continue>
block, it may be more entertaining.
}
### last always comes here
-Omitting the C<continue> section is semantically equivalent to using an
-empty one, logically enough. In that case, C<next> goes directly back
+Omitting the C<continue> section is equivalent to using an
+empty one, logically enough, so C<next> goes directly back
to check the condition at the top of the loop.
-If the "switch" feature is enabled, C<continue> is also a
-function that will break out of the current C<when> or C<default>
-block, and fall through to the next case. See L<feature> and
-L<perlsyn/"Switch statements"> for more information.
-
+If the C<"switch"> feature is enabled, C<continue> is also a function that
+falls through the current C<when> or C<default> block instead of iterating
+a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
+See L<feature> and L<perlsyn/"Switch statements"> for more
+information.
=item cos EXPR
X<cos> X<cosine> X<acos> X<arccosine>
=item cos
Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
-takes cosine of C<$_>.
+takes the cosine of C<$_>.
For the inverse cosine operation, you may use the C<Math::Trig::acos()>
function, or use this relation:
library (assuming that you actually have a version there that has not
been extirpated as a potential munition).
-crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
+crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
into a short string, called a digest, which is returned. The same
PLAINTEXT and SALT will always return the same string, but there is no
(known) way to get the original PLAINTEXT from the hash. Small
if a correct password is given. The digest of the password is stored,
not the password itself. The user types in a password that is
crypt()'d with the same salt as the stored digest. If the two digests
-match the password is correct.
+match, the password is correct.
When verifying an existing digest string you should use the digest as
the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
to create the digest is visible as part of the digest. This ensures
crypt() will hash the new string with the same salt as the digest.
This allows your code to work with the standard L<crypt|/crypt> and
-with more exotic implementations. In other words, do not assume
-anything about the returned string itself, or how many bytes in the
-digest matter.
+with more exotic implementations. In other words, assume
+nothing about the returned string itself nor about how many bytes
+of SALT may matter.
Traditionally the result is a string of 13 bytes: two first bytes of
the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
If using crypt() on a Unicode string (which I<potentially> has
characters with codepoints above 255), Perl tries to make sense
-of the situation by trying to downgrade (a copy of the string)
+of the situation by trying to downgrade (a copy of)
the string back to an eight-bit byte string before calling crypt()
(on that copy). If that works, good. If not, crypt() dies with
C<Wide character in crypt>.
+Portability issues: L<perlport/crypt>.
+
=item dbmclose HASH
X<dbmclose>
Breaks the binding between a DBM file and a hash.
+Portability issues: L<perlport/dbmclose>.
+
=item dbmopen HASH,DBNAME,MASK
X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
If you don't have write access to the DBM file, you can only read hash
variables, not set them. If you want to test whether you can write,
-either use file tests or try setting a dummy hash entry inside an C<eval>,
-which will trap the error.
+either use file tests or try setting a dummy hash entry inside an C<eval>
+to trap the error.
Note that functions such as C<keys> and C<values> may return huge lists
when used on large DBM files. You may prefer to use the C<each>
dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
or die "Can't open netscape history file: $!";
+Portability issues: L<perlport/dbmopen>.
+
+=item default BLOCK
+
+Within a C<foreach> or a C<given>, a C<default> BLOCK acts like a C<when>
+that's always true. Only available after Perl 5.10, and only if the
+C<switch> feature has been requested. See L</when>.
+
=item defined EXPR
X<defined> X<undef> X<undefined>
=item defined
Returns a Boolean value telling whether EXPR has a value other than
-the undefined value C<undef>. If EXPR is not present, C<$_> will be
+the undefined value C<undef>. If EXPR is not present, C<$_> is
checked.
Many operations return C<undef> to indicate failure, end of file,
has ever been defined. The return value is unaffected by any forward
declarations of C<&func>. A subroutine that is not defined
may still be callable: its package may have an C<AUTOLOAD> method that
-makes it spring into existence the first time that it is called--see
+makes it spring into existence the first time that it is called; see
L<perlsub>.
Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
-used to report whether memory for that aggregate has ever been
+used to report whether memory for that aggregate had ever been
allocated. This behavior may disappear in future versions of Perl.
You should instead use a simple test for size:
Examples:
- print if defined $switch{'D'};
+ print if defined $switch{D};
print "$val\n" while defined($val = pop(@ary));
die "Can't readlink $sym: $!"
unless defined($value = readlink $sym);
sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
$debugging = 0 unless defined $debugging;
-Note: Many folks tend to overuse C<defined>, and then are surprised to
+Note: Many folks tend to overuse C<defined> and are then surprised to
discover that the number C<0> and C<""> (the zero-length string) are, in fact,
defined values. For example, if you say
matched something that happened to be zero characters long. This is all
very above-board and honest. When a function returns an undefined value,
it's an admission that it couldn't give you an honest answer. So you
-should use C<defined> only when you're questioning the integrity of what
+should use C<defined> only when questioning the integrity of what
you're trying to do. At other times, a simple comparison to C<0> or C<""> is
what you want.
=item delete EXPR
X<delete>
-Given an expression that specifies an element or slice of an aggregate (a
-hash or an array), deletes the specified elements from that aggregate so
-that exists() on that element no longer returns true. Setting an aggregate
-element to the undefined value does not remove its key, but deleting it
-does; see L</exists>.
+Given an expression that specifies an element or slice of a hash, C<delete>
+deletes the specified elements from that hash so that exists() on that element
+no longer returns true. Setting a hash element to the undefined value does
+not remove its key, but deleting it does; see L</exists>.
-Returns the value or values deleted in list context, or the last such
+In list context, returns the value or values deleted, or the last such
element in scalar context. The return list's length always matches that of
-the argument list: deleting non-existent elements returns the undefined
-value in their corresponding positions.
+the argument list: deleting non-existent elements returns the undefined value
+in their corresponding positions.
+
+delete() may also be used on arrays and array slices, but its behavior is less
+straightforward. Although exists() will return false for deleted entries,
+deleting array elements never changes indices of existing values; use shift()
+or splice() for that. However, if all deleted elements fall at the end of an
+array, the array's size shrinks to the position of the highest element that
+still tests true for exists(), or to 0 if none do.
-Deleting array elements never changes indices of existing values; use
-shift() or splice() for that. However, if all deleted elements fall at
-the end of an array, the array's size shrinks to the position of the
-highest element that still tests true for exists(), or to 0 if none do.
+B<WARNING:> Calling delete on array values is deprecated and likely to
+be removed in a future version of Perl.
Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
=item die LIST
X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
-Outside an C<eval>, prints the value of LIST to C<STDERR> and
-exits with the current value of C<$!> (errno). If C<$!> is C<0>,
-exits with the value of C<<< ($? >> 8) >>> (backtick `command`
-status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
-an C<eval(),> the error message is stuffed into C<$@> and the
-C<eval> is terminated with the undefined value. This makes
-C<die> the way to raise an exception.
+C<die> raises an exception. Inside an C<eval> the error message is stuffed
+into C<$@> and the C<eval> is terminated with the undefined value.
+If the exception is outside of all enclosing C<eval>s, then the uncaught
+exception prints LIST to C<STDERR> and exits with a non-zero value. If you
+need to exit the process with a specific exit code, see L</exit>.
Equivalent examples:
/etc/games is no good at canasta line 123.
/etc/games is no good, stopped at canasta line 123.
-See also exit(), warn(), and the Carp module.
-
If the output is empty and C<$@> already contains a value (typically from a
previous eval) that value is reused after appending C<"\t...propagated">.
This is useful for propagating exceptions:
If the output is empty and C<$@> contains an object reference that has a
C<PROPAGATE> method, that method will be called with additional file
and line number parameters. The return value replaces the value in
-C<$@>. i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
+C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
were called.
If C<$@> is empty then the string C<"Died"> is used.
+If an uncaught exception results in interpreter exit, the exit code is
+determined from the values of C<$!> and C<$?> with this pseudocode:
+
+ exit $! if $!; # errno
+ exit $? >> 8 if $? >> 8; # child exit status
+ exit 255; # last resort
+
+The intent is to squeeze as much possible information about the likely cause
+into the limited space of the system exit code. However, as C<$!> is the value
+of C's C<errno>, which can be set by any system call, this means that the value
+of the exit code used by C<die> can be non-predictable, so should not be relied
+upon, other than to be non-zero.
+
You can also call C<die> with a reference argument, and if this is trapped
within an C<eval>, C<$@> contains that reference. This permits more
elaborate exception handling using objects that maintain arbitrary state
}
}
-Because perl stringifies uncaught exception messages before display,
+Because Perl stringifies uncaught exception messages before display,
you'll probably want to overload stringification operations on
exception objects. See L<overload> for details about that.
You can arrange for a callback to be run just before the C<die>
does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
-handler will be called with the error text and can change the error
+handler is called with the error text and can change the error
message, if it sees fit, by calling C<die> again. See
-L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
+L<perlvar/%SIG> for details on setting C<%SIG> entries, and
L<"eval BLOCK"> for some examples. Although this feature was
to be run only right before your program was to exit, this is not
-currently the case--the C<$SIG{__DIE__}> hook is currently called
+currently so: the C<$SIG{__DIE__}> hook is currently called
even inside eval()ed blocks/strings! If one wants the hook to do
nothing in such situations, put
this promotes strange action at a distance, this counterintuitive
behavior may be fixed in a future release.
+See also exit(), warn(), and the Carp module.
+
=item do BLOCK
X<do> X<block>
=item do SUBROUTINE(LIST)
X<do>
-This form of subroutine call is deprecated. See L<perlsub>.
+This form of subroutine call is deprecated. SUBROUTINE can be a bareword,
+a scalar variable or a subroutine beginning with C<&>.
=item do EXPR
X<do>
eval `cat stat.pl`;
except that it's more efficient and concise, keeps track of the current
-filename for error messages, searches the @INC directories, and updates
-C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
-variables. It also differs in that code evaluated with C<do FILENAME>
+filename for error messages, searches the C<@INC> directories, and updates
+C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
+these variables. It also differs in that code evaluated with C<do FILENAME>
cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
same, however, in that it does reparse the file every time you call it,
so you probably don't want to do this inside a loop.
-If C<do> cannot read the file, it returns undef and sets C<$!> to the
-error. If C<do> can read the file but cannot compile it, it
-returns undef and sets an error message in C<$@>. If the file is
-successfully compiled, C<do> returns the value of the last expression
-evaluated.
+If C<do> can read the file but cannot compile it, it returns C<undef> and sets
+an error message in C<$@>. If C<do> cannot read the file, it returns undef
+and sets C<$!> to the error. Always check C<$@> first, as compilation
+could fail in a way that also sets C<$!>. If the file is successfully
+compiled, C<do> returns the value of the last expression evaluated.
Inclusion of library modules is better done with the
C<use> and C<require> operators, which also do automatic error checking
it as C<CORE::dump()>, if you don't want to be warned against a possible
typo.
+Portability issues: L<perlport/dump>.
+
=item each HASH
X<each> X<hash, iterator>
=item each ARRAY
X<array, iterator>
+=item each EXPR
+
When called in list context, returns a 2-element list consisting of the key
and value for the next element of a hash, or the index and value for the
next element of an array, so that you can iterate over it. When called in
in an array.
Hash entries are returned in an apparently random order. The actual random
-order is subject to change in future versions of perl, but it is
+order is subject to change in future versions of Perl, but it is
guaranteed to be in the same order as either the C<keys> or C<values>
function would produce on the same (unmodified) hash. Since Perl
5.8.2 the ordering can be different even between different runs of Perl
C<values> on the hash or array. If you add or delete a hash's elements
while iterating over it, entries may be skipped or duplicated--so don't do
that. Exception: It is always safe to delete the item most recently
-returned by C<each()>, so the following code will work properly:
+returned by C<each()>, so the following code works properly:
while (($key, $value) = each %hash) {
print $key, "\n";
}
This prints out your environment like the printenv(1) program,
-only in a different order:
+but in a different order:
while (($key,$value) = each %ENV) {
print "$key=$value\n";
}
-See also C<keys>, C<values> and C<sort>.
+Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
+reference to an unblessed hash or array. The argument will be dereferenced
+automatically. This aspect of C<each> is considered highly experimental.
+The exact behaviour may change in a future version of Perl.
+
+ while (($key,$value) = each $hashref) { ... }
+
+See also C<keys>, C<values>, and C<sort>.
=item eof FILEHANDLE
X<eof>
=item eof
-Returns 1 if the next read on FILEHANDLE will return end of file, or if
+Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
FILEHANDLE is not open. FILEHANDLE may be an expression whose value
gives the real filehandle. (Note that this function actually
reads a character and then C<ungetc>s it, so isn't useful in an
see L<perlop/"I/O Operators">.
In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
-detect the end of each file, C<eof()> will only detect the end of the
-last file. Examples:
+detect the end of each file, whereas C<eof()> will detect the end
+of the very last file only. Examples:
# reset line numbering on each input file
while (<>) {
}
Practical hint: you almost never need to use C<eof> in Perl, because the
-input operators typically return C<undef> when they run out of data, or if
-there was an error.
+input operators typically return C<undef> when they run out of data or
+encounter an error.
=item eval EXPR
X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
In the first form, the return value of EXPR is parsed and executed as if it
were a little Perl program. The value of the expression (which is itself
-determined within scalar context) is first parsed, and if there weren't any
-errors, executed in the lexical context of the current Perl program, so
-that any variable settings or subroutine and format definitions remain
-afterwards. Note that the value is parsed every time the C<eval> executes.
+determined within scalar context) is first parsed, and if there were no
+errors, executed as a block within the lexical context of the current Perl
+program. This means, that in particular, any outer lexical variables are
+visible to it, and any package variable settings or subroutine and format
+definitions remain afterwards.
+
+Note that the value is parsed every time the C<eval> executes.
If EXPR is omitted, evaluates C<$_>. This form is typically used to
delay parsing and subsequent execution of the text of EXPR until run time.
determined.
If there is a syntax error or runtime error, or a C<die> statement is
-executed, C<eval> returns an undefined value in scalar context
-or an empty list in list context, and C<$@> is set to the
-error message. If there was no error, C<$@> is guaranteed to be a null
-string. Beware that using C<eval> neither silences perl from printing
+executed, C<eval> returns C<undef> in scalar context
+or an empty list--or, for syntax errors, a list containing a single
+undefined value--in list context, and C<$@> is set to the error
+message. The discrepancy in the return values in list context is
+considered a bug by some, and will probably be fixed in a future
+release. If there was no error, C<$@> is guaranteed to be the empty
+string. Beware that using C<eval> neither silences Perl from printing
warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
Note that, because C<eval> traps otherwise-fatal errors, it is useful for
determining whether a particular feature (such as C<socket> or C<symlink>)
-is implemented. It is also Perl's exception trapping mechanism, where
+is implemented. It is also Perl's exception-trapping mechanism, where
the die operator is used to raise exceptions.
If you want to trap errors when loading an XS module, some problems with
particular situation, you can just use symbolic references instead, as
in case 6.
-The assignment to C<$@> occurs before restoration of localised variables,
-which means a temporary is required if you want to mask some but not all
+Before Perl 5.14, the assignment to C<$@> occurred before restoration
+of localised variables, which means that for your code to run on older
+versions, a temporary is required if you want to mask some but not all
errors:
# alter $@ on nefarious repugnancy only
{
local $@; # protect existing $@
eval { test_repugnancy() };
- # $@ =~ /nefarious/ and die $@; # DOES NOT WORK
+ # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
$@ =~ /nefarious/ and $e = $@;
}
die $e if defined $e
C<eval BLOCK> does I<not> count as a loop, so the loop control statements
C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
-Note that as a special case, an C<eval ''> executed within the C<DB>
-package doesn't see the usual surrounding lexical scope, but rather the
-scope of the first non-DB piece of code that called it. You don't normally
-need to worry about this unless you are writing a Perl debugger.
+An C<eval ''> executed within the C<DB> package doesn't see the usual
+surrounding lexical scope, but rather the scope of the first non-DB piece
+of code that called it. You don't normally need to worry about this unless
+you are writing a Perl debugger.
=item exec LIST
X<exec> X<execute>
=item exec PROGRAM LIST
-The C<exec> function executes a system command I<and never returns>--
+The C<exec> function executes a system command I<and never returns>;
use C<system> instead of C<exec> if you want it to return. It fails and
returns false only if the command does not exist I<and> it is executed
directly instead of via your system's command shell (see below).
Since it's a common mistake to use C<exec> instead of C<system>, Perl
warns you if there is a following statement that isn't C<die>, C<warn>,
-or C<exit> (if C<-w> is set --but you always do that). If you
+or C<exit> (if C<-w> is set--but you always do that, right?). If you
I<really> want to follow an C<exec> with some other statement, you
can use one of these styles to avoid the warning:
exec {'/bin/csh'} '-sh'; # pretend it's a login shell
-When the arguments get executed via the system shell, results will
-
be subject to its quirks and capabilities. See L<perlop/"`STRING`">
+When the arguments get executed via the system shell, results are
+subject to its quirks and capabilities. See L<perlop/"`STRING`">
for details.
Using an indirect object with C<exec> or C<system> is also more
Note that C<exec> will not call your C<END> blocks, nor will it invoke
C<DESTROY> methods on your objects.
+Portability issues: L<perlport/exec>.
+
=item exists EXPR
X<exists> X<autovivification>
-Given an expression that specifies a hash element or array element,
-returns true if the specified element in the hash or array has ever
-been initialized, even if the corresponding value is undefined.
+Given an expression that specifies an element of a hash, returns true if the
+specified element in the hash has ever been initialized, even if the
+corresponding value is undefined.
print "Exists\n" if exists $hash{$key};
print "Defined\n" if defined $hash{$key};
print "True\n" if $hash{$key};
+exists may also be called on array elements, but its behavior is much less
+obvious and is strongly tied to the use of L</delete> on arrays. B<Be aware>
+that calling exists on array values is deprecated and likely to be removed in
+a future version of Perl.
+
print "Exists\n" if exists $array[$index];
print "Defined\n" if defined $array[$index];
print "True\n" if $array[$index];
-A hash or array element can be true only if it's defined, and defined if
+A hash or array element can be true only if it's defined and defined only if
it exists, but the reverse doesn't necessarily hold true.
Given an expression that specifies the name of a subroutine,
does not count as declaring it. Note that a subroutine that does not
exist may still be callable: its package may have an C<AUTOLOAD>
method that makes it spring into existence the first time that it is
print "Exists\n" if exists &subroutine;
print "Defined\n" if defined &subroutine;
if (exists &{$ref->{A}{B}{$key}}) { }
-Although the deepest nested array or hash will not spring into existence
-just because its existence was tested, any intervening ones will.
+Although the mostly deeply nested array or hash will not spring into
+existence just because its existence was tested, any intervening ones will.
Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
into existence due to the existence test for the $key element above.
-This happens anywhere the arrow operator is used, including even:
+This happens anywhere the arrow operator is used, including even here:
undef $ref;
if (exists $ref->{"Some key"}) { }
The exit() function does not always exit immediately. It calls any
defined C<END> routines first, but these C<END> routines may not
themselves abort the exit. Likewise any object destructors that need to
-be called are called before the real exit. If this is a problem, you
+be called are called before the real exit. C<END> routines and destructors
+can change the exit status by modifying C<$?>. If this is a problem, you
can call C<POSIX:_exit($status)> to avoid END and destructor processing.
See L<perlmod> for details.
+Portability issues: L<perlport/exit>.
+
=item exp EXPR
X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
use Fcntl;
first to get the correct constant definitions. Argument processing and
-value return works just like C<ioctl> below.
+value returned work just like C<ioctl> below.
For example:
use Fcntl;
in numeric context. It is also exempt from the normal B<-w> warnings
on improper numeric conversions.
-Note that C<fcntl> will produce a fatal error if used on a machine that
+Note that C<fcntl> raises an exception if used on a machine that
doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
manpage to learn what functions are available on your system.
$flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
or die "Can't set flags for the socket: $!\n";
+Portability issues: L<perlport/fcntl>.
+
=item fileno FILEHANDLE
X<fileno>
Returns the file descriptor for a filehandle, or undefined if the
-filehandle is not open. This is mainly useful for constructing
+filehandle is not open. If there is no real file descriptor at the OS
+level, as can happen with filehandles connected to memory objects via
+C<open> with a reference for the third argument, -1 is returned.
+
+This is mainly useful for constructing
bitmaps for C<select> and low-level POSIX tty-handling operations.
If FILEHANDLE is an expression, the value is taken as an indirect
filehandle, generally its name.
print "THIS and THAT are dups\n";
}
-(Filehandles connected to memory objects via new features of C<open> may
-return undefined even though they are open.)
-
-
=item flock FILEHANDLE,OPERATION
X<flock> X<lock> X<locking>
Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
for success, false on failure. Produces a fatal error if used on a
machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
-C<flock> is Perl's portable file locking interface, although it locks
-only entire files, not records.
+C<flock> is Perl's portable file-locking interface, although it locks
+entire files only, not records.
Two potentially non-obvious but traditional C<flock> semantics are
that it waits indefinitely until the lock is granted, and that its locks
-B<merely advisory>. Such discretionary locks are more flexible, but offer
-fewer guarantees. This means that programs that do not also use C<flock>
-may modify files locked with C<flock>. See L<perlport>,
-your port's specific documentation, or your system-specific local manpages
+are B<merely advisory>. Such discretionary locks are more flexible, but
+offer fewer guarantees. This means that programs that do not also use
+
C<flock> may modify files locked with C<flock>. See L<perlport>,
+your port's specific documentation, and your system-specific local manpages
for details. It's best to assume traditional behavior if you're writing
portable programs. (But if you're not, you should as always feel perfectly
free to write for your own system's idiosyncrasies (sometimes called
OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
-you can use the symbolic names if you import them from the Fcntl module,
-either individually, or as a group using the ':flock' tag. LOCK_SH
+you can use the symbolic names if you import them from the L<Fcntl> module,
+either individually, or as a group using the C<:flock> tag. LOCK_SH
requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
-LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
-waiting for the lock (check the return status to see if you got it).
+LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
+waiting for the lock; check the return status to see if you got it.
To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
before locking or unlocking it.
that. If you like you can force Perl to ignore your system's flock(2)
function, and so provide its own fcntl(2)-based emulation, by passing
the switch C<-Ud_flock> to the F<Configure> program when you configure
-perl.
+and build a new Perl.
Here's a mailbox appender for BSD systems.
print $mbox $msg,"\n\n";
unlock($mbox);
-On systems that support a real flock(), locks are inherited across fork()
-calls, whereas those that must resort to the more capricious fcntl()
-function lose the locks, making it harder to write servers.
+On systems that support a real flock(2), locks are inherited across fork()
+calls, whereas those that must resort to the more capricious fcntl(2)
+function lose their locks, making it seriously harder to write servers.
