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, scalar arguments
come first and list argument follow, and there can only ever
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
+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
was never a list to start with.
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.) all return
+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>
=item Keywords related to the control flow of your Perl program
X<control flow>
-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>
+C<caller>, C<continue>, C<die>, C<do>,
+C<dump>, C<eval>, C<evalbytes> C<exit>,
+C<__FILE__>, C<goto>, C<last>, C<__LINE__>, C<next>, C<__PACKAGE__>,
+C<redo>, C<return>, C<sub>, C<__SUB__>, C<wantarray>
-=item Keywords related to switch
+C<__SUB__> is only available with a C<use v5.16> (or higher) declaration or
+with the C<"current_sub"> feature (see L<feature>).
-C<break>, C<continue>, C<given>, C<when>, C<default>
+=item Keywords related to the switch feature
-(These are available only if you enable the C<"switch"> feature.
-See L<feature> and L<perlsyn/"Switch statements">.)
+C<break>, C<continue>, C<default>, C<given>, C<when>
+
+Except for C<continue>, these are available only if you enable the
+C<"switch"> feature or use the C<CORE::> prefix.
+See L<feature> and L<perlsyn/"Switch statements">.
+Alternately, include a C<use v5.10> or later to the current scope. In Perl
+5.14 and earlier, C<continue> required the C<"switch"> feature, like the
+other keywords.
=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 available only if the C<"state"> feature is enabled. See
-L<feature>.)
+C<state> is available only if the C<"state"> feature
+is enabled or if it is prefixed with C<CORE::>. See
+L<feature>. Alternately, include a C<use v5.10> or later to the current scope.
=item Miscellaneous functions
-C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>,
+C<defined>, C<dump>, C<eval>, C<evalbytes>,
+C<formline>, C<local>, C<my>, C<our>,
C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray>
=item Functions for processes and process groups
X<process> X<pid> X<process id>
C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
-C<pipe>, C<qx//>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
+C<pipe>, C<qx//>, C<readpipe>, C<setpgrp>,
+C<setpriority>, C<sleep>, C<system>,
C<times>, C<wait>, C<waitpid>
=item Keywords related to Perl modules
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:
#...
}
+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 $file) + 1024 # correct
+
The interpretation of the file permission operators C<-r>, C<-R>,
C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
of the file and the uids and gids of the user. There may be other
If you are using ACLs, there is a pragma called C<filetest> that may
produce more accurate results than the bare stat() mode bits.
-When under the C<use filetest 'access'> the above-mentioned filetests
-test whether the permission can (not) be granted using the
+When under C<use filetest 'access'> the above-mentioned filetests
+test whether the permission can(not) be granted using the
access(2) family of system calls. Also note that the C<-x> and C<-X> may
under this pragma return true even if there are no execute permission
bits set (nor any extra execute permission ACLs). This strangeness is
in effect. Read the documentation for the C<filetest> pragma for more
information.
-Note that C<-s/a/b/> does not do a negated substitution. Saying
-C<-exp($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%)
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
the return value of C<-f $file> as an argument to another filetest
operator, no special magic will happen.)
+Portability issues: L<perlport/-X>.
+
=item abs VALUE
X<abs> X<absolute>
For more information see L<perlipc>.
+Portability issues: L<perlport/alarm>.
+
=item atan2 Y,X
X<atan2> X<arctangent> X<tan> X<tangent>
The return value for C<atan2(0,0)> is implementation-defined; consult
your atan2(3) manpage for more information.
+Portability issues: L<perlport/atan2>.
+
=item bind SOCKET,NAME
X<bind>
taken as the name of the filehandle. Returns true on success,
otherwise it returns C<undef> and sets C<$!> (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 filehandle.
-When LAYER is present using binmode on a text file makes sense.
+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
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() 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 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 important not only for readline() and print() operations,
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>
is used. Because a C<bless> is often the last thing in a constructor,
it returns the reference for convenience. Always use the two-argument
version if a derived class might inherit the function doing the blessing.
-See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
-of objects.
+SeeL<perlobj> for more about the blessing (and blessings) of objects.
Consider always blessing objects in CLASSNAMEs that are mixed case.
Namespaces with all lowercase names are considered reserved for
Break out of a C<given()> block.
-This keyword is enabled by the C<"switch"> feature: see L<feature>
-for more information.