See also L<DB_File> for other flock() examples.
+Portability issues: L<perlport/flock>.
+
=item fork
X<fork> X<child> X<parent>
backgrounded job launched from a remote shell) won't think you're done.
You should reopen those to F</dev/null> if it's any issue.
+On some platforms such as Windows, where the fork() system call is not available,
+Perl can be built to emulate fork() in the Perl interpreter. The emulation is designed to,
+at the level of the Perl program, be as compatible as possible with the "Unix" fork().
+However it has limitation that has to be considered in code intended to be portable.
+See L<perlfork> for more details.
+
+Portability issues: L<perlport/fork>.
+
=item format
X<format>
and then set C<$^A> back to C<"">. Note that a format typically
does one C<formline> per line of form, but the C<formline> function itself
doesn't care how many newlines are embedded in the PICTURE. This means
-that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
+that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
You may therefore need to use multiple formlines to implement a single
-record format, just like the format compiler.
+record format, just like the C<format> compiler.
Be careful if you put double quotes around the picture, because an C<@>
character may be taken to mean the beginning of an array name.
C<formline> always returns true. See L<perlform> for other examples.
+If you are trying to use this instead of C<write> to capture the output,
+you may find it easier to open a filehandle to a scalar
+(C<< open $fh, ">", \$output >>) and write to that instead.
+
=item getc FILEHANDLE
X<getc> X<getchar> X<character> X<file, read>
=item getc
Returns the next character from the input file attached to FILEHANDLE,
-or the undefined value at end of file, or if there was an error (in
+or the undefined value at end of file or if there was an error (in
the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
STDIN. This is not particularly efficient. However, it cannot be
used by itself to fetch single characters without waiting for the user
system "stty -cbreak </dev/tty >/dev/tty 2>&1";
}
else {
- system "stty", 'icanon', 'eol', '^@'; # ASCII null
+ system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
}
print "\n";
The C<POSIX::getattr> function can do this more portably on
systems purporting POSIX compliance. See also the C<Term::ReadKey>
-module from your nearest CPAN site; details on CPAN can be found on
+module from your nearest CPAN site; details on CPAN can be found under
L<perlmodlib/CPAN>.
=item getlogin
X<getlogin> X<login>
This implements the C library function of the same name, which on most
-systems returns the current login from F</etc/utmp>, if any. If null,
-use C<getpwuid>.
+systems returns the current login from F</etc/utmp>, if any. If it
+returns the empty string, use C<getpwuid>.
$login = getlogin || getpwuid($<) || "Kilroy";
Do not consider C<getlogin> for authentication: it is not as
secure as C<getpwuid>.
+Portability issues: L<perlport/getlogin>.
+
=item getpeername SOCKET
X<getpeername> X<peer>
-Returns the packed sockaddr address of other end of the SOCKET connection.
+Returns the packed sockaddr address of the other end of the SOCKET
+connection.
use Socket;
$hersockaddr = getpeername(SOCK);
Returns the current process group for the specified PID. Use
a PID of C<0> to get the current process group for the
current process. Will raise an exception if used on a machine that
-doesn't implement getpgrp(2). If PID is omitted, returns process
-group of current process. Note that the POSIX version of C<getpgrp>
+doesn't implement getpgrp(2). If PID is omitted, returns the process
+group of the current process. Note that the POSIX version of C<getpgrp>
does not accept a PID argument, so only C<PID==0> is truly portable.
+Portability issues: L<perlport/getpgrp>.
+
=item getppid
X<getppid> X<parent> X<pid>
Note for Linux users: on Linux, the C functions C<getpid()> and
C<getppid()> return different values from different threads. In order to
-be portable, this behavior is not reflected by the perl-level function
+be portable, this behavior is not reflected by the Perl-level function
C<getppid()>, that returns a consistent value across threads. If you want
to call the underlying C<getppid()>, you may use the CPAN module
C<Linux::Pid>.
+Portability issues: L<perlport/getppid>.
+
=item getpriority WHICH,WHO
X<getpriority> X<priority> X<nice>
(See C<getpriority(2)>.) Will raise a fatal exception if used on a
machine that doesn't implement getpriority(2).
+Portability issues: L<perlport/getpriority>.
+
=item getpwnam NAME
X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
($name,$aliases,$proto) = getproto*
($name,$aliases,$port,$proto) = getserv*
-(If the entry doesn't exist you get a null list.)
+(If the entry doesn't exist you get an empty list.)
The exact meaning of the $gcos field varies but it usually contains
the real name of the user (as opposed to the login name) and other
system users are able to change this information and therefore it
cannot be trusted and therefore the $gcos is tainted (see
L<perlsec>). The $passwd and $shell, user's encrypted password and
-login shell, are also tainted, because of the same reason.
+login shell, are also tainted, for the same reason.
In scalar context, you get the name, unless the function was a
lookup by name, in which case you get the other thing, whatever it is.
aging. In some systems the $comment field may be $class. The $expire
field, if present, encodes the expiration period of the account or the
password. For the availability and the exact meaning of these fields
-in your system, please consult your getpwnam(3) documentation and your
+in your system, please consult getpwnam(3) and your system's
F<pwd.h> file. You can also find out from within Perl what your
$quota and $comment fields mean and whether you have the $expire field
by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
-files are only supported if your vendor has implemented them in the
+files are supported only if your vendor has implemented them in the
intuitive fashion that calling the regular C library routines gets the
shadow versions if you're running under privilege or if there exists
the shadow(3) functions as found in System V (this includes Solaris
-and Linux.) Those systems that implement a proprietary shadow password
+and Linux). Those systems that implement a proprietary shadow password
facility are unlikely to be supported.
-The $members value returned by I<getgr*()> is a space separated list of
+The $members value returned by I<getgr*()> is a space-separated list of
the login names of the members of the group.
For the I<gethost*()> functions, if the C<h_errno> variable is supported in
C, it will be returned to you via C<$?> if the function call fails. The
-C<@addrs> value returned by a successful call is a list of the raw
-addresses returned by the corresponding system library call. In the
-Internet domain, each address is four bytes long and you can unpack it
+C<@addrs> value returned by a successful call is a list of raw
+addresses returned by the corresponding library call. In the
+Internet domain, each address is four bytes long; you can unpack it
by saying something like:
($a,$b,$c,$d) = unpack('W4',$addr[0]);
use User::pwent;
$is_his = (stat($filename)->uid == pwent($whoever)->uid);
-Even though it looks like they're the same method calls (uid),
+Even though it looks as though they're the same method calls (uid),
they aren't, because a C<File::stat> object is different from
a C<User::pwent> object.
+Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
+
=item getsockname SOCKET
X<getsockname>
number of TCP, which you can get using C<getprotobyname>.
The function returns a packed string representing the requested socket
-option, or C<undef> if there is an error (the error reason will be in
-C<$!>). What exactly is in the packed string depends on LEVEL and OPTNAME;
+option, or C<undef> on error, with the reason for the error placed in
+C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
consult getsockopt(2) for details. A common case is that the option is an
integer, in which case the result is a packed integer, which you can decode
using C<unpack> with the C<i> (or C<I>) format.
-An example to test whether Nagle's algorithm is turned on on a socket:
+Here's an example to test whether Nagle's algorithm is enabled on a socket:
use Socket qw(:all);
my $nodelay = unpack("I", $packed);
print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
+Portability issues: L<perlport/getsockopt>.
+
+=item given EXPR BLOCK
+X<given>
+
+=item given BLOCK
+
+C<given> is analogous to the C<switch> keyword in other languages. C<given>
+and C<when> are used in Perl to implement C<switch>/C<case> like statements.
+Only available after Perl 5.10. For example:
+
+ use v5.10;
+ given ($fruit) {
+ when (/apples?/) {
+ print "I like apples."
+ }
+ when (/oranges?/) {
+ print "I don't like oranges."
+ }
+ default {
+ print "I don't like anything"
+ }
+ }
+
+See L<perlsyn/"Switch statements"> for detailed information.
=item glob EXPR
X<glob> X<wildcard> X<filename, expansion> X<expand>
Note that C<glob> splits its arguments on whitespace and treats
each segment as separate pattern. As such, C<glob("*.c *.h")>
matches all files with a F<.c> or F<.h> extension. The expression
-C<glob(".* *")> matchs all files in the current working directory.
+C<glob(".* *")> matches all files in the current working directory.
If non-empty braces are the only wildcard characters used in the
C<glob>, no filenames are matched, but potentially many strings
C<File::Glob> extension. See L<File::Glob> for details, including
C<bsd_glob> which does not treat whitespace as a pattern separator.
+Portability issues: L<perlport/glob>.
+
=item gmtime EXPR
X<gmtime> X<UTC> X<Greenwich>
=item gmtime
-Works just like L<localtime> but the returned values are
+Works just like L</localtime> but the returned values are
localized for the standard Greenwich time zone.
-Note: when called in list context, $isdst, the last value
-returned by gmtime is always C<0>. There is no
+Note: When called in list context, $isdst, the last value
+returned by gmtime, is always C<0>. There is no
Daylight Saving Time in GMT.
-See L<perlport/gmtime> for portability concerns.
+Portability issues: L<perlport/gmtime>.
=item goto LABEL
X<goto> X<jump> X<jmp>
else within the dynamic scope, including out of subroutines, but it's
usually better to use some other construct such as C<last> or C<die>.
The author of Perl has never felt the need to use this form of C<goto>
-(in Perl, that is--C is another matter). (The difference is that C
+(in Perl, that is; C is another matter). (The difference is that C
does not offer named loops combined with loop control. Perl does, and
this replaces most structured uses of C<goto> in other languages.)
goto ("FOO", "BAR", "GLARCH")[$i];
+As shown in this example, C<goto-EXPR> is exempt from the "looks like a
+function" rule. A pair of parentheses following it does not (necessarily)
+delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
+
Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
deprecated and will issue a warning. Even then, it may not be used to
go into any construct that requires initialization, such as a
routine was called first.
NAME needn't be the name of a subroutine; it can be a scalar variable
-containing a code reference, or a block that evaluates to a code
+containing a code reference or a block that evaluates to a code
reference.
=item grep BLOCK LIST
Hex strings may only represent integers. Strings that would cause
integer overflow trigger a warning. Leading whitespace is not stripped,
unlike oct(). To present something as hex, look into L</printf>,
-L</sprintf>, or L</unpack>.
+L</sprintf>, and L</unpack>.
=item import LIST
X<import>
Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
You should not use this function for rounding: one because it truncates
-towards C<0>, and two because machine representations of floating point
+towards C<0>, and two because machine representations of floating-point
numbers can sometimes produce counterintuitive results. For example,
C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
because it's really more like -268.99999999999994315658 instead. Usually,
own, based on your C header files such as F<< <sys/ioctl.h> >>.
(There is a Perl script called B<h2ph> that comes with the Perl kit that
may help you in this, but it's nontrivial.) SCALAR will be read and/or
-written depending on the FUNCTION--a pointer to the string value of SCALAR
+written depending on the FUNCTION; a C pointer to the string value of SCALAR
will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
has no string value but does have a numeric value, that value will be
passed rather than a pointer to the string value. To guarantee this to be
The special string C<"0 but true"> is exempt from B<-w> complaints
about improper numeric conversions.
+Portability issues: L<perlport/ioctl>.
+
=item join EXPR,LIST
X<join>
=item keys ARRAY
+=item keys EXPR
+
Returns a list consisting of all the keys of the named hash, or the indices
of an array. (In scalar context, returns the number of keys or indices.)
The keys of a hash are returned in an apparently random order. The actual
-random order is subject to change in future versions of perl, but it
+random order is subject to change in future versions of Perl, but it
is guaranteed to be the same order as either the C<values> or C<each>
function produces (given that the hash has not been modified). Since
-Perl 5.8.1 the ordering is different even between different runs of
+Perl 5.8.1 the ordering can be different even between different runs of
Perl for security reasons (see L<perlsec/"Algorithmic Complexity
Attacks">).
-As a side effect, calling keys() resets the HASH or ARRAY's internal iterator
+As a side effect, calling keys() resets the internal interator of the HASH or ARRAY
(see L</each>). In particular, calling keys() in void context resets
the iterator with no other overhead.
printf "%4d %s\n", $hash{$key}, $key;
}
-As an lvalue C<keys> allows you to increase the number of hash buckets
+Used as an lvalue, C<keys> allows you to increase the number of hash buckets
allocated for the given hash. This can gain you a measure of efficiency if
you know the hash is going to get big. (This is similar to pre-extending
an array by assigning a larger number to $#array.) If you say
as trying has no effect). C<keys @array> in an lvalue context is a syntax
error.
-See also C<each>, C<values> and C<sort>.
+Starting with Perl 5.14, C<keys> can take a scalar EXPR, which must contain
+a reference to an unblessed hash or array. The argument will be
+dereferenced automatically. This aspect of C<keys> is considered highly
+experimental. The exact behaviour may change in a future version of Perl.
+
+ for (keys $hashref) { ... }
+ for (keys $obj->get_arrayref) { ... }
+
+See also C<each>, C<values>, and C<sort>.
=item kill SIGNAL, LIST
X<kill> X<signal>
$cnt = kill 1, $child1, $child2;
kill 9, @goners;
-If SIGNAL is zero, no signal is sent to the process, but the kill(2)
-system call will check whether it's possible to send a signal to it (that
+If SIGNAL is zero, no signal is sent to the process, but C<kill>
+checks whether it's I<possible> to send a signal to it (that
means, to be brief, that the process is owned by the same user, or we are
-the super-user). This is a useful way to check that a child process is
+the super-user). This is useful to check that a child process is still
alive (even if only as a zombie) and hasn't changed its UID. See
L<perlport> for notes on the portability of this construct.
See L<perlipc/"Signals"> for more details.
+On some platforms such as Windows where the fork() system call is not available.
+Perl can be built to emulate fork() at the interpreter level.
+This emulation has limitation related to kill that has to be considered,
+for code running on Windows and in code intended to be portable.
+
+See L<perlfork> for more details.
+
+Portability issues: L<perlport/kill>.
+
=item last LABEL
X<last> X<break>
}
C<last> cannot be used to exit a block that returns a value such as
-C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
+C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
a grep() or map() operation.
Note that a block by itself is semantically identical to a loop
=item Otherwise, If EXPR has the UTF8 flag set
If the current package has a subroutine named C<ToLower>, it will be used to
-change the case (See L<perlunicode/User-Defined Case Mappings>.)
+change the case
+(See L<perlunicode/"User-Defined Case Mappings (for serious hackers only)">.)
Otherwise Unicode semantics are used for the case change.
=item Otherwise, if C<use locale> is in effect
=item Otherwise, if C<use feature 'unicode_strings'> is in effect:
Unicode semantics are used for the case change. Any subroutine named
-C<ToLower> will not be used.
+C<ToLower> will be ignored.
=item Otherwise:
If EXPR is omitted, uses C<$_>.
-This function behaves the same way under various pragma, such as in a locale,
+This function behaves the same way under various pragmata, such as in a locale,
as L</lc> does.
=item length EXPR
=item length
Returns the length in I<characters> of the value of EXPR. If EXPR is
-omitted, returns length of C<$_>. If EXPR is undefined, returns C<undef>.
-Note that this cannot be used on an entire array or hash to find out how
-many elements these have. For that, use C<scalar @array> and C<scalar keys
-%hash> respectively.
-
-Note the I<characters>: if the EXPR is in Unicode, you will get the
-number of characters, not the number of bytes. To get the length
-of the internal string in bytes, use C<bytes::length(EXPR)>, see
-L<bytes>. Note that the internal encoding is variable, and the number
-of bytes usually meaningless. To get the number of bytes that the
-string would have when encoded as UTF-8, use
-C<length(Encoding::encode_utf8(EXPR))>.
+omitted, returns the length of C<$_>. If EXPR is undefined, returns
+C<undef>.
+
+This function cannot be used on an entire array or hash to find out how
+many elements these have. For that, use C<scalar @array> and C<scalar keys
+%hash>, respectively.
+
+Like all Perl character operations, length() normally deals in logical
+characters, not physical bytes. For how many bytes a string encoded as
+UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have
+to C<use Encode> first). See L<Encode> and L<perlunicode>.
=item link OLDFILE,NEWFILE
X<link>
Creates a new filename linked to the old filename. Returns true for
success, false otherwise.
+Portability issues: L<perlport/link>.
+
=item listen SOCKET,QUEUESIZE
X<listen>
-Does the same thing that the listen system call does. Returns true if
+Does the same thing that the listen(2) system call does. Returns true if
it succeeded, false otherwise. See the example in
L<perlipc/"Sockets: Client/Server Communication">.
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
localtime(time);
-All list elements are numeric, and come straight out of the C `struct
+All list elements are numeric and come straight out of the C `struct
tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
of the specified time.
-C<$mday> is the day of the month, and C<$mon> is the month itself, in
-the range C<0..11> with 0 indicating January and 11 indicating December.
+C<$mday> is the day of the month and C<$mon> the month in
+the range C<0..11>, with 0 indicating January and 11 indicating December.
This makes it easy to get a month name from a list:
my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
print "$abbr[$mon] $mday";
# $mon=9, $mday=18 gives "Oct 18"
-C<$year> is the number of years since 1900, not just the last two digits
+C<$year> is the number of years since 1900, B<not> just the last two digits
of the year. That is, C<$year> is C<123> in year 2023. The proper way
to get a 4-digit year is simply:
Otherwise you create non-Y2K-compliant programs--and you wouldn't want
to do that, would you?
-To get the last two digits of the year (e.g., '01' in 2001) do:
+To get the last two digits of the year (e.g., "01" in 2001) do:
$year = sprintf("%02d", $year % 100);
$now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
-This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
+This scalar value is B<not> locale-dependent but is a Perl builtin. For GMT
instead of local time use the L</gmtime> builtin. See also the
-C<Time::Local> module (to convert the second, minutes, hours, ... back to
+C<Time::Local> module (for converting seconds, minutes, hours, and such back to
the integer value returned by time()), and the L<POSIX> module's strftime(3)
and mktime(3) functions.
-To get somewhat similar but locale dependent date strings, set up your
+To get somewhat similar but locale-dependent date strings, set up your
locale environment variables appropriately (please see L<perllocale>) and
try for example:
Note that the C<%a> and C<%b>, the short forms of the day of the week
and the month of the year, may not necessarily be three characters wide.
-See L<perlport/localtime> for portability concerns.
-
-The L<Time::gmtime> and L<Time::localtime> modules provides a convenient,
+The L<Time::gmtime> and L<Time::localtime> modules provide a convenient,
by-name access mechanism to the gmtime() and localtime() functions,
respectively.
For a comprehensive date and time representation look at the
L<DateTime> module on CPAN.
+Portability issues: L<perlport/localtime>.
+
=item lock THING
X<lock>
-This function places an advisory lock on a shared variable, or referenced
+This function places an advisory lock on a shared variable or referenced
object contained in I<THING> until the lock goes out of scope.
lock() is a "weak keyword" : this means that if you've defined a function
by this name (before any calls to it), that function will be called
-instead. (However, if you've said C<use threads>, lock() is always a
-keyword.) See L<threads>.
+instead. If you are not under C<use threads::shared> this does nothing.
+See L<threads::shared>.
=item log EXPR
X<log> X<logarithm> X<e> X<ln> X<base>
=item log
Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
-returns log of C<$_>. To get the log of another base, use basic algebra:
+returns the log of C<$_>. To get the
+log of another base, use basic algebra:
The base-N log of a number is equal to the natural log of that number
divided by the natural log of N. For example:
If EXPR is omitted, stats C<$_>.
+Portability issues: L<perlport/lstat>.
+
=item m//
The match operator. See L<perlop/"Regexp Quote-Like Operators">.
list context, so each element of LIST may produce zero, one, or
more elements in the returned value.
- @chars = map(chr, @nums);
+ @chars = map(chr, @numbers);
+
+translates a list of numbers to the corresponding characters.
+
+ my @squares = map { $_ * $_ } @numbers;
+
+translates a list of numbers to their squared values.
+
+ my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;
+
+shows that number of returned elements can differ from the number of
+input elements. To omit an element, return an empty list ().
+This could also be achieved by writing
+
+ my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;
+
+which makes the intention more clear.
-translates a list of numbers to the corresponding characters. And
+Map always returns a list, which can be
+assigned to a hash such that the elements
+become key/value pairs. See L<perldata> for more details.
%hash = map { get_a_key_for($_) => $_ } @array;
can't be seen from the outside, avoiding any potential side-effects.
C<{> starts both hash references and blocks, so C<map { ...> could be either
-the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
+the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look
ahead for the closing C<}> it has to take a guess at which it's dealing with
based on what it finds just after the C<{>. Usually it gets it right, but if it
doesn't it won't realize something is wrong until it gets to the C<}> and
encounters the missing (or unexpected) comma. The syntax error will be
reported close to the C<}>, but you'll need to change something near the C<{>
-such as using a unary C<+> to give perl some help:
+such as using a unary C<+> to give Perl some help:
- %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
- %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
- %hash = map { ("\L$_", 1) } @array # this also works
- %hash = map { lc($_), 1 } @array # as does this.
- %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
+ %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong
+ %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right
+ %hash = map { ("\L$_" => 1) } @array # this also works
+ %hash = map { lc($_) => 1 } @array # as does this.
+ %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
- %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
+ %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
or to force an anon hash constructor use C<+{>:
- @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
+ @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs comma at end
-and you get list of anonymous hashes each with only 1 entry.
+to get a list of anonymous hashes each with only one entry apiece.
=item mkdir FILENAME,MASK
X<mkdir> X<md> X<directory, create>
Creates the directory specified by FILENAME, with permissions
specified by MASK (as modified by C<umask>). If it succeeds it
-returns true, otherwise it returns false and sets C<$!> (errno).
-If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
-to C<$_>.
+returns true; otherwise it returns false and sets C<$!> (errno).
+MASK defaults to 0777 if omitted, and FILENAME defaults
+to C<$_> if omitted.
-In general, it is better to create directories with permissive MASK,
-and let the user modify that with their C<umask>, than it is to supply
+In general, it is better to create directories with a permissive MASK
+and let the user modify that with their C<umask> than it is to supply
a restrictive MASK and give the user no way to be more permissive.
The exceptions to this rule are when the file or directory should be
kept private (mail files, for instance). The perlfunc(1) entry on
then ARG must be a variable that will hold the returned C<msqid_ds>
structure. Returns like C<ioctl>: the undefined value for error,
C<"0 but true"> for zero, or the actual return value otherwise. See also
-L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
+L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
+C<IPC::Semaphore>.
+
+Portability issues: L<perlport/msgctl>.
=item msgget KEY,FLAGS
X<msgget>
Calls the System V IPC function msgget(2). Returns the message queue
-id, or the undefined value if there is an error. See also
-L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
+id, or C<undef> on error. See also
+L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
+C<IPC::Msg>.
+
+Portability issues: L<perlport/msgget>.
=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
X<msgrcv>
SIZE. Note that when a message is received, the message type as a
native long integer will be the first thing in VAR, followed by the
actual message. This packing may be opened with C<unpack("l! a*")>.
-Taints the variable. Returns true if successful, or false if there is
-an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
-C<IPC::SysV::Msg> documentation.
+Taints the variable. Returns true if successful, false
+on error. See also L<perlipc/"SysV IPC"> and the documentation for
+C<IPC::SysV> and C<IPC::SysV::Msg>.
+
+Portability issues: L<perlport/msgrcv>.
=item msgsnd ID,MSG,FLAGS
X<msgsnd>
Calls the System V IPC function msgsnd to send the message MSG to the
message queue ID. MSG must begin with the native long integer message
-type, and be followed by the length of the actual message, and finally
+type, be followed by the length of the actual message, and then finally
the message itself. This kind of packing can be achieved with
C<pack("l! a*", $type, $message)>. Returns true if successful,
-or false if there is an error. See also C<IPC::SysV>
+false on error. See also the C<IPC::SysV>
and C<IPC::SysV::Msg> documentation.
+Portability issues: L<perlport/msgsnd>.
+
=item my EXPR
X<my>
the list must be placed in parentheses.
The exact semantics and interface of TYPE and ATTRS are still
-evolving. TYPE is currently bound to the use of C<fields> pragma,
+evolving. TYPE is currently bound to the use of the C<fields> pragma,
and attributes are handled using the C<attributes> pragma, or starting
from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
L<perlsub/"Private Variables via my()"> for details, and L<fields>,
}
Note that if there were a C<continue> block on the above, it would get
-executed even on discarded lines. If the LABEL is omitted, the command
+executed even on discarded lines. If LABEL is omitted, the command
refers to the innermost enclosing loop.
C<next> cannot be used to exit a block which returns a value such as
-C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
+C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
a grep() or map() operation.
Note that a block by itself is semantically identical to a loop
See also L</continue> for an illustration of how C<last>, C<next>, and
C<redo> work.
-=item no Module VERSION LIST
-X<no>
+=item no MODULE VERSION LIST
+X<no declarations>
+X<unimporting>
-=item no Module VERSION
+=item no MODULE VERSION
-=item no Module LIST
+=item no MODULE LIST
-=item no Module
+=item no MODULE
=item no VERSION
value. (If EXPR happens to start off with C<0x>, interprets it as a
hex string. If EXPR starts off with C<0b>, it is interpreted as a
binary string. Leading whitespace is ignored in all three cases.)
-The following will handle decimal, binary, octal, and hex in the standard
-Perl or C notation:
+The following will handle decimal, binary, octal, and hex in standard
+Perl notation:
$val = oct($val) if $val =~ /^0/;
If EXPR is omitted, uses C<$_>. To go the other way (produce a number
in octal), use sprintf() or printf():
- $perms = (stat("filename"))[2] & 07777;
- $oct_perms = sprintf "%lo", $perms;
+ $dec_perms = (stat("filename"))[2] & 07777;
+ $oct_perm_str = sprintf "%o", $perms;
The oct() function is commonly used when a string such as C<644> needs
-to be converted into a file mode, for example. (Although perl will
-automatically convert strings into numbers as needed, this automatic
-conversion assumes base 10.)
+to be converted into a file mode, for example. Although Perl
+automatically converts strings into numbers as needed, this automatic
+conversion assumes base 10.
+
+Leading white space is ignored without warning, as too are any trailing
+non-digits, such as a decimal point (C<oct> only handles non-negative
+integers, not negative integers or floating point).
=item open FILEHANDLE,EXPR
X<open> X<pipe> X<file, open> X<fopen>
Simple examples to open a file for reading:
- open(my $fh, '<', "input.txt") or die $!;
+ open(my $fh, "<", "input.txt")
+ or die "cannot open < input.txt: $!";
and for writing:
- open(my $fh, '>', "output.txt") or die $!;
+ open(my $fh, ">", "output.txt")
+ or die "cannot open > output.txt: $!";
(The following is a comprehensive reference to open(): for a gentler
introduction you may consider L<perlopentut>.)
-If FILEHANDLE is an undefined scalar variable (or array or hash element)
-the variable is assigned a reference to a new anonymous filehandle,
-otherwise if FILEHANDLE is an expression, its value is used as the name of
-the real filehandle wanted. (This is considered a symbolic reference, so
-C<use strict 'refs'> should I<not> be in effect.)
-
-If EXPR is omitted, the scalar variable of the same name as the
-FILEHANDLE contains the filename. (Note that lexical variables--those
-declared with C<my>--will not work for this purpose; so if you're
-using C<my>, specify EXPR in your call to open.)
-
-If three or more arguments are specified then the mode of opening and
-the file name are separate. If MODE is C<< '<' >> or nothing, the file
-is opened for input. If MODE is C<< '>' >>, the file is truncated and
-opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
-the file is opened for appending, again being created if necessary.
-
-You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
+If FILEHANDLE is an undefined scalar variable (or array or hash element), a
+new filehandle is autovivified, meaning that the variable is assigned a
+reference to a newly allocated anonymous filehandle. Otherwise if
+FILEHANDLE is an expression, its value is the real filehandle. (This is
+considered a symbolic reference, so C<use strict "refs"> should I<not> be
+in effect.)
+
+If EXPR is omitted, the global (package) scalar variable of the same
+name as the FILEHANDLE contains the filename. (Note that lexical
+variables--those declared with C<my> or C<state>--will not work for this
+purpose; so if you're using C<my> or C<state>, specify EXPR in your
+call to open.)
+
+If three (or more) arguments are specified, the open mode (including
+optional encoding) in the second argument are distinct from the filename in
+the third. If MODE is C<< < >> or nothing, the file is opened for input.
+If MODE is C<< > >>, the file is opened for output, with existing files
+first being truncated ("clobbered") and nonexisting files newly created.
+If MODE is C<<< >> >>>, the file is opened for appending, again being
+created if necessary.
+
+You can put a C<+> in front of the C<< > >> or C<< < >> to
indicate that you want both read and write access to the file; thus
-C<< '+<' >> is almost always preferred for read/write updates--the C<<
-'+>' >> mode would clobber the file first. You can't usually use
+C<< +< >> is almost always preferred for read/write updates--the
+C<< +> >> mode would clobber the file first. You cant usually use
either read-write mode for updating textfiles, since they have
-variable
length records. See the B<-i> switch in L<perlrun> for a
+variable
-length records. See the B<-i> switch in L<perlrun> for a
better approach. The file is created with permissions of C<0666>
modified by the process's C<umask> value.
-These various prefixes correspond to the fopen(3) modes of C<'r'>,
-C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
-
-In the 2-arguments (and 1-argument) form of the call the mode and
-filename should be concatenated (in this order), possibly separated by
-spaces. It is possible to omit the mode in these forms if the mode is
-C<< '<' >>.
+These various prefixes correspond to the fopen(3) modes of C<r>,
+C<r+>, C<w>, C<w+>, C<a>, and C<a+>.
-If the filename begins with C<'|'>, the filename is interpreted as a
-command to which output is to be piped, and if the filename ends with a
-C<'|'>, the filename is interpreted as a command that pipes output to
-us. See L<perlipc/"Using open() for IPC">
-for more examples of this. (You are not allowed to C<open> to a command
-that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
-and L<perlipc/"Bidirectional Communication with Another Process">
-for alternatives.)
+In the one- and two-argument forms of the call, the mode and filename
+should be concatenated (in that order), preferably separated by white
+space. You can--but shouldn't--omit the mode in these forms when that mode
+is C<< < >>. It is always safe to use the two-argument form of C<open> if
+the filename argument is a known literal.
-For three or more arguments if MODE is C<'|-'>, the filename is
+For three or more arguments if MODE is C<|->, the filename is
interpreted as a command to which output is to be piped, and if MODE
-is C<'-|'>, the filename is interpreted as a command that pipes
-output to us. In the 2-arguments (and 1-argument) form one should
-replace dash (C<'-'>) with the command.
+is C<-|>, the filename is interpreted as a command that pipes
+output to us. In the two-argument (and one-argument) form, one should
+replace dash (C<->) with the command.
See L<perlipc/"Using open() for IPC"> for more examples of this.
(You are not allowed to C<open> to a command that pipes both in I<and>
out, but see L<IPC::Open2>, L<IPC::Open3>, and
-L<perlipc/"Bidirectional Communication"> for alternatives.)
+L<perlipc/"Bidirectional Communication with Another Process"> for
+alternatives.)
-In the three-or-more argument form of pipe opens, if LIST is specified
+In the form of pipe opens taking three or more arguments, if LIST is specified
(extra arguments after the command name) then LIST becomes arguments
to the command invoked if the platform supports it. The meaning of
C<open> with more than three arguments for non-pipe modes is not yet
-specified. Experimental "layers" may give extra LIST arguments
+defined, but experimental "layers" may give extra LIST arguments
meaning.
-In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
-and opening C<< '>-' >> opens STDOUT.
+In the two-argument (and one-argument) form, opening C<< <- >>
+or C<-> opens STDIN and opening C<< >- >> opens STDOUT.
-You may use the three-argument form of open to specify IO "layers"
-(sometimes also referred to as "disciplines") to be applied to the handle
+You may (and usually should) use the three-argument form of open to specify
+I/O layers (sometimes referred to as "disciplines") to apply to the handle
that affect how the input and output are processed (see L<open> and
-L<PerlIO> for more details). For example
+L<PerlIO> for more details). For example:
- open(my $fh, "<:encoding(UTF-8)", "file")
+ open(my $fh, "<:encoding(UTF-8)", "filename")
+ || die "can't open UTF-8 encoded filename: $!";
-will open the UTF-8 encoded file containing Unicode characters,
+opens the UTF8-encoded file containing Unicode characters;
see L<perluniintro>. Note that if layers are specified in the
-three-arg
form then default layers stored in ${^OPEN} (see L<perlvar>;
+three-arg
ument form, then default layers stored in ${^OPEN} (see L<perlvar>;
usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
Open returns nonzero on success, the undefined value otherwise. If
modules that can help with that problem)) always check
the return value from opening a file.
-As a special case the 3-arg form with a read/write mode and the third
+As a special case the three-argument form with a read/write mode and the third
argument being C<undef>:
open(my $tmp, "+>", undef) or die ...
-opens a filehandle to an anonymous temporary file. Also using "+<"
+opens a filehandle to an anonymous temporary file. Also using C<< +< >>
works for symmetry, but you really should consider writing something
to the temporary file first. You will need to seek() to do the
reading.
-Since v5.8.0, perl has built using PerlIO by default. Unless you've
-changed this (i.e., Configure -Uuseperlio), you can open file handles to
-"in memory" files held in Perl scalars via:
+Since v5.8.0, Perl has built using PerlIO by default. Unless you've
+changed this (such as building Perl with C<Configure -Uuseperlio>), you can
+open filehandles directly to Perl scalars via:
- open($fh, '>', \$variable) || ..
+ open($fh, ">", \$variable) || ..
-Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
-file, you have to close it first:
+To (re)open C<STDOUT> or C<STDERR> as an in-memory file, close it first:
close STDOUT;
- open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
+ open(STDOUT, ">", \$variable)
+ or die "Can't open STDOUT: $!";
-Examples:
+General examples:
$ARTICLE = 100;
- open ARTICLE or die "Can't find article $ARTICLE: $!\n";
+ open(ARTICLE) or die "Can't find article $ARTICLE: $!\n";
while (<ARTICLE>) {...
- open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
+ open(LOG, ">>/usr/spool/news/twitlog"); # (log is reserved)
# if the open fails, output is discarded
- open(my $dbase, '+<', 'dbase.mine') # open for update
+ open(my $dbase, "+<", "dbase.mine") # open for update
or die "Can't open 'dbase.mine' for update: $!";
- open(my $dbase, '+<dbase.mine') # ditto
+ open(my $dbase, "+<dbase.mine") # ditto
or die "Can't open 'dbase.mine' for update: $!";
- open(ARTICLE, '-|', "caesar <$article") # decrypt article
+ open(ARTICLE, "-|", "caesar <$article") # decrypt article
or die "Can't start caesar: $!";
open(ARTICLE, "caesar <$article |") # ditto
open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
or die "Can't start sort: $!";
- # in memory files
- open(MEMORY,'>', \$var)
+ # in-memory files
+ open(MEMORY, ">", \$var)
or die "Can't open memory file: $!";
print MEMORY "foo!\n"; # output will appear in $var
# process argument list of files along with any includes
foreach $file (@ARGV) {
- process($file, 'fh00');
+ process($file, "fh00");
}
sub process {
my($filename, $input) = @_;
$input++; # this is a string increment
- unless (open($input, $filename)) {
+ unless (open($input, "<", $filename)) {
print STDERR "Can't open $filename: $!\n";
return;
}
See L<perliol> for detailed info on PerlIO.
You may also, in the Bourne shell tradition, specify an EXPR beginning
-with C<< '>&' >>, in which case the rest of the string is interpreted
+with C<< >& >>, in which case the rest of the string is interpreted
as the name of a filehandle (or file descriptor, if numeric) to be
duped (as C<dup(2)>) and opened. You may use C<&> after C<< > >>,
C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
The mode you specify should match the mode of the original filehandle.
(Duping a filehandle does not take into account any existing contents
-of IO buffers.) If you use the 3-arg form then you can pass either a
-number, the name of a filehandle or the normal "reference to a glob".
+of IO buffers.) If you use the three-argument form, then you can pass either a
+number, the name of a filehandle, or the normal "reference to a glob".
Here is a script that saves, redirects, and restores C<STDOUT> and
C<STDERR> using various methods:
#!/usr/bin/perl
- open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
- open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
+ open(my $oldout, ">&STDOUT") or die "Can't dup STDOUT: $!";
+ open(OLDERR, ">&", \*STDERR) or die "Can't dup STDERR: $!";
- open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
- open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
+ open(STDOUT, '>', "foo.out") or die "Can't redirect STDOUT: $!";
+ open(STDERR, ">&STDOUT") or die "Can't dup STDOUT: $!";
select STDERR; $| = 1; # make unbuffered
select STDOUT; $| = 1; # make unbuffered
print STDOUT "stdout 1\n"; # this works for
print STDERR "stderr 1\n"; # subprocesses too
- open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
- open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
+ open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!";
+ open(STDERR, ">&OLDERR") or die "Can't dup OLDERR: $!";
print STDOUT "stdout 2\n";
print STDERR "stderr 2\n";
Being parsimonious on filehandles is also useful (besides being
parsimonious) for example when something is dependent on file
descriptors, like for example locking using flock(). If you do just
-C<< open(A, '>>&B') >>, the filehandle A will not have the same file
-descriptor as B, and therefore flock(A) will not flock(B), and vice
-versa. But with C<< open(A, '>>&=B') >> the filehandles will share
-the same file descriptor.
-
-Note that if you are using Perls older than 5.8.0, Perl will be using
-the standard C libraries' fdopen() to implement the "=" functionality.
-On many Unix systems fdopen() fails when file descriptors exceed a
-certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
-most often the default.
-
-You can see whether Perl has been compiled with PerlIO or not by
-running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
-is C<define>, you have PerlIO, otherwise you don't.
-
-If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
-with 2-arguments (or 1-argument) form of open(), then
-there is an implicit fork done, and the return value of open is the pid
-of the child within the parent process, and C<0> within the child
-process. (Use C<defined($pid)> to determine whether the open was successful.)
-The filehandle behaves normally for the parent, but i/o to that
+C<< open(A, ">>&B") >>, the filehandle A will not have the same file
+descriptor as B, and therefore flock(A) will not flock(B) nor vice
+versa. But with C<< open(A, ">>&=B") >>, the filehandles will share
+the same underlying system file descriptor.
+
+Note that under Perls older than 5.8.0, Perl uses the standard C library's'
+fdopen() to implement the C<=> functionality. On many Unix systems,
+fdopen() fails when file descriptors exceed a certain value, typically 255.
+For Perls 5.8.0 and later, PerlIO is (most often) the default.
+
+You can see whether your Perl was built with PerlIO by running C<perl -V>
+and looking for the C<useperlio=> line. If C<useperlio> is C<define>, you
+have PerlIO; otherwise you don't.
+
+If you open a pipe on the command C<-> (that is, specify either C<|-> or C<-|>
+with the one- or two-argument forms of C<open>),
+an implicit C<fork> is done, so C<open> returns twice: in the parent
+process it returns the pid
+of the child process, and in the child process it returns (a defined) C<0>.
+Use C<defined($pid)> or C<//> to determine whether the open was successful.
+
+For example, use either
+
+ $child_pid = open(FROM_KID, "-|") // die "can't fork: $!";
+
+or
+ $child_pid = open(TO_KID, "|-") // die "can't fork: $!";
+
+followed by
+
+ if ($child_pid) {
+ # am the parent:
+ # either write TO_KID or else read FROM_KID
+ ...
+ wait $child_pid;
+ } else {
+ # am the child; use STDIN/STDOUT normally
+ ...
+ exit;
+ }
+
+The filehandle behaves normally for the parent, but I/O to that
filehandle is piped from/to the STDOUT/STDIN of the child process.
-In the child process the filehandle isn't opened--i/o happens from/to
-the new STDOUT or STDIN. Typically this is used like the normal
+In the child process, the filehandle isn't opened--I/O happens from/to
+the new STDOUT/STDIN. Typically this is used like the normal
piped open when you want to exercise more control over just how the
-pipe command gets executed, such as when you are running setuid, and
-don't want to have to scan shell commands for metacharacters.
-The following triples are more or less equivalent:
+pipe command gets executed, such as when running setuid and
+you don't want to have to scan shell commands for metacharacters.
+
+The following blocks are more or less equivalent:
open(FOO, "|tr '[a-z]' '[A-Z]'");
- open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
- open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
- open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
+ open(FOO, "|-", "tr '[a-z]' '[A-Z]'");
+ open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
+ open(FOO, "|-", "tr", '[a-z]', '[A-Z]');
open(FOO, "cat -n '$file'|");
- open(FOO, '-|', "cat -n '$file'");
- open(FOO, '-|') || exec 'cat', '-n', $file;
- open(FOO, '-|', "cat", '-n', $file);
+ open(FOO, "-|", "cat -n '$file'");
+ open(FOO, "-|") || exec "cat", "-n", $file;
+ open(FOO, "-|", "cat", "-n", $file);
-The last example in each block shows the pipe as "list form", which is
+The last two examples in each block show the pipe as "list form", which is
not yet supported on all platforms. A good rule of thumb is that if
-your platform has true C<fork()> (in other words, if your platform is
-Unix) you can use the list form.
+your platform has a real C<fork()> (in other words, if your platform is
+Unix, including Linux and MacOS X), you can use the list form. You would
+want to use the list form of the pipe so you can pass literal arguments
+to the command without risk of the shell interpreting any shell metacharacters
+in them. However, this also bars you from opening pipes to commands
+that intentionally contain shell metacharacters, such as:
+
+ open(FOO, "|cat -n | expand -4 | lpr")
+ // die "Can't open pipeline to lpr: $!";
See L<perlipc/"Safe Pipe Opens"> for more examples of this.
On systems that support a close-on-exec flag on files, the flag will
be set for the newly opened file descriptor as determined by the value
-of
$^F. See L<perlvar/$^F>.
+of
C<$^F>. See L<perlvar/$^F>.
Closing any piped filehandle causes the parent process to wait for the
-child to finish, and returns the status value in C<$?> and
+child to finish, then returns the status value in C<$?> and
C<${^CHILD_ERROR_NATIVE}>.
-The filename passed to 2-argument (or 1-argument) form of open() will
-have leading and trailing whitespace deleted, and the normal
+The filename passed to the one- and two-argument forms of open() will
+have leading and trailing whitespace deleted and normal
redirection characters honored. This property, known as "magic open",
can often be used to good effect. A user could specify a filename of
F<"rsh cat file |">, or you could change certain filenames as needed:
$filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
open(FH, $filename) or die "Can't open $filename: $!";
-Use 3-argument form to open a file with arbitrary weird characters in it,
+Use the three-argument form to open a file with arbitrary weird characters in it,
- open(FOO, '<', $file);
+ open(FOO, "<", $file)
+ || die "can't open < $file: $!";
otherwise it's necessary to protect any leading and trailing whitespace:
$file =~ s#^(\s)#./$1#;
- open(FOO, "< $file\0");
+ open(FOO, "< $file\0")
+ || die "open failed: $!";
(this may not work on some bizarre filesystems). One should
-conscientiously choose between the I<magic> and 3-arguments form
+conscientiously choose between the I<magic> and I<three-argument> form
of open():
- open IN, $ARGV[0];
+ open(IN, $ARGV[0]) || die "can't open $ARGV[0]: $!";
will allow the user to specify an argument of the form C<"rsh cat file |">,
but will not work on a filename that happens to have a trailing space, while
- open IN, '<', $ARGV[0];
+ open(IN, "<", $ARGV[0])
+ || die "can't open < $ARGV[0]: $!";
will have exactly the opposite restrictions.
If you want a "real" C C<open> (see C<open(2)> on your system), then you
-should use the C<sysopen> function, which involves no such magic (but
-may use subtly different filemodes than Perl open(), which is mapped
-to C fopen()). This is
-another way to protect your filenames from interpretation. For example:
+should use the C<sysopen> function, which involves no such magic (but may
+use subtly different filemodes than Perl open(), which is mapped to C
+fopen()). This is another way to protect your filenames from
+interpretation. For example:
use IO::Handle;
sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
Using the constructor from the C<IO::Handle> package (or one of its
subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
-filehandles that have the scope of whatever variables hold references to
-them, and automatically close whenever and however you leave that scope:
+filehandles that have the scope of the variables used to hold them, then
+automatically (but silently) close once their reference counts become
+zero, typically at scope exit:
use IO::File;
#...
sub read_myfile_munged {
my $ALL = shift;
+ # or just leave it undef to autoviv
my $handle = IO::File->new;
- open($handle, "myfile") or die "myfile: $!";
+ open($handle, "<", "myfile") or die "myfile: $!";
$first = <$handle>
or return (); # Automatically closed here.
- mung $first or die "mung failed"; # Or here.
- return $first, <$handle> if $ALL; # Or here.
- $first; # Or here.
+ mung($first) or die "mung failed"; # Or here.
+ return (first, <$handle>) if $ALL; # Or here.
+ return $first; # Or here.
}
+B<WARNING:> The previous example has a bug because the automatic
+close that happens when the refcount on C<handle> does not
+properly detect and report failures. I<Always> close the handle
+yourself and inspect the return value.