+This keyword is enabled by the C<"switch"> feature: see
+L<feature> for more information. You can also access it by
+prefixing it with C<CORE::>. 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.
-Also be aware that setting C<@DB::args> is I<best effort>, intended for
+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
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 means that the effects of C<shift @_> can
-I<normally> be undone (but not C<pop @_> or other splicing, and not if a
-reference to C<@_> has been taken, and subject to the caveat about reallocated
+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.
false otherwise. See the example under C<die>.
On systems that support fchdir(2), you may pass a filehandle or
-directory handle as argument. On systems that don't support fchdir(2),
+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, but C<"0644"> is not. Returns the number of files
-successfully changed. See also L</oct>, if all you have is a string.
+successfully changed. See also L</oct> if all you have is a string.
$cnt = chmod 0755, "foo", "bar";
chmod 0755, @executables;
chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
# Identical to the chmod 0755 of the example above.
+Portability issues: L<perlport/chmod>.
+
=item chomp VARIABLE
X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> 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:
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>
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> closes it for you. (See
-C<open>.) However, an explicit C<close> on an input file resets the line
+L<open|/open FILEHANDLE>.) However, an explicit C<close> on an input file resets the line
counter (C<$.>), while the implicit close done by C<open> does not.
If the filehandle came from a piped open, C<close> returns false if one of
afterwards--and implicitly puts the exit status value of that command into
C<$?> 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
the other end can't handle that, be sure to read all the data before
or die "Can't open 'foo' for input: $!";
FILEHANDLE may be an expression whose value can be used as an indirect
-filehandle, usually the real filehandle name.
+filehandle, usually the real filehandle name or an autovivified handle.
=item closedir DIRHANDLE
X<closedir>
=item continue
-C<continue> is actually a flow control statement rather than a function. If
+When followed by a BLOCK, C<continue> is actually a
+flow control statement rather than a function. If
there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
C<foreach>), it is always executed just before the conditional is about to
be evaluated again, just like the third part of a C<for> loop in C. Thus
empty one, logically enough, so C<next> goes directly back
to check the condition at the top of the loop.
-If the C<"switch"> feature is enabled, C<continue> is also a
-function that exits the current C<when> (or C<default>) block and
-falls through to the next one. See L<feature> and
-L<perlsyn/"Switch statements"> for more information.
-
+When there is no BLOCK, C<continue> is a function that
+falls through the current C<when> or C<default> block instead of iterating
+a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
+In Perl 5.14 and earlier, this form of C<continue> was
+only available when the C<"switch"> feature was enabled.
+See L<feature> and L<perlsyn/"Switch statements"> for more
+information.
=item cos EXPR
X<cos> X<cosine> X<acos> X<arccosine>
=item cos
Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
-takes cosine of C<$_>.
+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 $digest>). The SALT used
to create the digest is visible as part of the digest. This ensures
crypt() will hash the new string with the same salt as the digest.
This allows your code to work with the standard L<crypt|/crypt> and
-with more exotic implementations. In other words, do not assume
-anything about the returned string itself, or how many bytes in the
-digest matter.
+with more exotic implementations. In other words, assume
+nothing about the returned string itself nor about how many bytes
+of SALT may matter.
Traditionally the result is a string of 13 bytes: two first bytes of
the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
If using crypt() on a Unicode string (which I<potentially> has
characters with codepoints above 255), Perl tries to make sense
-of the situation by trying to downgrade (a copy of the string)
+of the situation by trying to downgrade (a copy of)
the string back to an eight-bit byte string before calling crypt()
(on that copy). If that works, good. If not, crypt() dies with
C<Wide character in crypt>.
+Portability issues: L<perlport/crypt>.
+
=item dbmclose HASH
X<dbmclose>
Breaks the binding between a DBM file and a hash.
+Portability issues: L<perlport/dbmclose>.
+
=item dbmopen HASH,DBNAME,MASK
X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
-[This function has been largely superseded by the C<tie> function.]
+[This function has been largely superseded by the
+L<tie|/tie VARIABLE,CLASSNAME,LIST> function.]
This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
hash. HASH is the name of the hash. (Unlike normal C<open>, the first
dbmopen(%NS_Hist, "$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 or if the keyword is prefixed with
+C<CORE::>. See L</when>.