+
+ close($handle)
+ || warn "close failed: $!";
+
See L</seek> for some details about mixing reading and writing.
+Portability issues: L<perlport/open>.
+
=item opendir DIRHANDLE,EXPR
X<opendir>
DIRHANDLE may be an expression whose value can be used as an indirect
dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
scalar variable (or array or hash element), the variable is assigned a
-reference to a new anonymous dirhandle.
+reference to a new anonymous dirhandle; that is, it's autovivified.
DIRHANDLEs have their own namespace separate from FILEHANDLEs.
-See example at C<readdir>.
+See the example at C<readdir>.
=item ord EXPR
X<ord> X<encoding>
=item ord
Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
-or Unicode) value of the first character of EXPR. If EXPR is omitted,
-uses C<$_>.
+or Unicode) value of the first character of EXPR.
+If EXPR is an empty string, returns 0. If EXPR is omitted, uses C<$_>.
+(Note I<character>, not byte.)
For the reverse, see L</chr>.
See L<perlunicode> for more about Unicode.
package for use within the current scope. When C<use strict 'vars'> is in
effect, C<our> lets you use declared global variables without qualifying
them with package names, within the lexical scope of the C<our> declaration.
-In this way C<our> differs from C<use vars>, which is package scoped.
+In this way C<our> differs from C<use vars>, which is package-scoped.
-Unlike C<my>, which both allocates storage for a variable and associates
-a simple name with that storage for use within the current scope, C<our>
-associates a simple name with a package variable in the current package,
-for use within the current scope. In other words, C<our> has the same
-scoping rules as C<my>, but does not necessarily create a
-variable.
+Unlike C<my> or C<state>, which allocates storage for a variable and
+associates a simple name with that storage for use within the current
+scope, C<our> associates a simple name with a package (read: global)
+variable in the current package, for use within the current lexical scope.
+In other words, C<our> has the same scoping rules as C<my> or C<state>, but
+does not necessarily create a variable.
If more than one value is listed, the list must be placed
in parentheses.
given by the TEMPLATE. The resulting string is the concatenation of
the converted values. Typically, each converted value looks
like its machine-level representation. For example, on 32-bit machines
-an integer may be represented by a sequence of 4 bytes that will be
-converted to a sequence of 4 characters.
+an integer may be represented by a sequence of 4 bytes, which will in
+Perl be presented as a string that's 4 characters long.
See L<perlpacktut> for an introduction to this function.
a A string with arbitrary binary data, will be null padded.
A A text (ASCII) string, will be space padded.
- Z A null terminated (ASCIZ) string, will be null padded.
+ Z A null-terminated (ASCIZ) string, will be null padded.
b A bit string (ascending bit order inside each byte, like vec()).
B A bit string (descending bit order inside each byte).
c A signed char (8-bit) value.
C An unsigned char (octet) value.
- W An unsigned char value (can be greater than 255).
+ W An unsigned char value (can be greater than 255).
s A signed short (16-bit) value.
S An unsigned short value.
Q An unsigned quad value.
(Quads are available only if your system supports 64-bit
integer values _and_ if Perl has been compiled to support those.
- Causes a fatal error otherwise.)
+ Raises an exception otherwise.)
i A signed integer value.
I A unsigned integer value.
j A Perl internal signed integer value (IV).
J A Perl internal unsigned integer value (UV).
- f A single-precision float in the native format.
- d A double-precision float in the native format.
+ f A single-precision float in native format.
+ d A double-precision float in native format.
- F A Perl internal floating point value (NV) in the native format
- D A long double-precision float in the native format.
+ F A Perl internal floating-point value (NV) in native format
+ D A float of long-double precision in native format.
(Long doubles are available only if your system supports long
double values _and_ if Perl has been compiled to support those.
- Causes a fatal error otherwise.)
+ Raises an exception otherwise.)
p A pointer to a null-terminated string.
P A pointer to a structure (fixed-length string).
and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in byte mode.
w A BER compressed integer (not an ASN.1 BER, see perlpacktut for
- details). Its bytes represent an unsigned integer in base 128,
- most significant digit first, with as few digits as possible. Bit
- eight (the high bit) is set on each byte except the last.
+ details). Its bytes represent an unsigned integer in base 128,
+ most significant digit first, with as few digits as possible. Bit
+ eight (the high bit) is set on each byte except the last.
- x A null byte.
+ x A null byte (a.k.a ASCII NUL, "\000", chr(0))
X Back up a byte.
- @ Null fill or truncate to absolute position, counted from the
- start of the innermost ()-group.
- . Null fill or truncate to absolute position specified by value.
+ @ Null-fill or truncate to absolute position, counted from the
+ start of the innermost ()-group.
+ . Null-fill or truncate to absolute position specified by the value.
( Start of a ()-group.
-One or more of the modifiers below may optionally follow some letters in the
-TEMPLATE (the second column lists the letters for which the modifier is
-valid):
+One or more modifiers below may optionally follow certain letters in the
+TEMPLATE (the second column lists letters for which the modifier is valid):
! sSlLiI Forces native (short, long, int) sizes instead
of fixed (16-/32-bit) sizes.
< sSiIlLqQ Force little-endian byte-order on the type.
jJfFdDpP (The "little end" touches the construct.)
-The C<E<gt>> and C<E<lt>> modifiers can also be used on C<()>-groups,
-in which case they force a certain byte-order on all components of
-that group, including subgroups.
+The C<< > >> and C<< < >> modifiers can also be used on C<()> groups
+to force a particular byte-order on all components in that group,
+including all its subgroups.
The following rules apply:
-=over 8
+=over
=item *
-Each letter may optionally be followed by a number giving a repeat
-count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
-C<H>, C<@>, C<.>, C<x>, C<X> and C<P> the pack function will gobble up
-that many values from the LIST. A C<*> for the repeat count means to
-use however many items are left, except for C<@>, C<x>, C<X>, where it
-is equivalent to C<0>, for <.> where it means relative to string start
-and C<u>, where it is equivalent to 1 (or 45, which is the same).
-A numeric repeat count may optionally be enclosed in brackets, as in
-C<pack 'C[80]', @arr>.
-
-One can replace the numeric repeat count by a template enclosed in brackets;
-then the packed length of this template in bytes is used as a count.
-For example, C<x[L]> skips a long (it skips the number of bytes in a long);
-the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
-If the template in brackets contains alignment commands (such as C<x![d]>),
-its packed length is calculated as if the start of the template has the maximal
-possible alignment.
-
-When used with C<Z>, C<*> results in the addition of a trailing null
-byte (so the packed result will be one longer than the byte C<length>
-of the item).
+Each letter may optionally be followed by a number indicating the repeat
+count. A numeric repeat count may optionally be enclosed in brackets, as
+in C<pack("C[80]", @arr)>. The repeat count gobbles that many values from
+the LIST when used with all format types other than C<a>, C<A>, C<Z>, C<b>,
+C<B>, C<h>, C<H>, C<@>, C<.>, C<x>, C<X>, and C<P>, where it means
+something else, dscribed below. Supplying a C<*> for the repeat count
+instead of a number means to use however many items are left, except for:
+
+=over
+
+=item *
+
+C<@>, C<x>, and C<X>, where it is equivalent to C<0>.
+
+=item *
+
+<.>, where it means relative to the start of the string.
+
+=item *
+
+C<u>, where it is equivalent to 1 (or 45, which here is equivalent).
+
+=back
+
+One can replace a numeric repeat count with a template letter enclosed in
+brackets to use the packed byte length of the bracketed template for the
+repeat count.
+
+For example, the template C<x[L]> skips as many bytes as in a packed long,
+and the template C<"$t X[$t] $t"> unpacks twice whatever $t (when
+variable-expanded) unpacks. If the template in brackets contains alignment
+commands (such as C<x![d]>), its packed length is calculated as if the
+start of the template had the maximal possible alignment.
+
+When used with C<Z>, a C<*> as the repeat count is guaranteed to add a
+trailing null byte, so the resulting string is always one byte longer than
+the byte length of the item itself.
When used with C<@>, the repeat count represents an offset from the start
-of the innermost () group.
+of the innermost C<()> group.
+
+When used with C<.>, the repeat count determines the starting position to
+calculate the value offset as follows:
-When used with C<.>, the repeat count is used to determine the starting
-position from where the value offset is calculated. If the repeat count
-is 0, it's relative to the current position. If the repeat count is C<*>,
-the offset is relative to the start of the packed string. And if its an
-integer C<n> the offset is relative to the start of the n-th innermost
-() group (or the start of the string if C<n> is bigger then the group
-level).
+=over
+
+=item *
+
+If the repeat count is C<0>, it's relative to the current position.
+
+=item *
+
+If the repeat count is C<*>, the offset is relative to the start of the
+packed string.
+
+=item *
+
+And if it's an integer I<n>, the offset is relative to the start of the
+I<n>th innermost C<( )> group, or to the start of the string if I<n> is
+bigger then the group level.
+
+=back
The repeat count for C<u> is interpreted as the maximal number of bytes
to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
=item *
The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
-string of length count, padding with nulls or spaces as necessary. When
+string of length count, padding with nulls or spaces as needed. When
unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
-after the first null, and C<a> returns data verbatim.
+after the first null, and C<a> returns data with no stripping at all.
-If the value-to-pack is too long, it is truncated. If too long and an
-explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
-by a null byte. Thus C<Z> always packs a trailing null (except when the
-count is 0).
+If the value to pack is too long, the result is truncated. If it's too
+long and an explicit count is provided, C<Z> packs only C<$count-1> bytes,
+followed by a null byte. Thus C<Z> always packs a trailing null, except
+when the count is 0.
=item *
-Likewise, the C<b> and C<B> fields pack a string that many bits long.
-Each character of the input field of pack() generates 1 bit of the result.
+Likewise, the C<b> and C<B> formats pack a string that's that many bits long.
+Each such format generates 1 bit of the result. These are typically followed
+by a repeat count like C<B8> or C<B64>.
+
Each result bit is based on the least-significant bit of the corresponding
input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
-and C<"1"> generate bits 0 and 1, as do characters C<"\0"> and C<"\1">.
+and C<"1"> generate bits 0 and 1, as do characters C<"\000"> and C<"\001">.
-Starting from the beginning of the input string of pack(), each 8-tuple
-of characters is converted to 1 character of output. With format C<b>
+Starting from the beginning of the input string, each 8-tuple
+of characters is converted to 1 character of output. With format C<b>,
the first character of the 8-tuple determines the least-significant bit of a
-character, and with format C<B> it determines the most-significant bit of
+character; with format C<B>, it determines the most-significant bit of
a character.
-If the length of the input string is not exactly divisible by 8, the
+If the length of the input string is not evenly divisible by 8, the
remainder is packed as if the input string were padded by null characters
-at the end. Similarly, during unpack()ing the "extra" bits are ignored.
+at the end. Similarly during unpacking, "extra" bits are ignored.
-If the input string of pack() is longer than needed, extra characters are
-ignored. A C<*> for the repeat count of pack() means to use all the
-characters of the input field. On unpack()ing the bits are converted to a
-string of C<"0">s and C<"1">s.
+If the input string is longer than needed, remaining characters are ignored.
+
+A C<*> for the repeat count uses all characters of the input field.
+On unpacking, bits are converted to a string of C<0>s and C<1>s.
=item *
-The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
-representable as hexadecimal digits, 0-9a-f) long.
+The C<h> and C<H> formats pack a string that many nybbles (4-bit groups,
+representable as hexadecimal digits, C<"0".."9"> C<"a".."f">) long.
-Each character of the input field of pack() generates 4 bits of the result.
-For non-alphabetical characters the result is based on the 4 least-significant
+For each such format, pack() generates 4 bits of result.
+With non-alphabetical characters, the result is based on the 4 least-significant
bits of the input character, i.e., on C<ord($char)%16>. In particular,
characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
-C<"\0"> and C<"\1">. For characters C<"a".."f"> and C<"A".."F"> the result
+C<"\000"> and C<"\001">. For characters C<"a".."f"> and C<"A".."F">, the result
is compatible with the usual hexadecimal digits, so that C<"a"> and
-C<"A"> both generate the nybble C<0xa==10>. The result for characters
-C<"g".."z"> and C<"G".."Z"> is not well-defined.
+C<"A"> both generate the nybble C<0xA==10>. Use only these specific hex
+characters with this format.
-Starting from the beginning of the input string of pack(), each pair
-of characters is converted to 1 character of output. With format C<h> the
+Starting from the beginning of the template to pack(), each pair
+of characters is converted to 1 character of output. With format C<h>, the
first character of the pair determines the least-significant nybble of the
-output character, and with format C<H> it determines the most-significant
+output character; with format C<H>, it determines the most-significant
nybble.
-If the length of the input string is not even, it behaves as if padded
-by a null character at the end. Similarly, during unpack()ing the "extra"
-nybbles are ignored.
+If the length of the input string is not even, it behaves as if padded by
+a null character at the end. Similarly, "extra" nybbles are ignored during
+unpacking.
+
+If the input string is longer than needed, extra characters are ignored.
-If the input string of pack() is longer than needed, extra characters are
-ignored.
-A C<*> for the repeat count of pack() means to use all the characters of
-the input field. On unpack()ing the nybbles are converted to a string
-of hexadecimal digits.
+A C<*> for the repeat count uses all characters of the input field. For
+unpack(), nybbles are converted to a string of hexadecimal digits.
=item *
-The C<p> type packs a pointer to a null-terminated string. You are
-responsible for ensuring the string is not a temporary value (which can
-potentially get deallocated before you get around to using the packed result).
-The C<P> type packs a pointer to a structure of the size indicated by the
-length. A NULL pointer is created if the corresponding value for C<p> or
-C<P> is C<undef>, similarly for unpack().
+The C<p> format packs a pointer to a null-terminated string. You are
+responsible for ensuring that the string is not a temporary value, as that
+could potentially get deallocated before you got around to using the packed
+result. The C<P> format packs a pointer to a structure of the size indicated
+by the length. A null pointer is created if the corresponding value for
+C<p> or C<P> is C<undef>; similarly with unpack(), where a null pointer
+unpacks into C<undef>.
-If your system has a strange pointer size (i.e., a pointer is neither as
-big as an int nor as big as a long), it may not be possible to pack or
+If your system has a strange pointer size--meaning a pointer is neither as
+big as an int nor as big as a long--it may not be possible to pack or
unpack pointers in big- or little-endian byte order. Attempting to do
-so will result in a fatal error.
+so raises an exception.
=item *
The C</> template character allows packing and unpacking of a sequence of
-items where the packed structure contains a packed item count followed by
-the packed items themselves.
-
-For C<pack> you write I<length-item>C</>I<sequence-item> and the
-I<length-item> describes how the length value is packed. The ones likely
-to be of most use are integer-packing ones like C<n> (for Java strings),
-C<w> (for ASN.1 or SNMP) and C<N> (for Sun XDR).
-
-For C<pack>, the I<sequence-item> may have a repeat count, in which case
-the minimum of that and the number of available items is used as argument
-for the I<length-item>. If it has no repeat count or uses a '*', the number
+items where the packed structure contains a packed item count followed by
+the packed items themselves. This is useful when the structure you're
+unpacking has encoded the sizes or repeat counts for some of its fields
+within the structure itself as separate fields.
+
+For C<pack>, you write I<length-item>C</>I<sequence-item>, and the
+I<length-item> describes how the length value is packed. Formats likely
+to be of most use are integer-packing ones like C<n> for Java strings,
+C<w> for ASN.1 or SNMP, and C<N> for Sun XDR.
+
+For C<pack>, I<sequence-item> may have a repeat count, in which case
+the minimum of that and the number of available items is used as the argument
+for I<length-item>. If it has no repeat count or uses a '*', the number
of available items is used.
-For C<unpack> an internal stack of integer arguments unpacked so far is
+For C<unpack>, an internal stack of integer arguments unpacked so far is
used. You write C</>I<sequence-item> and the repeat count is obtained by
popping off the last element from the stack. The I<sequence-item> must not
have a repeat count.
-If the I<sequence-item> refers to a string type (C<"A">, C<"a"> or C<"Z">),
-the I<length-item> is the string length, not the number of strings. If there is
-an explicit repeat count for pack, the packed string will be adjusted to that length.
+If I<sequence-item> refers to a string type (C<"A">, C<"a">, or C<"Z">),
+the I<length-item> is the string length, not the number of strings. With
+an explicit repeat count for pack, the packed string is adjusted to that
+length. For example:
- unpack 'W/a', "\04Gurusamy"; gives ('Guru')
- unpack 'a3/A A*', '007 Bond J '; gives (' Bond', 'J')
- unpack 'a3 x2 /A A*', '007: Bond, J.'; gives ('Bond, J', '.')
- pack 'n/a* w/a','hello,','world'; gives "\000\006hello,\005world"
- pack 'a/W2', ord('a') .. ord('z'); gives '2ab'
+ unpack("W/a", "\004Gurusamy") gives ("Guru")
+ unpack("a3/A A*", "007 Bond J ") gives (" Bond", "J")
+ unpack("a3 x2 /A A*", "007: Bond, J.") gives ("Bond, J", ".")
+
+ pack("n/a* w/a","hello,","world") gives "\000\006hello,\005world"
+ pack("a/W2", ord("a") .. ord("z")) gives "2ab"
The I<length-item> is not returned explicitly from C<unpack>.
-Adding a count to the I<length-item> letter is unlikely to do anything
-useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
-I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
-which Perl does not regard as legal in numeric strings.
+Supplying a count to the I<length-item> format letter is only useful with
+C<A>, C<a>, or C<Z>. Packing with a I<length-item> of C<a> or C<Z> may
+introduce C<"\000"> characters, which Perl does not regard as legal in
+numeric strings.
=item *
The integer types C<s>, C<S>, C<l>, and C<L> may be
-followed by a C<!> modifier to signify native shorts or
-longs--as you can see from above for example a bare C<l> does mean
-exactly 32 bits, the native C<long> (as seen by the local C compiler)
-may be larger. This is an issue mainly in 64-bit platforms. You can
-see whether using C<!> makes any difference by
+followed by a C<!> modifier to specify native shorts or
+longs. As shown in the example above, a bare C<l> means
+exactly 32 bits, although the native C<long> as seen by the local C compiler
+may be larger. This is mainly an issue on 64-bit platforms. You can
+see whether using C<!> makes any difference this way:
+
+ printf "format s is %d, s! is %d\n",
+ length pack("s"), length pack("s!");
- print length(pack("s")), " ", length(pack("s!")), "\n";
- print length(pack("l")), " ", length(pack("l!")), "\n";
+ printf "format l is %d, l! is %d\n",
+ length pack("l"), length pack("l!");
-C<i!> and C<I!> also work but only because of completeness;
+
+C<i!> and C<I!> are also allowed, but only for completeness' sake:
they are identical to C<i> and C<I>.
The actual sizes (in bytes) of native shorts, ints, longs, and long
-longs on the platform where Perl was built are also available via
-L<Config>:
+longs on the platform where Perl was built are also available from
+the command line:
+
+ $ perl -V:{short,int,long{,long}}size
+ shortsize='2';
+ intsize='4';
+ longsize='4';
+ longlongsize='8';
+
+or programmatically via the C<Config> module:
use Config;
print $Config{shortsize}, "\n";
print $Config{longsize}, "\n";
print $Config{longlongsize}, "\n";
-(The C<$Config{longlongsize}> will be undefined if your system does
-not support long longs.)
+C<$Config{longlongsize}> is undefined on systems without
+long long support.
=item *
-The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
-are inherently non-portable between processors and operating systems
-because they obey the native byteorder and endianness. For example a
-4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
-(arranged in and handled by the CPU registers) into bytes as
+The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J> are
+inherently non-portable between processors and operating systems because
+they obey native byteorder and endianness. For example, a 4-byte integer
+0x12345678 (305419896 decimal) would be ordered natively (arranged in and
+handled by the CPU registers) into bytes as
0x12 0x34 0x56 0x78 # big-endian
0x78 0x56 0x34 0x12 # little-endian
-Basically, the Intel and VAX CPUs are little-endian, while everybody
-else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
-Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
-used/uses them in little-endian mode; SGI/Cray uses them in big-endian
-mode.
+Basically, Intel and VAX CPUs are little-endian, while everybody else,
+including Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray, are
+big-endian. Alpha and MIPS can be either: Digital/Compaq uses (well, used)
+them in little-endian mode, but SGI/Cray uses them in big-endian mode.
-The names `big-endian' and `little-endian' are comic references to
-the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
-Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
-the egg-eating habits of the Lilliputians.
+The names I<big-endian> and I<little-endian> are comic references to the
+egg-eating habits of the little-endian Lilliputians and the big-endian
+Blefuscudians from the classic Jonathan Swift satire, I<Gulliver's Travels>.
+This entered computer lingo via the paper "On Holy Wars and a Plea for
+Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980.
Some systems may have even weirder byte orders such as
0x56 0x78 0x12 0x34
0x34 0x12 0x78 0x56
-You can see your system's preference with
+You can determine your system endianness with this incantation:
- print join(" ", map { sprintf "%#02x", $_ }
- unpack("W*",pack("L",0x12345678))), "\n";
+ printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678);
The byteorder on the platform where Perl was built is also available
via L<Config>:
use Config;
- print $Config{byteorder}, "\n";
+ print "$Config{byteorder}\n";
+
+or from the command line:
-Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
-and C<'87654321'> are big-endian.
+ $ perl -V:byteorder
-If you want portable packed integers you can either use the formats
-C<n>, C<N>, C<v>, and C<V>, or you can use the C<E<gt>> and C<E<lt>>
-modifiers. These modifiers are only available as of perl 5.9.2.
-See also L<perlport>.
+Byteorders C<"1234"> and C<"12345678"> are little-endian; C<"4321">
+and C<"87654321"> are big-endian.
+
+For portably packed integers, either use the formats C<n>, C<N>, C<v>,
+and C<V> or else use the C<< > >> and C<< < >> modifiers described
+immediately below. See also L<perlport>.
=item *
-All integer and floating point formats as well as C<p> and C<P> and
-C<()>-groups may be followed by the C<E<gt>> or C<E<lt>> modifiers
-to force big- or little- endian byte-order, respectively.
-This is especially useful, since C<n>, C<N>, C<v> and C<V> don't cover
-signed integers, 64-bit integers and floating point values. However,
-there are some things to keep in mind.
+Starting with Perl 5.9.2, integer and floating-point formats, along with
+the C<p> and C<P> formats and C<()> groups, may all be followed by the
+C<< > >> or C<< < >> endianness modifiers to respectively enforce big-
+or little-endian byte-order. These modifiers are especially useful
+given how C<n>, C<N>, C<v>, and C<V> don't cover signed integers,
+64-bit integers, or floating-point values.
+
+Here are some concerns to keep in mind when using an endianness modifier:
-Exchanging signed integers between different platforms only works
-if all platforms store them in the same format. Most platforms store
-signed integers in two's complement, so usually this is not an issue.
+=over
+
+=item *
+
+Exchanging signed integers between different platforms works only
+when all platforms store them in the same format. Most platforms store
+signed integers in two's-complement notation, so usually this is not an issue.
+
+=item *
-The C<E<gt>> or C<E<lt>> modifiers can only be used on floating point
+The C<< > >> or C<< < >> modifiers can only be used on floating-point
formats on big- or little-endian machines. Otherwise, attempting to
-do so will result in a fatal error.
+use them raises an exception.
-Forcing big- or little-endian byte-order on floating point values for
-data exchange can only work if all platforms are using the same
-binary representation (e.g. IEEE floating point format). Even if all
-platforms are using IEEE, there may be subtle differences. Being able
-to use C<E<gt>> or C<E<lt>> on floating point values can be useful,
+=item *
+
+Forcing big- or little-endian byte-order on floating-point values for
+data exchange can work only if all platforms use the same
+binary representation such as IEEE floating-point. Even if all
+platforms are using IEEE, there may still be subtle differences. Being able
+to use C<< > >> or C<< < >> on floating-point values can be useful,
but also dangerous if you don't know exactly what you're doing.
-It is not a general way to portably store floating point values.
+It is not a general way to portably store floating-point values.
+
+=item *
-When using C<E<gt>> or C<E<lt>> on an C<()>-group, this will affect
-all types inside the group that accept the byte-order modifiers,
-including all subgroups. It will silently be ignored for all other
+When using C<< > >> or C<< < >> on a C<()> group, this affects
+all types inside the group that accept byte-order modifiers,
+including all subgroups. It is silently ignored for all other
types. You are not allowed to override the byte-order within a group
that already has a byte-order modifier suffix.
+=back
+
=item *
-Real numbers (floats and doubles) are in the native machine format only;
-due to the multiplicity of floating formats around, and the lack of a
-standard "network" representation, no facility for interchange has been
-made. This means that packed floating point data written on one machine
-may not be readable on another - even if both use IEEE floating point
-arithmetic (as the endian-ness of the memory representation is not part
+Real numbers (floats and doubles) are in native machine format only.
+Due to the multiplicity of floating-point formats and the lack of a
+standard "network" representation for them, no facility for interchange has been
+made. This means that packed floating-point data written on one machine
+may not be readable on another, even if both use IEEE floating-point
+arithmetic (because the endianness of the memory representation is not part
of the IEEE spec). See also L<perlport>.
-If you know exactly what you're doing, you can use the C<E<gt>> or C<E<lt>>
-modifiers to force big- or little-endian byte-order on floating point values.
+If you know I<exactly> what you're doing, you can use the C<< > >> or C<< < >>
+modifiers to force big- or little-endian byte-order on floating-point values.
-Note that Perl uses doubles (or long doubles, if configured) internally for
-all numeric calculation, and converting from double into float and thence back
-to double again will lose precision (i.e., C<unpack("f", pack("f", $foo)>)
-will not in general equal $foo).
+Because Perl uses doubles (or long doubles, if configured) internally for
+all numeric calculation, converting from double into float and thence
+to double again loses precision, so C<unpack("f", pack("f", $foo)>)
+will not in general equal $foo.
=item *
-Pack and unpack can operate in two modes, character mode (C<C0> mode) where
-the packed string is processed per character and UTF-8 mode (C<U0> mode)
+Pack and unpack can operate in two modes: character mode (C<C0> mode) where
+the packed string is processed per character, and UTF-8 mode (C<U0> mode)
where the packed string is processed in its UTF-8-encoded Unicode form on
-a byte by byte basis. Character mode is the default unless the format string
-starts with an C<U>. You can switch mode at any moment with an explicit
-C<C0> or C<U0> in the format. A mode is in effect until the next mode switch
-or until the end of the ()-group in which it was entered.
+a byte-by-byte basis. Character mode is the default unless the format string
+starts with C<U>. You can always switch mode mid-format with an explicit
+C<C0> or C<U0> in the format. This mode remains in effect until the next
+mode change, or until the end of the C<()> group it (directly) applies to.
+
+Using C<C0> to get Unicode characters while using C<U0> to get I<non>-Unicode
+bytes is not necessarily obvious. Probably only the first of these
+is what you want:
+
+ $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
+ perl -CS -ne 'printf "%v04X\n", $_ for unpack("C0A*", $_)'
+ 03B1.03C9
+ $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
+ perl -CS -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
+ CE.B1.CF.89
+ $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
+ perl -C0 -ne 'printf "%v02X\n", $_ for unpack("C0A*", $_)'
+ CE.B1.CF.89
+ $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
+ perl -C0 -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
+ C3.8E.C2.B1.C3.8F.C2.89
+
+Those examples also illustrate that you should not try to use
+C<pack>/C<unpack> as a substitute for the L<Encode> module.
=item *
-You must yourself do any alignment or padding by inserting for example
-enough C<'x'>es while packing. There is no way to pack() and unpack()
-could know where the characters are going to or coming from. Therefore
-C<pack> (and C<unpack>) handle their output and input as flat
-sequences of characters.
+You must yourself do any alignment or padding by inserting, for example,
+enough C<"x">es while packing. There is no way for pack() and unpack()
+to know where characters are going to or coming from, so they
+handle their output and input as flat sequences of characters.
=item *
-A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
-take a repeat count, both as postfix, and for unpack() also via the C</>
-template character. Within each repetition of a group, positioning with
-C<@> starts again at 0. Therefore, the result of
+A C<()> group is a sub-TEMPLATE enclosed in parentheses. A group may
+take a repeat count either as postfix, or for unpack(), also via the C</>
+template character. Within each repetition of a group, positioning with
+C<@> starts over at 0. Therefore, the result of
- pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
+ pack("@1A((@2A)@3A)", qw[X Y Z])
-is the string "\0a\0\0bc".
+is the string C<"\0X\0\0YZ">.
=item *
-C<x> and C<X> accept C<!> modifier. In this case they act as
-alignment commands: they jump forward/back to the closest position
-aligned at a multiple of C<count> characters. For example, to pack() or
-unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
-use the template C<W x![d] d W[2]>; this assumes that doubles must be
-aligned on the double's size.
+C<x> and C<X> accept the C<!> modifier to act as alignment commands: they
+jump forward or back to the closest position aligned at a multiple of C<count>
+characters. For example, to pack() or unpack() a C structure like
-For alignment commands C<count> of 0 is equivalent to C<count> of 1;
-both result in no-ops.
+ struct {
+ char c; /* one signed, 8-bit character */
+ double d;
+ char cc[2];
+ }
+
+one may need to use the template C<c x![d] d c[2]>. This assumes that
+doubles must be aligned to the size of double.
+
+For alignment commands, a C<count> of 0 is equivalent to a C<count> of 1;
+both are no-ops.
=item *
-C<n>, C<N>, C<v> and C<V> accept the C<!> modifier. In this case they
-will represent signed 16-/32-bit integers in big-/little-endian order.
-This is only portable if all platforms sharing the packed data use the
-same binary representation for signed integers (e.g. all platforms are
-using two's complement representation).
+C<n>, C<N>, C<v> and C<V> accept the C<!> modifier to
+represent signed 16-/32-bit integers in big-/little-endian order.
+This is portable only when all platforms sharing packed data use the
+same binary representation for signed integers; for example, when all
+platforms use two's-complement representation.
=item *
-A comment in a TEMPLATE starts with C<#> and goes to the end of line.
-White space may be used to separate pack codes from each other, but
-modifiers and a repeat count must follow immediately.
+Comments can be embedded in a TEMPLATE using C<#> through the end of line.
+White space can separate pack codes from each other, but modifiers and
+repeat counts must follow immediately. Breaking complex templates into
+individual line-by-line components, suitably annotated, can do as much to
+improve legibility and maintainability of pack/unpack formats as C</x> can
+for complicated pattern matches.
=item *
-If TEMPLATE requires more arguments to pack() than actually given, pack()
+If TEMPLATE requires more arguments than pack() is given, pack()
assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
-to pack() than actually given, extra arguments are ignored.
+than given, extra arguments are ignored.
=back
$foo = pack("ccxxcc",65,66,67,68);
# foo eq "AB\0\0CD"
- # note: the above examples featuring "W" and "c" are true
+ # NOTE: The examples above featuring "W" and "c" are true
# only on ASCII and ASCII-derived systems such as ISO Latin 1
- # and UTF-8. In EBCDIC the first example would be
- # $foo = pack("WWWW",193,194,195,196);
+ # and UTF-8. On EBCDIC systems, the first example would be
+ # $foo = pack("WWWW",193,194,195,196);
$foo = pack("s2",1,2);
- # "\1\0\2\0" on little-endian
- # "\0\1\0\2" on big-endian
+ # "\001\000\002\000" on little-endian
+ # "\000\001\000\002" on big-endian
$foo = pack("a4","abcd","x","y","z");
# "abcd"
The same template may generally also be used in unpack().
+=item package NAMESPACE
+
=item package NAMESPACE VERSION
X<package> X<module> X<namespace> X<version>
-=item package NAMESPACE
+=item package NAMESPACE BLOCK
+
+=item package NAMESPACE VERSION BLOCK
+X<package> X<module> X<namespace> X<version>
-Declares the compilation unit as being in the given namespace. The scope
-of the package declaration is from the declaration itself through the end
-of the enclosing block, file, or eval (the same as the C<my> operator).
-All further unqualified dynamic identifiers will be in this namespace.
-A package statement affects only dynamic variables--including those
-you've used C<local> on--but I<not> lexical variables, which are created
-with C<my>. Typically it would be the first declaration in a file to
-be included by the C<require> or C<use> operator. You can switch into a
-package in more than one place; it merely influences which symbol table
-is used by the compiler for the rest of that block. You can refer to
-variables and filehandles in other packages by prefixing the identifier
-with the package name and a double colon: C<$Package::Variable>.
-If the package name is null, the C<main> package as assumed. That is,
-C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
-still seen in older code).
-
-If VERSION is provided, C<package> also sets the C<$VERSION> variable in the
-given namespace. VERSION must be a numeric literal or v-string; it is
-parsed exactly the same way as a VERSION argument to C<use MODULE VERSION>.
-C<$VERSION> should only be set once per package.
+Declares the BLOCK or the rest of the compilation unit as being in the
+given namespace. The scope of the package declaration is either the
+supplied code BLOCK or, in the absence of a BLOCK, from the declaration
+itself through the end of current scope (the enclosing block, file, or
+C<eval>). That is, the forms without a BLOCK are operative through the end
+of the current scope, just like the C<my>, C<state>, and C<our> operators.
+All unqualified dynamic identifiers in this scope will be in the given
+namespace, except where overridden by another C<package> declaration or
+when they're one of the special identifiers that qualify into C<main::>,
+like C<STDOUT>, C<ARGV>, C<ENV>, and the punctuation variables.
+
+A package statement affects dynamic variables only, including those
+you've used C<local> on, but I<not> lexical variables, which are created
+with C<my>, C<state>, or C<our>. Typically it would be the first
+declaration in a file included by C<require> or C<use>. You can switch into a
+package in more than one place, since this only determines which default
+symbol table the compiler uses for the rest of that block. You can refer to
+identifiers in other packages than the current one by prefixing the identifier
+with the package name and a double colon, as in C<$SomePack::var>
+or C<ThatPack::INPUT_HANDLE>. If package name is omitted, the C<main>
+package as assumed. That is, C<$::sail> is equivalent to
+C<$main::sail> (as well as to C<$main'sail>, still seen in ancient
+code, mostly from Perl 4).
+
+If VERSION is provided, C<package> sets the C<$VERSION> variable in the given
+namespace to a L<version> object with the VERSION provided. VERSION must be a
+"strict" style version number as defined by the L<version> module: a positive
+decimal number (integer or decimal-fraction) without exponentiation or else a
+dotted-decimal v-string with a leading 'v' character and at least three
+components. You should set C<$VERSION> only once per package.
See L<perlmod/"Packages"> for more information about packages, modules,
and classes. See L<perlsub> for other scoping issues.
IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
after each command, depending on the application.
-See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
+See L<IPC::Open2>, L<IPC::Open3>, and
+L<perlipc/"Bidirectional Communication with Another Process">
for examples of such things.
-On systems that support a close-on-exec flag on files, the flag will be set
-for the newly opened file descriptors as determined by the value of $^F.
-See L<perlvar/$^F>.
+On systems that support a close-on-exec flag on files, that flag is set
+on all newly opened file descriptors whose C<fileno>s are I<higher> than
+
the current value of $^F (by default 2 for C<STDERR>). See L<perlvar/$^F>.
=item pop ARRAY
X<pop> X<stack>
+=item pop EXPR
+
=item pop
Pops and returns the last value of the array, shortening the array by
one element.
-If there are no elements in the array, returns the undefined value
-(although this may happen at other times as well). If ARRAY is
-omitted, pops the C<@ARGV> array in the main program, and the C<@_>
-array in subroutines, just like C<shift>.
+Returns the undefined value if the array is empty, although this may also
+happen at other times. If ARRAY is omitted, pops the C<@ARGV> array in the
+main program, but the C<@_> array in subroutines, just like C<shift>.
+
+Starting with Perl 5.14, C<pop> can take a scalar EXPR, which must hold a
+reference to an unblessed array. The argument will be dereferenced
+automatically. This aspect of C<pop> is considered highly experimental.
+The exact behaviour may change in a future version of Perl.
=item pos SCALAR
X<pos> X<match, position>
=item pos
-Returns the offset of where the last C<m//g> search left off for the variable
-in question (C<$_> is used when the variable is not specified). Note that
-0 is a valid match offset. C<undef> indicates that the search position
-is reset (usually due to match failure, but can also be because no match has
-yet been run on the scalar). C<pos> directly accesses the location used
-by the regexp engine to store the offset, so assigning to C<pos> will change
-that offset, and so will also influence the C<\G> zero-width assertion in
-regular expressions. Because a failed C<m//gc> match doesn't reset the offset,
-the return from C<pos> won't change either in this case. See L<perlre> and
+Returns the offset of where the last C<m//g> search left off for the
+variable in question (C<$_> is used when the variable is not
+specified). Note that 0 is a valid match offset. C<undef> indicates
+that the search position is reset (usually due to match failure, but
+can also be because no match has yet been run on the scalar).
+
+C<pos> directly accesses the location used by the regexp engine to
+store the offset, so assigning to C<pos> will change that offset, and
+so will also influence the C<\G> zero-width assertion in regular
+expressions. Both of these effects take place for the next match, so
+you can't affect the position with C<pos> during the current match,
+such as in C<(?{pos() = 5})> or C<s//pos() = 5/e>.
+
+Setting C<pos> also resets the I<matched with zero-length> flag, described
+under L<perlre/"Repeated Patterns Matching a Zero-length Substring">.
+
+Because a failed C<m//gc> match doesn't reset the offset, the return
+from C<pos> won't change either in this case. See L<perlre> and
L<perlop>.
=item print FILEHANDLE LIST
X<print>
+=item print FILEHANDLE
+
=item print LIST
=item print
Prints a string or a list of strings. Returns true if successful.
-FILEHANDLE may be a scalar variable name, in which case the variable
-contains the name of or a reference to the filehandle, thus introducing
-one level of indirection. (NOTE: If FILEHANDLE is a variable and
-the next token is a term, it may be misinterpreted as an operator
-unless you interpose a C<+> or put parentheses around the arguments.)
-If FILEHANDLE is omitted, prints by default to standard output (or
-to the last selected output channel--see L</select>). If LIST is
-also omitted, prints C<$_> to the currently selected output channel.
-To set the default output channel to something other than STDOUT
-use the select operation. The current value of C<$,> (if any) is
-printed between each LIST item. The current value of C<$\> (if
-any) is printed after the entire LIST has been printed. Because
-print takes a LIST, anything in the LIST is evaluated in list
-context, and any subroutine that you call will have one or more of
-its expressions evaluated in list context. Also be careful not to
-follow the print keyword with a left parenthesis unless you want
-the corresponding right parenthesis to terminate the arguments to
-the print--interpose a C<+> or put parentheses around all the
-arguments.
-
-Note that if you're storing FILEHANDLEs in an array, or if you're using
-any other expression more complex than a scalar variable to retrieve it,
-you will have to use a block returning the filehandle value instead:
+FILEHANDLE may be a scalar variable containing the name of or a reference
+to the filehandle, thus introducing one level of indirection. (NOTE: If
+FILEHANDLE is a variable and the next token is a term, it may be
+misinterpreted as an operator unless you interpose a C<+> or put
+parentheses around the arguments.) If FILEHANDLE is omitted, prints to the
+last selected (see L</select>) output handle. If LIST is omitted, prints
+C<$_> to the currently selected output handle. To use FILEHANDLE alone to
+print the content of C<$_> to it, you must use a real filehandle like
+C<FH>, not an indirect one like C<$fh>. To set the default output handle
+to something other than STDOUT, use the select operation.
+
+The current value of C<$,> (if any) is printed between each LIST item. The
+current value of C<$\> (if any) is printed after the entire LIST has been
+printed. Because print takes a LIST, anything in the LIST is evaluated in
+list context, including any subroutines whose return lists you pass to
+C<print>. Be careful not to follow the print keyword with a left
+parenthesis unless you want the corresponding right parenthesis to
+terminate the arguments to the print; put parentheses around all arguments
+(or interpose a C<+>, but that doesn't look as good).
+
+If you're storing handles in an array or hash, or in general whenever
+you're using any expression more complex than a bareword handle or a plain,
+unsubscripted scalar variable to retrieve it, you will have to use a block
+returning the filehandle value instead, in which case the LIST may not be
+omitted:
print { $files[$i] } "stuff\n";
print { $OK ? STDOUT : STDERR } "stuff\n";
=item printf FILEHANDLE FORMAT, LIST
X<printf>
+=item printf FILEHANDLE
+
=item printf FORMAT, LIST
+=item printf
+
Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
-(the output record separator) is not appended. The first argument
-of the list will be interpreted as the C<printf> format. See C<sprintf>
-for an explanation of the format argument. If C<use locale> is in effect,
-and POSIX::setlocale() has been called, the character used for the decimal
-separator in formatted floating point numbers is affected by the LC_NUMERIC
-locale. See L<perllocale> and L<POSIX>.
+(the output record separator) is not appended. The first argument of the
+list will be interpreted as the C<printf> format. See C<sprintf> for an
+explanation of the format argument. If you omit the LIST, C<$_> is used;
+to use FILEHANDLE without a LIST, you must use a real filehandle like
+C<FH>, not an indirect one like C<$fh>. If C<use locale> is in effect and
+POSIX::setlocale() has been called, the character used for the decimal
+separator in formatted floating-point numbers is affected by the LC_NUMERIC
+locale setting. See L<perllocale> and L<POSIX>.
Don't fall into the trap of using a C<printf> when a simple
C<print> would do. The C<print> is more efficient and less
=item push ARRAY,LIST
X<push> X<stack>
-Treats ARRAY as a stack, and pushes the values of LIST
-onto the end of ARRAY. The length of ARRAY increases by the length of
-LIST. Has the same effect as
+=item push EXPR,LIST
+
+Treats ARRAY as a stack by appending the values of LIST to the end of
+ARRAY. The length of ARRAY increases by the length of LIST. Has the same
+effect as
for $value (LIST) {
$ARRAY[++$#ARRAY] = $value;
but is more efficient. Returns the number of elements in the array following
the completed C<push>.
+Starting with Perl 5.14, C<push> can take a scalar EXPR, which must hold a
+reference to an unblessed array. The argument will be dereferenced
+automatically. This aspect of C<push> is considered highly experimental.
+The exact behaviour may change in a future version of Perl.
+
=item q/STRING/
=item qq/STRING/
my $quoted_substring = quotemeta($substring);
$sentence =~ s{$quoted_substring}{big bad wolf};
-Will both leave the sentence as is. Normally, when accepting string input from
-the user, quotemeta() or C<\Q> must be used.
+Will both leave the sentence as is. Normally, when accepting literal string
+input from the user, quotemeta() or C<\Q> must be used.
+
+In Perl 5.14, all characters whose code points are above 127 are not
+quoted in UTF8-encoded strings, but all are quoted in UTF-8 strings.
+It is planned to change this behavior in 5.16, but the exact rules
+haven't been determined yet.
=item rand EXPR
X<rand> X<random>
Returns a random fractional number greater than or equal to C<0> and less
than the value of EXPR. (EXPR should be positive.) If EXPR is
omitted, the value C<1> is used. Currently EXPR with the value C<0> is
-also special-cased as C<1> - this has not been documented before perl 5.8.0
-and is subject to change in future versions of perl. Automatically calls
+also special-cased as C<1> (this was undocumented before Perl 5.8.0
+and is subject to change in future versions of Perl). Automatically calls
C<srand> unless C<srand> has already been called. See also C<srand>.
Apply C<int()> to the value returned by C<rand()> if you want random
large or too small, then your version of Perl was probably compiled
with the wrong number of RANDBITS.)
+B<C<rand()> is not cryptographically secure. You should not rely
+on it in security-sensitive situations.> As of this writing, a
+number of third-party CPAN modules offer random number generators
+intended by their authors to be cryptographically secure,
+including: L<Math::Random::Secure>, L<Math::Random::MT::Perl>, and
+L<Math::TrulyRandom>.
+
=item read FILEHANDLE,SCALAR,LENGTH,OFFSET
X<read> X<file, read>
fread(3) library function. To get a true read(2) system call, see C<sysread>.
Note the I<characters>: depending on the status of the filehandle,
-either (8-bit) bytes or characters are read. By default all
+either (8-bit) bytes or characters are read. By default, all
filehandles operate on bytes, but for example if the filehandle has
been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
-pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
+pragma, L<open>), the I/O will operate on UTF8-encoded Unicode
characters, not bytes. Similarly for the C<:encoding> pragma:
in that case pretty much any characters can be read.
Returns the next directory entry for a directory opened by C<opendir>.
If used in list context, returns all the rest of the entries in the
-directory. If there are no more entries, returns an undefined value in
-scalar context or a null list in list context.
+directory. If there are no more entries, returns the undefined value in
+scalar context and the empty list in list context.
If you're planning to filetest the return values out of a C<readdir>, you'd
better prepend the directory in question. Otherwise, because we didn't
X<readline> X<gets> X<fgets>
Reads from the filehandle whose typeglob is contained in EXPR (or from
-*ARGV if EXPR is not provided). In scalar context, each call reads and
+C<*ARGV> if EXPR is not provided). In scalar context, each call reads and
returns the next line until end-of-file is reached, whereupon the
subsequent call returns C<undef>. In list context, reads until end-of-file
is reached and returns a list of lines. Note that the notion of "line"
=item readlink
Returns the value of a symbolic link, if symbolic links are
-implemented. If not, gives a fatal error. If there is some system
+implemented. If not, raises an exception. If there is a system
error, returns the undefined value and sets C<$!> (errno). If EXPR is
omitted, uses C<$_>.