+
=item defined EXPR
X<defined> X<undef> X<undefined>
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 $debugging;
-Note: Many folks tend to overuse C<defined>, and then are surprised to
+Note: Many folks tend to overuse C<defined> and are then surprised to
discover that the number C<0> and C<""> (the zero-length string) are, in fact,
defined values. For example, if you say
no longer returns true. Setting a hash element to the undefined value does
not remove its key, but deleting it does; see L</exists>.
-It returns the value or values deleted in list context, or the last such
+In list context, returns the value or values deleted, or the last such
element in scalar context. The return list's length always matches that of
the argument list: deleting non-existent elements returns the undefined value
in their corresponding positions.
array, the array's size shrinks to the position of the highest element that
still tests true for exists(), or to 0 if none do.
-B<Be aware> that calling delete on array values is deprecated and likely to
+B<WARNING:> Calling delete on array values is deprecated and likely to
be removed in a future version of Perl.
Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
into C<$@> 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>.
+need to exit the process with a specific exit code, see L</exit>.
Equivalent examples:
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.
does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
handler is called with the error text and can change the error
message, if it sees fit, by calling C<die> again. See
-L<perlvar/$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 so: the C<$SIG{__DIE__}> hook is currently called
=item do SUBROUTINE(LIST)
X<do>
-This form of subroutine call is deprecated. See L<perlsub>.
+This form of subroutine call is deprecated. SUBROUTINE can be a bareword,
+a scalar variable or a subroutine beginning with C<&>.
=item do EXPR
X<do>
eval `cat stat.pl`;
except that it's more efficient and concise, keeps track of the current
-filename for error messages, searches the @INC directories, and updates
-C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
-variables. It also differs in that code evaluated with C<do FILENAME>
+filename for error messages, searches the C<@INC> directories, and updates
+C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
+these variables. It also differs in that code evaluated with C<do FILENAME>
cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
same, however, in that it does reparse the file every time you call it,
so you probably don't want to do this inside a loop.
-If C<do> cannot read the file, it returns undef and sets C<$!> 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<$!>. If the file is successfully
+compiled, C<do> returns the value of the last expression evaluated.
Inclusion of library modules is better done with the
C<use> and C<require> operators, which also do automatic error checking
it as C<CORE::dump()>, if you don't want to be warned against a possible
typo.
+Portability issues: L<perlport/dump>.
+
=item each HASH
X<each> X<hash, iterator>
=item each ARRAY
X<array, iterator>
+=item each EXPR
+
When called in list context, returns a 2-element list consisting of the key
and value for the next element of a hash, or the index and value for the
next element of an array, so that you can iterate over it. When called in
print "$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 detect the end of only 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 (<>) {
print "--------------\n";
}
print;
- last if eof(); # needed if we're reading from a terminal
+ last if eof(); # needed if we're reading from a terminal
}
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.
+If the C<unicode_eval> feature is enabled (which is the default under a
+C<use 5.16> or higher declaration), EXPR or C<$_> is treated as a string of
+characters, so C<use utf8> declarations have no effect, and source filters
+are forbidden. In the absence of the C<unicode_eval> feature, the string
+will sometimes be treated as characters and sometimes as bytes, depending
+on the internal encoding, and source filters activated within the C<eval>
+exhibit the erratic, but historical, behaviour of affecting some outer file
+scope that is still compiling. See also the L</evalbytes> keyword, which
+always treats its input as a byte stream and works properly with source
+filters, and the L<feature> pragma.
+
In the second form, the code within the BLOCK is parsed only once--at the
same time the code surrounding the C<eval> itself was parsed--and executed
within the context of the current Perl program. This form is typically
determined.
If there is a syntax error or runtime error, or a C<die> statement is
-executed, C<eval> returns an undefined value in scalar context
-or an empty list in list context, and C<$@> is set to the
-error message. If there was no error, C<$@> is guaranteed to be the empty
-string. Beware that using C<eval> neither silences Perl from printing
+executed, C<eval> returns C<undef> in scalar context
+or an empty list in list context, and C<$@> is set to the error
+message. (Prior to 5.16, a bug caused C<undef> to be returned
+in list context for syntax errors, but not for runtime errors.)
+If there was no error, C<$@> is set to the empty string. A
+control flow operator like C<last> or C<goto> can bypass the setting of
+C<$@>. 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
the binary interface (such as Perl version skew) may be fatal even with
-C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
+C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set.