+Portability issues: L<perlport/readlink>.
+
=item readpipe EXPR
=item readpipe
(8-bit) bytes or characters are received. By default all sockets
operate on bytes, but for example if the socket has been changed using
binmode() to operate with the C<:encoding(utf8)> I/O layer (see the
-C<open> pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
+C<open> pragma, L<open>), the I/O will operate on UTF8-encoded Unicode
characters, not bytes. Similarly for the C<:encoding> pragma: in that
case pretty much any characters can be read.
}
C<redo> cannot be used to retry a block that returns a value such as
-C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
+C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
a grep() or map() operation.
Note that a block by itself is semantically identical to a loop
For a platform independent C<move> function look at the L<File::Copy>
module.
+Portability issues: L<perlport/rename>.
+
=item require VERSION
X<require>
VERSION may be either a numeric argument such as 5.006, which will be
compared to C<$]>, or a literal of the form v5.6.1, which will be compared
-to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
+to C<$^V> (aka $PERL_VERSION). An exception is raised if
VERSION is greater than the version of the current Perl interpreter.
Compare with L</use>, which can do a similar check at compile time.
eval "require $class";
-Now that you understand how C<require> looks for files in the case of a
+Now that you understand how C<require> looks for files with a
bareword argument, there is a little extra functionality going on behind
the scenes. Before C<require> looks for a "F<.pm>" extension, it will
first look for a similar filename with a "F<.pmc>" extension. If this file
is found, it will be loaded in place of any file ending in a "F<.pm>"
extension.
-You can also insert hooks into the import facility, by putting directly
-Perl code into the @INC array. There are three forms of hooks: subroutine
-references, array references and blessed objects.
+You can also insert hooks into the import facility by putting Perl code
+directly into the @INC array. There are three forms of hooks: subroutine
+references, array references, and blessed objects.
Subroutine references are the simplest case. When the inclusion system
walks through @INC and encounters a subroutine, this subroutine gets
-called with two parameters, the first being a reference to itself, and the
-second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
-subroutine should return nothing, or a list of up to three values in the
-following order:
+called with two parameters, the first a reference to itself, and the
+second the name of the file to be included (e.g., "F<Foo/Bar.pm>"). The
+subroutine should return either nothing or else a list of up to three
+values in the following order:
=over
A reference to a subroutine. If there is no filehandle (previous item),
then this subroutine is expected to generate one line of source code per
-call, writing the line into C<$_> and returning 1, then returning 0 at
-"end of file". If there is a filehandle, then the subroutine will be
+call, writing the line into C<$_> and returning 1, then finally at end of
+file returning 0. If there is a filehandle, then the subroutine will be
called to act as a simple source filter, with the line as read in C<$_>.
Again, return 1 for each valid line, and 0 after all lines have been
returned.
=back
If an empty list, C<undef>, or nothing that matches the first 3 values above
-is returned then C<require> will look at the remaining elements of @INC.
-Note that this file handle must be a real file handle (strictly a typeglob,
-or reference to a typeglob, blessed or unblessed) - tied file handles will be
-ignored and return value processing will stop there.
+is returned, then C<require> looks at the remaining elements of @INC.
+Note that this filehandle must be a real filehandle (strictly a typeglob
+or reference to a typeglob, whether blessed or unblessed); tied filehandles
+will be ignored and processing will stop there.
If the hook is an array reference, its first element must be a subroutine
reference. This subroutine is called as above, but the first parameter is
-the array reference. This enables to pass indirectly some arguments to
+the array reference. This lets you indirectly pass arguments to
the subroutine.
In other words, you can write:
# In the main program
push @INC, Foo->new(...);
-Note that these hooks are also permitted to set the %INC entry
+These hooks are also permitted to set the %INC entry
corresponding to the files they have loaded. See L<perlvar/%INC>.
For a yet-more-powerful import facility, see L</use> and L<perlmod>.
expression is interpreted as a list of single characters (hyphens
allowed for ranges). All variables and arrays beginning with one of
those letters are reset to their pristine state. If the expression is
-omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
-only variables or searches in the current package. Always returns
-1. Examples:
+omitted, one-match searches (C<?pattern?>) are reset to match again.
+Only resets variables or searches in the current package. Always returns
+1. Examples:
reset 'X'; # reset all X variables
reset 'a-z'; # reset lower case variables
Resetting C<"A-Z"> is not recommended because you'll wipe out your
C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
-variables--lexical variables are unaffected, but they clean themselves
+variables; lexical variables are unaffected, but they clean themselves
up on scope exit anyway, so you'll probably want to use them instead.
See L</my>.
context, depending on how the return value will be used, and the context
may vary from one execution to the next (see C<wantarray>). If no EXPR
is given, returns an empty list in list context, the undefined value in
-scalar context, and (of course) nothing at all in a void context.
+scalar context, and (of course) nothing at all in void context.
-(Note that in the absence of an explicit C<return>, a subroutine, eval,
-or do FILE will automatically return the value of the last expression
+(In the absence of an explicit C<return>, a subroutine, eval,
+or do FILE automatically returns the value of the last expression
evaluated.)
=item reverse LIST
Sets the current position to the beginning of the directory for the
C<readdir> routine on DIRHANDLE.
+Portability issues: L<perlport/rewinddir>.
+
=item rindex STR,SUBSTR,POSITION
X<rindex>
=item rmdir
Deletes the directory specified by FILENAME if that directory is
-empty. If it succeeds it returns true, otherwise it returns false and
+empty. If it succeeds it returns true; otherwise it returns false and
sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
To remove a directory tree recursively (C<rm -rf> on Unix) look at
=item say FILEHANDLE LIST
X<say>
+=item say FILEHANDLE
+
=item say LIST
=item say
-Just like C<print>, but implicitly appends a newline.
-C<say LIST> is simply an abbreviation for C<{ local $\ = "\n"; print
-LIST }>.
+Just like C<print>, but implicitly appends a newline. C<say LIST> is
+simply an abbreviation for C<{ local $\ = "\n"; print LIST }>. To use
+FILEHANDLE without a LIST to print the contents of C<$_> to it, you must
+use a real filehandle like C<FH>, not an indirect one like C<$fh>.
-This keyword is only available when the "say" feature is
-enabled: see L<feature>.
+This keyword is available only when the C<"say"> feature is enabled; see
+L<feature>. Alternately, include a C<use v5.10> or later to the current
+scope.
=item scalar EXPR
X<scalar> X<context>
the construction C<@{[ (some expression) ]}>, but usually a simple
C<(some expression)> suffices.
-Because C<scalar> is unary operator, if you accidentally use for EXPR a
-parenthesized list, this behaves as a scalar comma expression, evaluating
-all but the last element in void context and returning the final element
-evaluated in scalar context. This is seldom what you want.
+Because C<scalar> is a unary operator, if you accidentally use a
+parenthesized list for the EXPR, this behaves as a scalar comma expression,
+evaluating all but the last element in void context and returning the final
+element evaluated in scalar context. This is seldom what you want.
The following single statement:
Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
FILEHANDLE may be an expression whose value gives the name of the
filehandle. The values for WHENCE are C<0> to set the new position
-I<in bytes> to POSITION, C<1> to set it to the current position plus
-POSITION, and C<2> to set it to EOF plus POSITION (typically
-negative). For WHENCE you may use the constants C<SEEK_SET>,
+I<in bytes> to POSITION; C<1> to set it to the current position plus
+POSITION; and C<2> to set it to EOF plus POSITION, typically
+negative. For WHENCE you may use the constants C<SEEK_SET>,
C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
-of the file) from the Fcntl module. Returns C<1> on success, C<0>
+of the file) from the L<Fcntl> module. Returns C<1> on success, false
otherwise.
Note the I<in bytes>: even if the filehandle has been set to
layer), tell() will return byte offsets, not character offsets
(because implementing that would render seek() and tell() rather slow).
-If you want to position file for C<sysread> or C<syswrite>, don't use
-C<seek>--buffering makes its effect on the file's system position
+If you want to position the file for C<sysread> or C<syswrite>, don't use
+C<seek>, because buffering makes its effect on the file's read-write position
unpredictable and non-portable. Use C<sysseek> instead.
Due to the rules and rigors of ANSI C, on some systems you have to do a
seek(TEST,0,1);
This is also useful for applications emulating C<tail -f>. Once you hit
-EOF on your read, and then sleep for a while, you might have to stick in a
-seek() to reset things. The C<seek> doesn't change the current position,
+EOF on your read and then sleep for a while, you (probably) have to stick in a
+dummy seek() to reset things. The C<seek> doesn't change the position,
but it I<does> clear the end-of-file condition on the handle, so that the
-next C<< <FILE> >> makes Perl try again to read something. We hope.
+next C<< <FILE> >> makes Perl try again to read something. (We hope.)
-If that doesn't work (some IO implementations are particularly
-cantankerous), then you may need something more like this:
+If that doesn't work (some I/O implementations are particularly
+cantankerous), you might need something like this:
for (;;) {
for ($curpos = tell(FILE); $_ = <FILE>;
Returns the currently selected filehandle. If FILEHANDLE is supplied,
sets the new current default filehandle for output. This has two
-effects: first, a C<write> or a C<print> without a filehandle will
+effects: first, a C<write> or a C<print> without a filehandle
default to this FILEHANDLE. Second, references to variables related to
-output will refer to this output channel. For example, if you have to
-set the top of form format for more than one output channel, you might
-do the following:
+output will refer to this output channel.
+
+For example, to set the top-of-form format for more than one
+output channel, you might do the following:
select(REPORT1);
$^ = 'report1_top';
use IO::Handle;
STDERR->autoflush(1);
+Portability issues: L<perlport/select>.
+
=item select RBITS,WBITS,EBITS,TIMEOUT
X<select>
-This calls the select(2) system call with the bit masks specified, which
+This calls the select(2) syscall with the bit masks specified, which
can be constructed using C<fileno> and C<vec>, along these lines:
$rin = $win = $ein = '';
- vec($rin,fileno(STDIN),1) = 1;
- vec($win,fileno(STDOUT),1) = 1;
+ vec($rin, fileno(STDIN), 1) = 1;
+ vec($win, fileno(STDOUT), 1) = 1;
$ein = $rin | $win;
-If you want to select on many filehandles you might wish to write a
-subroutine:
+If you want to select on many filehandles, you may wish to write a
+subroutine like this:
sub fhbits {
- my(@fhlist) = split(' ',$_[0]);
- my($bits);
- for (@fhlist) {
- vec($bits,fileno($_),1) = 1;
+ my @fhlist = @_;
+ my $bits = "";
+ for my $fh (@fhlist) {
+ vec($bits, fileno($fh), 1) = 1;
}
- $bits;
+ return $bits;
}
- $rin = fhbits('STDIN TTY SOCK');
+ $rin = fhbits(*STDIN, *TTY, *MYSOCK);
The usual idiom is:
is implementation-dependent. See also L<perlport> for notes on the
portability of C<select>.
-On error, C<select> behaves like the select(2) system call : it returns
+On error, C<select> behaves just like select(2): it returns
-1 and sets C<$!>.
-Note: on some Unixes, the select(2) system call may report a socket file
-descriptor as "ready for reading", when actually no data is available,
-thus a subsequent read blocks. It can be avoided using always the
-O_NONBLOCK flag on the socket. See select(2) and fcntl(2) for further
-details.
+On some Unixes, select(2) may report a socket file descriptor as "ready for
+reading" even when no data is available, and thus any subsequent C<read>
+would block. This can be avoided if you always use O_NONBLOCK on the
+socket. See select(2) and fcntl(2) for further details.
+
+The standard C<IO::Select> module provides a user-friendlier interface
+to C<select>, mostly because it does all the bit-mask work for you.
B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
or <FH>) with C<select>, except as permitted by POSIX, and even
then only on POSIX systems. You have to use C<sysread> instead.
+Portability issues: L<perlport/select>.
+
=item semctl ID,SEMNUM,CMD,ARG
X<semctl>
-Calls the System V IPC function C<semctl>. You'll probably have to say
+Calls the System V IPC function semctl(2). You'll probably have to say
use IPC::SysV;
See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
documentation.
+Portability issues: L<perlport/semctl>.
+
=item semget KEY,NSEMS,FLAGS
X<semget>
-Calls the System V IPC function semget. Returns the semaphore id, or
-the undefined value if there is an error. See also
+Calls the System V IPC function semget(2). Returns the semaphore id, or
+the undefined value on error. See also
L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
documentation.
+Portability issues: L<perlport/semget>.
+
=item semop KEY,OPSTRING
X<semop>
semop structures. Each semop structure can be generated with
C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
implies the number of semaphore operations. Returns true if
-successful, or false if there is an error. As an example, the
+successful, false on error. As an example, the
following code waits on semaphore $semnum of semaphore id $semid:
$semop = pack("s!3", $semnum, -1, 0);
L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
documentation.
+Portability issues: L<perlport/semop>.
+
=item send SOCKET,MSG,FLAGS,TO
X<send>
=item send SOCKET,MSG,FLAGS
-Sends a message on a socket. Attempts to send the scalar MSG to the
-SOCKET filehandle. Takes the same flags as the system call of the
-same name. On unconnected sockets you must specify a destination to
-send TO, in which case it does a C C<sendto>. Returns the number of
-characters sent, or the undefined value if there is an error. The C
-system call sendmsg(2) is currently unimplemented. See
-L<perlipc/"UDP: Message Passing"> for examples.
+Sends a message on a socket. Attempts to send the scalar MSG to the SOCKET
+filehandle. Takes the same flags as the system call of the same name. On
+unconnected sockets, you must specify a destination to I<send to>, in which
+case it does a sendto(2) syscall. Returns the number of characters sent,
+or the undefined value on error. The sendmsg(2) syscall is currently
+unimplemented. See L<perlipc/"UDP: Message Passing"> for examples.
Note the I<characters>: depending on the status of the socket, either
(8-bit) bytes or characters are sent. By default all sockets operate
X<setpgrp> X<group>
Sets the current process group for the specified PID, C<0> for the current
-process. Will produce a fatal error if used on a machine that doesn't
+process. Raises an exception when used on a machine that doesn't
implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
accept any arguments, so only C<setpgrp(0,0)> is portable. See also
C<POSIX::setsid()>.
+Portability issues: L<perlport/setpgrp>.
+
=item setpriority WHICH,WHO,PRIORITY
X<setpriority> X<priority> X<nice> X<renice>
Sets the current priority for a process, a process group, or a user.
-(See setpriority(2).) Will produce a fatal error if used on a machine
+(See setpriority(2).) Raises an exception when used on a machine
that doesn't implement setpriority(2).
+Portability issues: L<perlport/setpriority>.
+
=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
X<setsockopt>
-Sets the socket option requested. Returns undefined if there is an
-error. Use integer constants provided by the C<Socket> module for
+Sets the socket option requested. Returns C<undef> on error.
+Use integer constants provided by the C<Socket> module for
LEVEL and OPNAME. Values for LEVEL can also be obtained from
getprotobyname. OPTVAL might either be a packed string or an integer.
An integer OPTVAL is shorthand for pack("i", OPTVAL).
-An example disabling the Nagle's algorithm for a socket:
+An example disabling Nagle's algorithm on a socket:
use Socket qw(IPPROTO_TCP TCP_NODELAY);
setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
+Portability issues: L<perlport/setsockopt>.
+
=item shift ARRAY
X<shift>
+=item shift EXPR
+
=item shift
Shifts the first value of the array off and returns it, shortening the
C<@_> array within the lexical scope of subroutines and formats, and the
C<@ARGV> array outside a subroutine and also within the lexical scopes
established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>,
-C<UNITCHECK {}> and C<END {}> constructs.
+C<UNITCHECK {}>, and C<END {}> constructs.
+
+Starting with Perl 5.14, C<shift> can take a scalar EXPR, which must hold a
+reference to an unblessed array. The argument will be dereferenced
+automatically. This aspect of C<shift> is considered highly experimental.
+The exact behaviour may change in a future version of Perl.
See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
same thing to the left end of an array that C<pop> and C<push> do to the
first to get the correct constant definitions. If CMD is C<IPC_STAT>,
then ARG must be a variable that will hold the returned C<shmid_ds>
-structure. Returns like ioctl: the undefined value for error, "C<0> but
-true" for zero, or the actual return value otherwise.
+structure. Returns like ioctl: C<undef> for error; "C<0> but
+true" for zero; and the actual return value otherwise.
See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
+Portability issues: L<perlport/shmctl>.
+
=item shmget KEY,SIZE,FLAGS
X<shmget>
Calls the System V IPC function shmget. Returns the shared memory
-segment id, or the undefined value if there is an error.
+segment id, or C<undef> on error.
See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
+Portability issues: L<perlport/shmget>.
+
=item shmread ID,VAR,POS,SIZE
X<shmread>
X<shmwrite>
detaching from it. When reading, VAR must be a variable that will
hold the data read. When writing, if STRING is too long, only SIZE
bytes are used; if STRING is too short, nulls are written to fill out
-SIZE bytes. Return true if successful, or false if there is an error.
+SIZE bytes. Return true if successful, false on error.
shmread() taints the variable. See also L<perlipc/"SysV IPC">,
-C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
+C<IPC::SysV>, and the C<IPC::Shareable> module from CPAN.
+
+Portability issues: L<perlport/shmread> and L<perlport/shmwrite>.
=item shutdown SOCKET,HOW
X<shutdown>
Shuts down a socket connection in the manner indicated by HOW, which
-has the same interpretation as in the system call of the same name.
+has the same interpretation as in the syscall of the same name.
shutdown(SOCKET, 0); # I/we have stopped reading data
shutdown(SOCKET, 1); # I/we have stopped writing data
disables the file descriptor in any forked copies in other
processes.
-Returns C<1> for success. In the case of error, returns C<undef> if
+Returns C<1> for success; on error, returns C<undef> if
the first argument is not a valid filehandle, or returns C<0> and sets
C<$!> for any other failure.
Opens a socket of the specified kind and attaches it to filehandle
SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
-the system call of the same name. You should C<use Socket> first
+the syscall of the same name. You should C<use Socket> first
to get the proper definitions imported. See the examples in
L<perlipc/"Sockets: Client/Server Communication">.
Creates an unnamed pair of sockets in the specified domain, of the
specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
-for the system call of the same name. If unimplemented, yields a fatal
-error. Returns true if successful.
+for the syscall of the same name. If unimplemented, raises an exception.
+Returns true if successful.
On systems that support a close-on-exec flag on files, the flag will
be set for the newly opened file descriptors, as determined by the value
emulate socketpair using IP sockets to localhost if your system implements
sockets but not socketpair.
+Portability issues: L<perlport/socketpair>.
+
=item sort SUBNAME LIST
X<sort> X<qsort> X<quicksort> X<mergesort>
If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
order. If SUBNAME is specified, it gives the name of a subroutine
that returns an integer less than, equal to, or greater than C<0>,
-depending on how the elements of the list are to be ordered. (The C<<
-<=> >> and C<cmp> operators are extremely useful in such routines.)
+depending on how the elements of the list are to be ordered. (The
+C<< <=> >> and C<cmp> operators are extremely useful in such routines.)
SUBNAME may be a scalar variable name (unsubscripted), in which case
the value provides the name of (or a reference to) the actual
subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
an anonymous, in-line sort subroutine.
-If the subroutine's prototype is C<($$)>, the elements to be compared
-are passed by reference in C<@_>, as for a normal subroutine. This is
-slower than unprototyped subroutines, where the elements to be
-compared are passed into the subroutine
-as the package global variables $a and $b (see example below). Note that
-in the latter case, it is usually counter-productive to declare $a and
-$b as lexicals.
+If the subroutine's prototype is C<($$)>, the elements to be compared are
+passed by reference in C<@_>, as for a normal subroutine. This is slower
+than unprototyped subroutines, where the elements to be compared are passed
+into the subroutine as the package global variables $a and $b (see example
+below). Note that in the latter case, it is usually highly counter-productive
+to declare $a and $b as lexicals.
The values to be compared are always passed by reference and should not
be modified.
something to be avoided when writing clear code.
Perl 5.6 and earlier used a quicksort algorithm to implement sort.
-That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
+That algorithm was not stable, so I<could> go quadratic. (A I<stable> sort
preserves the input order of elements that compare equal. Although
quicksort's run time is O(NlogN) when averaged over all arrays of
length N, the time can be O(N**2), I<quadratic> behavior, for some
well-defined.
Because C<< <=> >> returns C<undef> when either operand is C<NaN>
-(not-a-number), and because C<sort> will trigger a fatal error unless the
-result of a comparison is defined, when sorting with a comparison function
-like C<< $a <=> $b >>, be careful about lists that might contain a C<NaN>.
-The following example takes advantage that C<NaN != NaN> to
-eliminate any C<NaN>s from the input.
+(not-a-number), and laso because C<sort> raises an exception unless the
+result of a comparison is defined, be careful when sorting with a
+comparison function like C<< $a <=> $b >> any lists that might contain a
+C<NaN>. The following example takes advantage that C<NaN != NaN> to
+eliminate any C<NaN>s from the input list.
@result = sort { $a <=> $b } grep { $_ == $_ } @input;
-=item splice ARRAY,OFFSET,LENGTH,LIST
+=item splice ARRAY or EXPR,OFFSET,LENGTH,LIST
X<splice>
-=item splice ARRAY,OFFSET,LENGTH
+=item splice ARRAY or EXPR,OFFSET,LENGTH
-=item splice ARRAY,OFFSET
+=item splice ARRAY or EXPR,OFFSET
-=item splice ARRAY
+=item splice ARRAY or EXPR
Removes the elements designated by OFFSET and LENGTH from an array, and
replaces them with the elements of LIST, if any. In list context,
If LENGTH is negative, removes the elements from OFFSET onward
except for -LENGTH elements at the end of the array.
If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
-past the end of the array, perl issues a warning, and splices at the
+past the end of the array, Perl issues a warning, and splices at the
end of the array.
The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
}
if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
+Starting with Perl 5.14, C<splice> can take scalar EXPR, which must hold a
+reference to an unblessed array. The argument will be dereferenced
+automatically. This aspect of C<splice> is considered highly experimental.
+The exact behaviour may change in a future version of Perl.
+
=item split /PATTERN/,EXPR,LIMIT
X<split>
empty string always returns the empty list, regardless of the LIMIT
specified.
-A pattern matching the null string (not to be confused with
-a null pattern C<//>, which is just one member of the set of patterns
-matching a null string) will split the value of EXPR into separate
-characters at each point it matches that way. For example:
+A pattern matching the empty string (not to be confused with
+an empty pattern C<//>, which is just one member of the set of patterns
+matching the epmty string), splits EXPR into individual
+characters. For example:
print join(':', split(/ */, 'hi there')), "\n";
produces the output 'h:i:t:h:e:r:e'.
-As a special case for C<split>, using the empty pattern C<//> specifically
-matches only the null string, and is not be confused with the regular use
-of C<//> to mean "the last successful pattern match". So, for C<split>,
-the following:
+As a special case for C<split>, the empty pattern C<//> specifically
+matches the empty string; this is not be confused with the normal use
+of an empty pattern to mean the last successful match. So to split
+a string into individual characters, the following:
print join(':', split(//, 'hi there')), "\n";
As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
-will give you as many null initial fields as there are leading spaces.
+will give you as many initial null fields (empty string) as there are leading spaces.
A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
whitespace produces a null first field. A C<split> with no arguments
really does a S<C<split(' ', $_)>> internally.
# Round number to 3 digits after decimal point
$rounded = sprintf("%.3f", $number);
-Perl does its own C<sprintf> formatting--it emulates the C
-function C<sprintf>, but it doesn't use it (except for floating-point
-numbers, and even then only the standard modifiers are allowed). As a
-result, any non-standard extensions in your local C<sprintf> are not
-available from Perl.
+Perl does its own C<sprintf> formatting: it emulates the C
+function sprintf(3), but doesn't use it except for floating-point
+numbers, and even then only standard modifiers are allowed.
+Non-standard extensions in your local sprintf(3) are
+therefore unavailable from Perl.
Unlike C<printf>, C<sprintf> does not do what you probably mean when you
pass it an array as your first argument. The array is given scalar context,
An explicit format parameter index, such as C<2$>. By default sprintf
will format the next unused argument in the list, but this allows you
-to take the arguments out of order, e.g.:
+to take the arguments out of order:
printf '%2$d %1$d', 12, 34; # prints "34 12"
printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
=item vector flag
-This flag tells perl to interpret the supplied string as a vector of
+This flag tells Perl to interpret the supplied string as a vector of
integers, one for each character in the string. Perl applies the format to
each integer in turn, then joins the resulting strings with a separator (a
dot C<.> by default). This can be useful for displaying ordinal values of
printf "bits are %0*v8b\n", " ", $bits; # random bitstring
You can also explicitly specify the argument number to use for
-the join string using e.g. C<*2$v>:
+the join string using something like C<*2$v>; for example:
printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
Arguments are usually formatted to be only as wide as required to
display the given value. You can override the width by putting
a number here, or get the width from the next argument (with C<*>)
-or from a specified argument (with e.g. C<*2$>):
+or from a specified argument (e.g., with C<*2$>):
- printf
'<%s>', "a"; # prints "<a>"
- printf '<%6s>', "a"; # prints "< a>"
- printf '<%*s>', 6, "a"; # prints "< a>"
- printf '<%*2$s>', "a", 6; # prints "< a>"
- printf '<%2s>', "long"; # prints "<long>" (does not truncate)
+ printf
"<%s>", "a"; # prints "<a>"
+ printf "<%6s>", "a"; # prints "< a>"
+ printf "<%*s>", 6, "a"; # prints "< a>"
+ printf "<%*2$s>", "a", 6; # prints "< a>"
+ printf "<%2s>", "long"; # prints "<long>" (does not truncate)
If a field width obtained through C<*> is negative, it has the same
effect as the C<-> flag: left-justification.
You can specify a precision (for numeric conversions) or a maximum
width (for string conversions) by specifying a C<.> followed by a number.
-For floating point formats except 'g' and 'G', this specifies
-how many places right of the decimal point to show (the default being 6); e.g.,
+For floating-point formats except C<g> and C<G>, this specifies
+how many places right of the decimal point to show (the default being 6).
+For example:
# these examples are subject to system-specific variation
printf '<%f>', 1; # prints "<1.000000>"
printf '<%e>', 10; # prints "<1.000000e+01>"
printf '<%.1e>', 10; # prints "<1.0e+01>"
-For 'g' and 'G', this specifies the maximum number of digits to show,
-including prior to the decimal point as well as after it, e.g.:
+For "g" and "G", this specifies the maximum number of digits to show,
+including thoe prior to the decimal point and those after it; for
+example:
- # these examples are subject to system-specific variation
+ # These examples are subject to system-specific variation.
printf '<%g>', 1; # prints "<1>"
printf '<%.10g>', 1; # prints "<1>"
printf '<%g>', 100; # prints "<100>"
printf '<%#10.6x>', 1; # prints "< 0x000001>"
For string conversions, specifying a precision truncates the string
-to fit in the specified width:
+to fit the specified width:
printf '<%.5s>', "truncated"; # prints "<trunc>"
printf '<%10.5s>', "truncated"; # prints "< trunc>"
printf '<%.6x>', 1; # prints "<000001>"
printf '<%.*x>', 6, 1; # prints "<000001>"
-If a precision obtained through C<*> is negative, it has the same
-effect as no precision.
+If a precision obtained through C<*> is negative, it counts
+as having no precision at all.
printf '<%.*s>', 7, "string"; # prints "<string>"
printf '<%.*s>', 3, "string"; # prints "<str>"
printf '<%.*d>', -1, 0; # prints "<0>"
You cannot currently get the precision from a specified number,
-but it is intended that this will be possible in the future using
-e.g. C<.*2$>:
+but it is intended that this will be possible in the future, for
+example using C<.*2$>:
- printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
+ printf "<%.*2$x>", 1, 6; # INVALID, but in future will print "<000001>"
=item size
bits), but you can override this to use instead one of the standard C types,
as supported by the compiler used to build Perl:
- l interpret integer as C type "long" or "unsigned long"
+ hh interpret integer as C type "char" or "unsigned char"
+ on Perl 5.14 or later
h interpret integer as C type "short" or "unsigned short"
- q, L or ll interpret integer as C type "long long", "unsigned long long".
- or "quads" (typically 64-bit integers)
+ j interpret integer as C type "intmax_t" on Perl 5.14
+ or later, and only with a C99 compiler (unportable)
+ l interpret integer as C type "long" or "unsigned long"
+ q, L, or ll interpret integer as C type "long long", "unsigned long long",
+ or "quad" (typically 64-bit integers)
+ t interpret integer as C type "ptrdiff_t" on Perl 5.14 or later
+ z interpret integer as C type "size_t" on Perl 5.14 or later
-The last will produce errors if Perl does not understand "quads" in your
-installation. (This requires that either the platform natively supports quads
-or Perl was specifically compiled to support quads.) You can find out
-whether your Perl supports quads via L<Config>:
+As of 5.14, none of these raises an exception if they are not supported on
+your platform. However, if warnings are enabled, a warning of the
+C<printf> warning class is issued on an unsupported conversion flag.
+Should you instead prefer an exception, do this:
+
+ use warnings FATAL => "printf";
+
+If you would like to know about a version dependency before you
+start running the program, put something like this at its top:
+
+ use 5.014; # for hh/j/t/z/ printf modifiers
+
+You can find out whether your Perl supports quads via L<Config>:
use Config;
- ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
- print "quads\n";
+ if ($Config{use64bitint} eq "define" || $Config{longsize} >= 8) {
+ print "Nice quads!\n";
+ }
-For floating point conversions (C<e f g E F G>), numbers are usually assumed
-to be the default floating point size on your platform (double or long double),
-but you can force 'long double' with C<q>, C<L>, or C<ll> if your
+For floating-point conversions (C<e f g E F G>), numbers are usually assumed
+to be the default floating-point size on your platform (double or long double),
+but you can force "long double" with C<q>, C<L>, or C<ll> if your
platform supports them. You can find out whether your Perl supports long
doubles via L<Config>:
use Config;
- $Config{d_longdbl} eq 'define' && print "long doubles\n";
+ print "long doubles\n" if $Config{d_longdbl} eq "define";
-You can find out whether Perl considers 'long double' to be the default
-floating point size to use on your platform via L<Config>:
+You can find out whether Perl considers "long double" to be the default
+floating-point size to use on your platform via L<Config>:
- use Config;
- ($Config{uselongdouble} eq 'define') &&
- print "long doubles by default\n";
+ use Config;
+ if ($Config{uselongdouble} eq "define") {
+ print "long doubles by default\n";
+ }
-It can also be the case that long doubles and doubles are the same thing:
+It can also be that long doubles and doubles are the same thing:
use Config;
($Config{doublesize} == $Config{longdblsize}) &&
print "doubles are long doubles\n";
-The size specifier C<V> has no effect for Perl code, but it is supported
-for compatibility with XS code; it means 'use the standard size for
-a Perl integer (or floating-point number)', which is already the
-default for Perl code.
+The size specifier C<V> has no effect for Perl code, but is supported for
+compatibility with XS code. It means "use the standard size for a Perl
+integer or floating-point number", which is the default.
=item order of arguments
-Normally, sprintf takes the next unused argument as the value to
+Normally, sprintf() takes the next unused argument as the value to
format for each format specification. If the format specification
uses C<*> to require additional arguments, these are consumed from
-the argument list in the order in which they appear in the format
-specification I<before> the value to format. Where an argument is
-specified using an explicit index, this does not affect the normal
-order for the arguments (even when the explicitly specified index
-would have been the next argument in any case).
+the argument list in the order they appear in the format
+specification I<before> the value to format. Where an argument is
+specified by an explicit index, this does not affect the normal
+order for the arguments, even when the explicitly specified index
+would have been the next argument.
So:
- printf '<%*.*s>', $a, $b, $c;
+ printf "<%*.*s>", $a, $b, $c;
-would use C<$a> for the width, C<$b> for the precision and C<$c>
-as the value to format, while:
+uses C<$a> for the width, C<$b> for the precision, and C<$c>
+as the value to format; while:
- printf '<%*1$.*s>', $a, $b;
+ printf "<%*1$.*s>", $a, $b;
-would use C<$a> for the width and the precision, and C<$b> as the
+would use C<$a> for the width and precision, and C<$b> as the
value to format.
-Here are some more examples - beware that when using an explicit
-index, the C<$> may need to be escaped:
+Here are some more examples; be aware that when using an explicit
+index, the C<$> may need escaping:
printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
=back
-If C<use locale> is in effect, and POSIX::setlocale() has been called,
-the character used for the decimal separator in formatted floating
-
point numbers is affected by the LC_NUMERIC locale. See L<perllocale>
+If C<use locale> is in effect and POSIX::setlocale() has been called,
+the character used for the decimal separator in formatted floating-point
+numbers is affected by the LC_NUMERIC locale. See L<perllocale>
and L<POSIX>.
=item sqrt EXPR
=item sqrt
-Return the square root of EXPR. If EXPR is omitted, returns square
-root of C<$_>. Only works on non-negative operands, unless you've
-loaded the standard Math::Complex module.
+Return the positive square root of EXPR. If EXPR is omitted, uses
+C<$_>. Works only for non-negative operands unless you've
+loaded the C<Math::Complex> module.
use Math::Complex;
- print sqrt(-2); # prints 1.4142135623731i
+ print sqrt(-4); # prints 2i
=item srand EXPR
X<srand> X<seed> X<randseed>
=item srand
-Sets the random number seed for the C<rand> operator.
+Sets and returns the random number seed for the C<rand> operator.
The point of the function is to "seed" the C<rand> function so that
C<rand> can produce a different sequence each time you run your
-program.
-
-If srand() is not called explicitly, it is called implicitly at the
-first use of the C<rand> operator. However, this was not the case in
-versions of Perl before 5.004, so if your script will run under older
-Perl versions, it should call C<srand>.
-
-Most programs won't even call srand() at all, except those that
-need a cryptographically-strong starting point rather than the
-generally acceptable default, which is based on time of day,
-process ID, and memory allocation, or the F</dev/urandom> device,
-if available.
-
-You can call srand($seed) with the same $seed to reproduce the
-I<same> sequence from rand(), but this is usually reserved for
-generating predictable results for testing or debugging.
-Otherwise, don't call srand() more than once in your program.
-
-Do B<not> call srand() (i.e., without an argument) more than once in
-a script. The internal state of the random number generator should
+program. When called with a parameter, C<srand> uses that for the seed;
+otherwise it (semi-)randomly chooses a seed. In either case, starting with
+Perl 5.14, it returns the seed.
+
+If C<srand()> is not called explicitly, it is called implicitly without a
+parameter at the first use of the C<rand> operator. However, this was not true
+of versions of Perl before 5.004, so if your script will run under older
+Perl versions, it should call C<srand>; otherwise most programs won't call
+C<srand()> at all.
+
+But there are a few situations in recent Perls where programs are likely to
+want to call C<srand>. One is for generating predictable results generally for
+testing or debugging. There, you use C<srand($seed)>, with the same C<$seed>
+each time. Another other case is where you need a cryptographically-strong
+starting point rather than the generally acceptable default, which is based on
+time of day, process ID, and memory allocation, or the F</dev/urandom> device
+if available. And still another case is that you may want to call C<srand()>
+after a C<fork()> to avoid child processes sharing the same seed value as the
+parent (and consequently each other).
+
+Do B<not> call C<srand()> (i.e., without an argument) more than once per
+process. The internal state of the random number generator should
contain more entropy than can be provided by any seed, so calling
-srand() again actually I<loses> randomness.
+C<srand()> again actually I<loses> randomness.
Most implementations of C<srand> take an integer and will silently
truncate decimal numbers. This means C<srand(42)> will usually
srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip -f`);
-If you're particularly concerned with this, see the C<Math::TrulyRandom>
-module in CPAN.
+If you're particularly concerned with this, search the CPAN for
+random number generator modules instead of rolling out your own.
Frequently called programs (like CGI scripts) that simply use
one-third of the time. So don't do that.
+A typical use of the returned seed is for a test program which has too many
+combinations to test comprehensively in the time available to it each run. It
+can test a random subset each time, and should there be a failure, log the seed
+used for that run so that it can later be used to reproduce the same results.
+
=item stat FILEHANDLE
X<stat> X<file, status> X<ctime>
Returns a 13-element list giving the status info for a file, either
the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
-omitted, it stats C<$_>. Returns a null list if the stat fails. Typically
+omitted, it stats C<$_> (not C<_>!). Returns the empty list if C<stat> fails. Typically
used as follows:
($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
(*) Not all fields are supported on all filesystem types. Notably, the
ctime field is non-portable. In particular, you cannot expect it to be a
-"creation time"
, see L<perlport/"Files and Filesystems"> for details.
+"creation time"
; see L<perlport/"Files and Filesystems"> for details.
If C<stat> is passed the special filehandle consisting of an underline, no
stat is done, but the current contents of the stat structure from the
$is_directory = S_ISDIR($mode);
You could write the last two using the C<-u> and C<-d> operators.
-The commonly available C<S_IF*> constants are
+Commonly available C<S_IF*> constants are:
# Permissions: read, write, execute, for user, group, others.
and the setuid/setgid/sticky bits
S_IFMT($mode) the part of $mode containing the file type
- which can be bit-anded with e.g. S_IFREG
+ which can be bit-anded with (for example) S_IFREG
or with the following functions
# The operators -f, -d, -l, -b, -c, -p, and -S.
about the C<S_*> constants. To get status info for a symbolic link
instead of the target file behind the link, use the C<lstat> function.
+Portability issues: L<perlport/stat>.
+
=item state EXPR
X<state>
lexical variables that are reinitialized each time their enclosing block
is entered.
-C<state> variables are only enabled when the C<feature 'state'> pragma is
-in effect. See L<feature>.
+C<state> variables are enabled only when the C<use feature "state"> pragma
+is in effect. See L<feature>.
=item study SCALAR
X<study>
Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
doing many pattern matches on the string before it is next modified.
This may or may not save time, depending on the nature and number of
-patterns you are searching on, and on the distribution of character
-frequencies in the string to be searched--you probably want to compare
-run times with and without it to see which runs faster. Those loops
+patterns you are searching and the distribution of character
+frequencies in the string to be searched; you probably want to compare
+run times with and without it to see which is faster. Those loops
that scan for many short constant strings (including the constant
parts of more complex patterns) will benefit most. You may have only
-one C<study> active at a time--if you study a different scalar the first
+one C<study> active at a time: if you study a different scalar the first
is "unstudied". (The way C<study> works is this: a linked list of every
character in the string to be searched is made, so we know, for
example, where all the C<'k'> characters are. From each search string,
print;
}
-In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
+In searching for C</\bfoo\b/>, only locations in C<$_> that contain C<f>
will be looked at, because C<f> is rarer than C<o>. In general, this is
a big win except in pathological cases. The only question is whether
it saves you more time than it took to build the linked list in the
=item sub NAME (PROTO) : ATTRS BLOCK
-This is subroutine definition, not a real function I<per se>.
-Without a BLOCK it's just a forward declaration. Without a NAME,
-it's an anonymous function declaration, and does actually return
-a value: the CODE ref of the closure you just created.
+This is subroutine definition, not a real function I<per se>. Without a
+BLOCK it's just a forward declaration. Without a NAME, it's an anonymous
+function declaration, so does return a value: the CODE ref of the closure
+just created.
See L<perlsub> and L<perlref> for details about subroutines and
-references
, and L<attributes> and L<Attribute::Handlers> for more
+references
; see L<attributes> and L<Attribute::Handlers> for more
information about attributes.
=item substr EXPR,OFFSET,LENGTH,REPLACEMENT
=item substr EXPR,OFFSET
Extracts a substring out of EXPR and returns it. First character is at
-offset C<0>, or whatever you've set C<$[> to (but don't do that).
+offset C<0> (or whatever you've set C<$[> to (but B<<don't do that>)).
If OFFSET is negative (or more precisely, less than C<$[>), starts
-that far from the end of the string. If LENGTH is omitted, returns
-everything to the end of the string. If LENGTH is negative, leaves that
+that far back from the end of the string. If LENGTH is omitted, returns
+everything through the end of the string. If LENGTH is negative, leaves that
many characters off the end of the string.
my $s = "The black cat climbed the green tree";
must itself be an lvalue. If you assign something shorter than LENGTH,
the string will shrink, and if you assign something longer than LENGTH,
the string will grow to accommodate it. To keep the string the same
-length you may need to pad or chop your value using C<sprintf>.
+length, you may need to pad or chop your value using C<sprintf>.
If OFFSET and LENGTH specify a substring that is partly outside the
string, only the part within the string is returned. If the substring
is beyond either end of the string, substr() returns the undefined
value and produces a warning. When used as an lvalue, specifying a
-substring that is entirely outside the string is a fatal error.
+substring that is entirely outside the string raises an exception.
Here's an example showing the behavior for boundary cases:
my $name = 'fred';
substr($name, 4) = 'dy'; # $name is now 'freddy'
- my $null = substr $name, 6, 2; # returns '' (no warning)
+ my $null = substr $name, 6, 2; # returns "" (no warning)
my $oops = substr $name, 7; # returns undef, with warning
- substr($name, 7) = 'gap'; # fatal error
+ substr($name, 7) = 'gap'; # raises an exception
An alternative to using substr() as an lvalue is to specify the
replacement string as the 4th argument. This allows you to replace
my $z = substr $s, 14, 7, "jumped from"; # climbed
# $s is now "The black cat jumped from the green tree"
-Note that the lvalue returned by the 3-arg version of substr() acts as
+Note that the lvalue returned by the three-argument version of substr() acts as
a 'magic bullet'; each time it is assigned to, it remembers which part
of the original string is being modified; for example:
Creates a new filename symbolically linked to the old filename.
Returns C<1> for success, C<0> otherwise. On systems that don't support
-symbolic links, produces a fatal error at run time. To check for that,
+symbolic links, raises an exception. To check for that,
use eval:
$symlink_exists = eval { symlink("",""); 1 };
+Portability issues: L<perlport/symlink>.
+
=item syscall NUMBER, LIST
X<syscall> X<system call>
Calls the system call specified as the first element of the list,
passing the remaining elements as arguments to the system call. If
-unimplemented, produces a fatal error. The arguments are interpreted
+unimplemented, raises an exception. The arguments are interpreted
as follows: if a given argument is numeric, the argument is passed as
an int. If not, the pointer to the string value is passed. You are
responsible to make sure a string is pre-extended long enough to
$s = "hi there\n";
syscall(&SYS_write, fileno(STDOUT), $s, length $s);
-Note that Perl supports passing of up to only 14 arguments to your system call,
-which in practice should usually suffice.
+Note that Perl supports passing of up to only 14 arguments to your syscall,
+which in practice should (usually) suffice.
Syscall returns whatever value returned by the system call it calls.
If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
-Note that some system calls can legitimately return C<-1>. The proper
-way to handle such calls is to assign C<$!=0;> before the call and
-check the value of C<$!> if syscall returns C<-1>.
+Note that some system calls I<can> legitimately return C<-1>. The proper
+way to handle such calls is to assign C<$!=0> before the call, then
+check the value of C<$!> if C<syscall> returns C<-1>.
There's a problem with C<syscall(&SYS_pipe)>: it returns the file
-number of the read end of the pipe it creates. There is no way
+number of the read end of the pipe it creates, but there is no way
to retrieve the file number of the other end. You can avoid this
problem by using C<pipe> instead.
+Portability issues: L<perlport/syscall>.
+
=item sysopen FILEHANDLE,FILENAME,MODE
X<sysopen>
=item sysopen FILEHANDLE,FILENAME,MODE,PERMS
-Opens the file whose filename is given by FILENAME, and associates it
-with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
-the name of the real filehandle wanted. This function calls the
-underlying operating system's C<open> function with the parameters
-FILENAME, MODE, PERMS.
+Opens the file whose filename is given by FILENAME, and associates it with
+FILEHANDLE. If FILEHANDLE is an expression, its value is used as the real
+filehandle wanted; an undefined scalar will be suitably autovivified. This
+function calls the underlying operating system's I<open>(2) function with the
+parameters FILENAME, MODE, and PERMS.
The possible values and flag bits of the MODE parameter are
-system-dependent; they are available via the standard module C<Fcntl>.
-See the documentation of your operating system's C<open> to see which
-values and flag bits are available. You may combine several flags
+system-dependent; they are available via the standard module C<Fcntl>. See
+the documentation of your operating system's I<open>(2) syscall to see
+which values and flag bits are available. You may combine several flags
using the C<|>-operator.
Some of the most common values are C<O_RDONLY> for opening the file in
X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
For historical reasons, some values work on almost every system
-supported by perl: zero means read-only, one means write-only, and two
+supported by Perl: 0 means read-only, 1 means write-only, and 2
means read/write. We know that these values do I<not> work under
OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
use them in new code.
See L<perlopentut> for a kinder, gentler explanation of opening files.
+Portability issues: L<perlport/sysopen>.
+
=item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
X<sysread>
=item sysread FILEHANDLE,SCALAR,LENGTH
Attempts to read LENGTH bytes of data into variable SCALAR from the
-specified FILEHANDLE, using the system call read(2). It bypasses
+specified FILEHANDLE, using the read(2). It bypasses
buffered IO, so mixing this with other kinds of reads, C<print>,
C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
perlio or stdio layers usually buffers data. Returns the number of
=item sysseek FILEHANDLE,POSITION,WHENCE
X<sysseek> X<lseek>
-Sets FILEHANDLE's system position in bytes using the system call
-lseek(2). FILEHANDLE may be an expression whose value gives the name
-of the filehandle. The values for WHENCE are C<0> to set the new
-position to POSITION, C<1> to set the it to the current position plus
-POSITION, and C<2> to set it to EOF plus POSITION (typically
-negative).
+Sets FILEHANDLE's system position in bytes using lseek(2). FILEHANDLE may
+be an expression whose value gives the name of the filehandle. The values
+for WHENCE are C<0> to set the new position to POSITION; C<1> to set the it
+to the current position plus POSITION; and C<2> to set it to EOF plus
+POSITION, typically negative.
Note the I<in bytes>: even if the filehandle has been set to operate
on characters (for example by using the C<:encoding(utf8)> I/O layer),
tell() will return byte offsets, not character offsets (because
implementing that would render sysseek() unacceptably slow).
-sysseek() bypasses normal buffered IO, so mixing this with reads (other
-than C<sysread>
, for example C<< <> >> or read()) C<print>, C<write>,
+sysseek() bypasses normal buffered IO, so mixing it with reads other
+than C<sysread>
(for example C<< <> >> or read()) C<print>, C<write>,
C<seek>, C<tell>, or C<eof> may cause confusion.
For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
=item system PROGRAM LIST
Does exactly the same thing as C<exec LIST>, except that a fork is
-done first, and the parent process waits for the child process to
+done first and the parent process waits for the child process to
exit. Note that argument processing varies depending on the
number of arguments. If there is more than one argument in LIST,
or if LIST is an array with more than one value, starts the program
The return value is the exit status of the program as returned by the
C<wait> call. To get the actual exit value, shift right by eight (see
below). See also L</exec>. This is I<not> what you want to use to capture
-the output from a command, for that you should use merely backticks or
+the output from a command; for that you should use merely backticks or
C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
indicates a failure to start the program or an error of the wait(2) system
call (inspect $! for the reason).
printf "child exited with value %d\n", $? >> 8;
}
-Alternatively you might inspect the value of C<${^CHILD_ERROR_NATIVE}>
-with the W*() calls of the POSIX extension.
+Alternatively, you may inspect the value of C<${^CHILD_ERROR_NATIVE}>
+with the C<W*()> calls from the POSIX module.
-When the arguments get executed via the system shell, results
-and return codes will be subject to its quirks and capabilities.
+When C<system>'s arguments are executed indirectly by the shell,
+results and return codes are subject to its quirks.
See L<perlop/"`STRING`"> and L</exec> for details.
+Since C<system> does a C<fork> and C<wait> it may affect a C<SIGCHLD>
+handler. See L<perlipc> for details.
+
+Portability issues: L<perlport/system>.
+
=item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
X<syswrite>
=item syswrite FILEHANDLE,SCALAR
Attempts to write LENGTH bytes of data from variable SCALAR to the
-specified FILEHANDLE, using the system call write(2). If LENGTH is
+specified FILEHANDLE, using write(2). If LENGTH is
not specified, writes whole SCALAR. It bypasses buffered IO, so
mixing this with reads (other than C<sysread())>, C<print>, C<write>,
C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
-stdio layers usually buffers data. Returns the number of bytes
+stdio layers usually buffer data. Returns the number of bytes
actually written, or C<undef> if there was an error (in this case the
errno variable C<$!> is also set). If the LENGTH is greater than the
-available data in the SCALAR after the OFFSET, only as much data as is
+data available in the SCALAR after the OFFSET, only as much data as is
available will be written.
An OFFSET may be specified to write the data from some part of the
string other than the beginning. A negative OFFSET specifies writing
that many characters counting backwards from the end of the string.
-In the case the SCALAR is empty you can use OFFSET but only zero offset.
+If SCALAR is of length zero, you can only use an OFFSET of 0.
-Note that if the filehandle has been marked as C<:utf8>, Unicode
-characters are written instead of bytes (the LENGTH, OFFSET, and the
-return value of syswrite() are in UTF-8 encoded Unicode characters).
+B<WARNING>: If the filehandle is marked C<:utf8>, Unicode characters
+encoded in UTF-8 are written instead of bytes, and the LENGTH, OFFSET, and
+return value of syswrite() are in (UTF8-encoded Unicode) characters.
The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
+Alternately, if the handle is not marked with an encoding but you
+attempt to write characters with code points over 255, raises an exception.
See L</binmode>, L</open>, and the C<open> pragma, L<open>.
=item tell FILEHANDLE
There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
-Do not use tell() (or other buffered I/O operations) on a file handle
-that has been manipulated by sysread(), syswrite() or sysseek().
+Do not use tell() (or other buffered I/O operations) on a filehandle
+that has been manipulated by sysread(), syswrite(), or sysseek().
Those functions ignore the buffering, while tell() does not.
=item telldir DIRHANDLE
DESTROY this
UNTIE this
-A class implementing a file handle should have the following methods:
+A class implementing a filehandle should have the following methods:
TIEHANDLE classname, LIST
READ this, scalar, length, offset
Not all methods indicated above need be implemented. See L<perltie>,
L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
-Unlike C<dbmopen>, the C<tie> function will not use or require a module
-for you--you need to do that explicitly yourself. See L<DB_File>
+Unlike C<dbmopen>, the C<tie> function will not C<use> or C<require> a module
+for you; you need to do that explicitly yourself. See L<DB_File>
or the F<Config> module for interesting C<tie> implementations.
For further details see L<perltie>, L<"tied VARIABLE">.
a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
1904 in the current local time zone for its epoch.
-For measuring time in better granularity than one second,
-you may use either the L<Time::HiRes> module (from CPAN, and starting from
-Perl 5.8 part of the standard distribution), or if you have
-gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
-See L<perlfaq8> for details.
+For measuring time in better granularity than one second, use the
+L<Time::HiRes> module from Perl 5.8 onwards (or from CPAN before then), or,
+if you have gettimeofday(2), you may be able to use the C<syscall>
+interface of Perl. See L<perlfaq8> for details.
For date and time processing look at the many related modules on CPAN.
For a comprehensive date and time representation look at the
=item times
X<times>
-Returns a four-element list giving the user and system times, in
-seconds, for this process and the children of this process.
+Returns a four-element list giving the user and system times in
+seconds for this process and any exited children of this process.
($user,$system,$cuser,$csystem) = times;
In scalar context, C<times> returns C<$user>.
-Note that times for children are included only after they terminate.
+Children's times are only included for terminated children.
+
+Portability issues: L<perlport/times>.
=item tr///
=item truncate EXPR,LENGTH
Truncates the file opened on FILEHANDLE, or named by EXPR, to the
-specified length. Produces a fatal error if truncate isn't implemented
-on your system. Returns true if successful, the undefined value
-otherwise.
+specified length. Raises an exception if truncate isn't implemented
+on your system. Returns true if successful, C<undef> on error.
The behavior is undefined if LENGTH is greater than the length of the
file.
The position in the file of FILEHANDLE is left unchanged. You may want to
-call L<seek> before writing to the file.
+call L<seek|/"seek FILEHANDLE,POSITION,WHENCE"> before writing to the file.
+
+Portability issues: L<perlport/truncate>.
=item uc EXPR
X<uc> X<uppercase> X<toupper>
Returns an uppercased version of EXPR. This is the internal function
implementing the C<\U> escape in double-quoted strings.
It does not attempt to do titlecase mapping on initial letters. See
-C<ucfirst> for that.
+L</ucfirst> for that.
If EXPR is omitted, uses C<$_>.
representing disabled permissions bits. The permission (or "mode")
values you pass C<mkdir> or C<sysopen> are modified by your umask, so
even if you tell C<sysopen> to create a file with permissions C<0777>,
-if your umask is C<0022> then the file will actually be created with
+if your umask is C<0022>, then the file will actually be created with
permissions C<0755>. If your C<umask> were C<0027> (group can't
write; others can't read, write, or execute), then passing
-C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
-027> is C<0640>).
+C<sysopen> C<0666> would create a file with mode C<0640> (because
+C<0666 &~ 027> is C<0640>).
Here's some advice: supply a creation mode of C<0666> for regular
files (in C<sysopen>) and one of C<0777> for directories (in
so on.
If umask(2) is not implemented on your system and you are trying to
-restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
-fatal error at run time. If umask(2) is not implemented and you are
+restrict access for I<yourself> (i.e., C<< (EXPR & 0700) > 0 >>),
+raises an exception. If umask(2) is not implemented and you are
not trying to restrict access for yourself, returns C<undef>.
Remember that a umask is a number, usually given in octal; it is I<not> a
string of octal digits. See also L</oct>, if all you have is a string.
+Portability issues: L<perlport/umask>.
+
=item undef EXPR
X<undef> X<undefine>
Undefines the value of EXPR, which must be an lvalue. Use only on a
scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
-(using C<&>), or a typeglob (using C<*>). (Saying C<undef $hash{$key}>
+(using C<&>), or a typeglob (using C<*>). Saying C<undef $hash{$key}>
will probably not do what you expect on most predefined variables or
-DBM list values, so don't do that; see L<delete>.) Always returns the
+DBM list values, so don't do that; see L</delete>. Always returns the
undefined value. You can omit the EXPR, in which case nothing is
undefined, but you still get an undefined value that you could, for
-instance, return from a subroutine, assign to a variable or pass as a
+instance, return from a subroutine, assign to a variable, or pass as a
parameter. Examples:
undef $foo;
foreach my $file ( @goners ) {
unlink $file or warn "Could not unlink $file: $!";
- }
+ }
Note: C<unlink> will not attempt to delete directories unless you are
superuser and the B<-U> flag is supplied to Perl. Even if these
and expands it out into a list of values.
(In scalar context, it returns merely the first value produced.)
-If EXPR is omitted, unpacks the C<$_> string. for an introduction to this function.
-
+If EXPR is omitted, unpacks the C<$_> string.
See L<perlpacktut> for an introduction to this function.
The string is broken into chunks described by the TEMPLATE. Each chunk
you want a <number>-bit checksum of the items instead of the items
themselves. Default is a 16-bit checksum. Checksum is calculated by
summing numeric values of expanded values (for string fields the sum of
-C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
+C<ord($char)> is taken; for bit fields the sum of zeroes and ones).
For example, the following
computes the same number as the System V sum program:
If there are more pack codes or if the repeat count of a field or a group
is larger than what the remainder of the input string allows, the result
-is not well defined: in some cases, the repeat count is decreased, or
-C<unpack()> will produce null strings or zeroes, or terminate with an
-error. If the input string is longer than one described by the TEMPLATE,
-the rest is ignored.
+is not well defined: the repeat count may be decreased, or
+C<unpack()> may produce empty strings or zeros, or it may raise an exception.
+If the input string is longer than one described by the TEMPLATE,
+the remainder of that input string is ignored.
See L</pack> for more examples and notes.
=item unshift ARRAY,LIST
X<unshift>
+=item unshift EXPR,LIST
+
Does the opposite of a C<shift>. Or the opposite of a C<push>,
depending on how you look at it. Prepends list to the front of the
-array, and returns the new number of elements in the array.
+array and returns the new number of elements in the array.
unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
prepended elements stay in the same order. Use C<reverse> to do the
reverse.
+Starting with Perl 5.14, C<unshift> can take a scalar EXPR, which must hold
+a reference to an unblessed array. The argument will be dereferenced
+automatically. This aspect of C<unshift> is considered highly
+experimental. The exact behaviour may change in a future version of Perl.
+
=item use Module VERSION LIST
X<use> X<module> X<import>
BEGIN { require Module; Module->import( LIST ); }
except that Module I<must> be a bareword.
+The importation can be made conditional; see L<if>.
-In the peculiar C<use VERSION> form, VERSION may be either a numeric
-argument such as 5.006, which will be compared to C<$]>, or a literal of
-the form v5.6.1, which will be compared to C<$^V> (aka $PERL_VERSION). A
-fatal error is produced if VERSION is greater than the version of the
+In the peculiar C<use VERSION> form, VERSION may be either a positive
+decimal fraction such as 5.006, which will be compared to C<$]>, or a v-string
+of the form v5.6.1, which will be compared to C<$^V> (aka $PERL_VERSION). An
+exception is raised if VERSION is greater than the version of the
current Perl interpreter; Perl will not attempt to parse the rest of the
file. Compare with L</require>, which can do a similar check at run time.
Symmetrically, C<no VERSION> allows you to specify that you want a version
-of perl older than the specified one.
+of Perl older than the specified one.
Specifying VERSION as a literal of the form v5.6.1 should generally be
avoided, because it leads to misleading error messages under earlier
C<use>ing library modules that won't work with older versions of Perl.
(We try not to do this more than we have to.)
-Also, if the specified perl version is greater than or equal to 5.9.5,
+Also, if the specified Perl version is greater than or equal to 5.9.5,
C<use VERSION> will also load the C<feature> pragma and enable all
features available in the requested version. See L<feature>.
-Similarly, if the specified perl version is greater than or equal to
+Similarly, if the specified Perl version is greater than or equal to
5.11.0, strictures are enabled lexically as with C<use strict> (except
that the F<strict.pm> file is not actually loaded).
The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
C<require> makes sure the module is loaded into memory if it hasn't been
-yet. The C<import> is not a builtin--it's just an ordinary static method
+yet. The C<import> is not a builtin; it's just an ordinary static method
call into the C<Module> package to tell the module to import the list of
features back into the current package. The module can implement its
C<import> method any way it likes, though most modules just choose to
Because C<use> takes effect at compile time, it doesn't respect the
ordinary flow control of the code being compiled. In particular, putting
a C<use> inside the false branch of a conditional doesn't prevent it
-from being processed. If a module or pragma needs to be loaded only
+from being processed. If a module or pragma only needs to be loaded
conditionally, this can be done using the L<if> pragma:
use if $] < 5.008, "utf8";
use if WANT_WARNINGS, warnings => qw(all);
-There's a corresponding C<no> command that unimports meanings imported
+There's a corresponding C<no> declaration that unimports meanings imported
by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
It behaves just as C<import> does with VERSION, an omitted or empty LIST,
or no unimport method being found.
no strict 'refs';
no warnings;
+Care should be taken when using the C<no VERSION> form of C<no>. It is
+I<only> meant to be used to assert that the running Perl is of a earlier
+version than its argument and I<not> to undo the feature-enabling side effects
+of C<use VERSION>.
+
See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
-for the C<-M> and C<-m> command-line options to perl that give C<use>
+for the C<-M> and C<-m> command-line options to Perl that give C<use>
functionality from the command-line.
=item utime LIST
X<utime>
Changes the access and modification times on each file of a list of
-files. The first two elements of the list must be the NUMERICAL access
+files. The first two elements of the list must be the NUMERIC access
and modification times, in that order. Returns the number of files
successfully changed. The inode change time of each file is set
to the current time. For example, this code has the same effect as the
$atime = $mtime = time;
utime $atime, $mtime, @ARGV;
-Since perl 5.7.2, if the first two elements of the list are C<undef>, then
-the utime(2) function in the C library will be called with a null second
+Since Perl 5.7.2, if the first two elements of the list are C<undef>,
+the utime(2) syscall from your C library is called with a null second
argument. On most systems, this will set the file's access and
modification times to the current time (i.e., equivalent to the example
-above) and will even work on other users' files where you have write
+above) and will work even on files you don't own provided you have write
permission:
- utime undef, undef, @ARGV;
+ for $file (@ARGV) {
+ utime(undef, undef, $file)
+ || warn "couldn't touch $file: $!";
+ }
Under NFS this will use the time of the NFS server, not the time of
the local machine. If there is a time synchronization problem, the
touch(1) command will in fact normally use this form instead of the
one shown in the first example.
-Note that only passing one of the first two elements as C<undef> will
-be equivalent of passing it as 0 and will not have the same effect as
-described when they are both C<undef>. This case will also trigger an
+Passing only one of the first two elements as C<undef> is
+equivalent to passing a 0 and will not have the effect
+described when both are C<undef>. This also triggers an
uninitialized warning.
-On systems that support futimes, you might pass file handles among the
-files. On systems that don't support futimes, passing file handles
-produces a fatal error at run time. The file handles must be passed
-as globs or references to be recognized. Barewords are considered
-file names.
+On systems that support futimes(2), you may pass filehandles among the
+files. On systems that don't support futimes(2), passing filehandles raises
+an exception. Filehandles must be passed as globs or glob references to be
+recognized; barewords are considered filenames.
+
+Portability issues: L<perlport/utime>.
=item values HASH
X<values>
=item values ARRAY
+=item values EXPR
+
Returns a list consisting of all the values of the named hash, or the values
-of an array. (In a scalar context, returns the number of values.)
+of an array. (In scalar context, returns the number of values.)
The values are returned in an apparently random order. The actual
-random order is subject to change in future versions of perl, but it
+random order is subject to change in future versions of Perl, but it
is guaranteed to be the same order as either the C<keys> or C<each>
function would produce on the same (unmodified) hash. Since Perl
5.8.1 the ordering is different even between different runs of Perl
for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
As a side effect, calling values() resets the HASH or ARRAY's internal
-iterator,
+iterator;
see L</each>. (In particular, calling values() in void context resets
the iterator with no other overhead. Apart from resetting the iterator,
C<values @array> in list context is the same as plain C<@array>.
that it taking C<values @array> out would require more documentation than
leaving it in.)
-
Note that the values are not copied, which means modifying them will
modify the contents of the hash:
for (values %hash) { s/foo/bar/g } # modifies %hash values
for (@hash{keys %hash}) { s/foo/bar/g } # same
+Starting with Perl 5.14, C<values> can take a scalar EXPR, which must hold
+a reference to an unblessed hash or array. The argument will be
+dereferenced automatically. This aspect of C<values> is considered highly
+experimental. The exact behaviour may change in a future version of Perl.
+
+ for (values $hashref) { ... }
+ for (values $obj->get_arrayref) { ... }
+
See also C<keys>, C<each>, and C<sort>.
=item vec EXPR,OFFSET,BITS
X<vec> X<bit> X<bit vector>
Treats the string in EXPR as a bit vector made up of elements of
-width BITS, and returns the value of the element specified by OFFSET
+width BITS and returns the value of the element specified by OFFSET
as an unsigned integer. BITS therefore specifies the number of bits
that are reserved for each element in the bit vector. This must
be a power of two from 1 to 32 (or 64, if your platform supports
=item wait
X<wait>
-Behaves like the wait(2) system call on your system: it waits for a child
+Behaves like wait(2) on your system: it waits for a child
process to terminate and returns the pid of the deceased process, or
C<-1> if there are no child processes. The status is returned in C<$?>
and C<${^CHILD_ERROR_NATIVE}>.
Note that a return value of C<-1> could mean that child processes are
being automatically reaped, as described in L<perlipc>.
+If you use wait in your handler for $SIG{CHLD} it may accidentally for the
+child created by qx() or system(). See L<perlipc> for details.
+
+Portability issues: L<perlport/wait>.
+
=item waitpid PID,FLAGS
X<waitpid>
then you can do a non-blocking wait for all pending zombie processes.
Non-blocking wait is available on machines supporting either the
-waitpid(2) or wait4(2) system calls. However, waiting for a particular
+waitpid(2) or wait4(2) syscalls. However, waiting for a particular
pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
system call by remembering the status values of processes that have
exited but have not been harvested by the Perl script yet.)
processes are being automatically reaped. See L<perlipc> for details,
and for other examples.
+Portability issues: L<perlport/waitpid>.
+
=item wantarray
X<wantarray> X<context>
# run-time warnings enabled after here
warn "\$foo is alive and $foo!"; # does show up
-See L<perlvar> for details on setting C<%SIG> entries, and for more
+See L<perlvar> for details on setting C<%SIG> entries and for more
examples. See the Carp module for other kinds of warnings using its
carp() and cluck() functions.
+=item when EXPR BLOCK
+X<when>
+
+=item when BLOCK
+
+C<when> is analogous to the C<case> keyword in other languages. Used with a
+C<foreach> loop or the experimental C<given> block, C<when> can be used in
+Perl to implement C<switch>/C<case> like statements. Available as a
+statement after Perl 5.10 and as a statement modifier after 5.14.
+Here are three examples:
+
+ use v5.10;
+ foreach (@fruits) {
+ when (/apples?/) {
+ say "I like apples."
+ }
+ when (/oranges?/) {
+ say "I don't like oranges."
+ }
+ default {
+ say "I don't like anything"
+ }
+ }
+
+ # require 5.14 for when as statement modifier
+ use v5.14;
+ foreach (@fruits) {
+ say "I like apples." when /apples?/;
+ say "I don't like oranges." when /oranges?;
+ default { say "I don't like anything" }
+ }
+
+ use v5.10;
+ given ($fruit) {
+ when (/apples?/) {
+ say "I like apples."
+ }
+ when (/oranges?/) {
+ say "I don't like oranges."
+ }
+ default {
+ say "I don't like anything"
+ }
+ }
+
+See L<perlsyn/"Switch statements"> for detailed information.
+
=item write FILEHANDLE
X<write>
format for the current output channel (see the C<select> function) may be set
explicitly by assigning the name of the format to the C<$~> variable.
-Top of form processing is handled automatically: if there is
-insufficient room on the current page for the formatted record, the
-page is advanced by writing a form feed, a special top-of-page format
-is used to format the new page header, and then the record is written.
-By default the top-of-page format is the name of the filehandle with
-"_TOP" appended, but it may be dynamically set to the format of your
-choice by assigning the name to the C<$^> variable while the filehandle is
-selected. The number of lines remaining on the current page is in
-variable C<$->, which can be set to C<0> to force a new page.
+Top of form processing is handled automatically: if there is insufficient
+room on the current page for the formatted record, the page is advanced by
+writing a form feed, a special top-of-page format is used to format the new
+page header before the record is written. By default, the top-of-page
+format is the name of the filehandle with "_TOP" appended. This would be a
+problem with autovivified filehandles, but it may be dynamically set to the
+format of your choice by assigning the name to the C<$^> variable while
+that filehandle is selected. The number of lines remaining on the current
+page is in variable C<$->, which can be set to C<0> to force a new page.
If FILEHANDLE is unspecified, output goes to the current default output
channel, which starts out as STDOUT but may be changed by the
L<perlop/"Quote and Quote-like Operators">.
=back
+
+=cut
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