See L<perlrun>.
If the code to be executed doesn't vary, you may use the eval-BLOCK
form to trap run-time errors without incurring the penalty of
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 $e
An C<eval ''> executed within the C<DB> package doesn't see the usual
surrounding lexical scope, but rather the scope of the first non-DB piece
-of code that called it. You don't normally need to worry about this unless
+of code that called it. You don't normally need to worry about this unless
you are writing a Perl debugger.
+=item evalbytes EXPR
+X<evalbytes>
+
+=item evalbytes
+
+This function is like L</eval> with a string argument, except it always
+parses its argument, or C<$_> if EXPR is omitted, as a string of bytes. A
+string containing characters whose ordinal value exceeds 255 results in an
+error. Source filters activated within the evaluated code apply to the
+code itself.
+
+This function is only available under the C<evalbytes> feature, a
+C<use v5.16> (or higher) declaration, or with a C<CORE::> prefix. See
+L<feature> for more information.
+
=item exec LIST
X<exec> X<execute>
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>
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>
+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 "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,
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>
$flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
or die "Can't set flags for the socket: $!\n";
+Portability issues: L<perlport/fcntl>.
+
+=item __FILE__
+X<__FILE__>
+
+A special token that returns the name of the file in which it occurs.
+
=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
+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> returns immediately rather than blocking
+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
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.
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 limitations that have to be considered in code intended to be portable.
+See L<perlfork> for more details.
+
+Portability issues: L<perlport/fork>.
+
=item format
X<format>
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>
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
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>
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>
Returns the current priority for a process, a process group, or a user.
-(See C<getpriority(2)>.) Will raise a fatal exception if used on a
+(See L<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>
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>,
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
$ip_address = inet_ntoa($packed_ip);
}
-Make sure <gethostbyname()> is called in SCALAR context and that
+Make sure C<gethostbyname()> is called in SCALAR context and that
its return value is checked for definedness.
+The C<getprotobynumber> function, even though it only takes one argument,
+has the precedence of a list operator, so beware:
+
+ getprotobynumber $number eq 'icmp' # WRONG
+ getprotobynumber($number eq 'icmp') # actually means this
+ getprotobynumber($number) eq 'icmp' # better this way
+
If you get tired of remembering which element of the return list
contains which return value, by-name interfaces are provided
in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
use User::pwent;
$is_his = (stat($filename)->uid == pwent($whoever)->uid);
-Even though it looks like they're the same method calls (uid),
+Even though it looks as though they're the same method calls (uid),
they aren't, because a C<File::stat> object is different from
a C<User::pwent> object.
+Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
+
=item getsockname SOCKET
X<getsockname>
The function returns a packed string representing the requested socket
option, or C<undef> on error, with the reason for the error placed in
-C<$!>). Just what is in the packed string depends on LEVEL and OPTNAME;
+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 you want to glob filenames that might contain whitespace, you'll
+have to use extra quotes around the spacey filename to protect it.
+For example, to glob filenames that have an C<e> followed by a space
+followed by an C<f>, use either of:
+
+ @spacies = <"*e f*">;
+ @spacies = glob '"*e f*"';
+ @spacies = glob q("*e f*");
+
+If you had to get a variable through, you could do this:
+
+ @spacies = glob "'*${var}e f*'";
+ @spacies = glob qq("*${var}e f*");
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, $isdst, the last value
+returned by gmtime, is always C<0>. There is no
Daylight Saving Time in GMT.
-See L<perlport/gmtime> for portability concerns.
+Portability issues: L<perlport/gmtime>.
=item goto LABEL
X<goto> X<jump> X<jmp>
goto ("FOO", "BAR", "GLARCH")[$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>
or after POSITION. If POSITION is omitted, starts searching from the
beginning of the string. POSITION before the beginning of the string
or after its end is treated as if it were the beginning or the end,
-respectively. POSITION and the return value are based at C<0> (or whatever
-you've set the C<$[> variable to--but don't do that). If the substring
-is not found, C<index> returns one less than the base, ordinarily C<-1>.
+respectively. POSITION and the return value are based at zero.
+If the substring is not found, C<index> returns -1.
=item int EXPR
X<int> X<integer> X<truncate> X<trunc> X<floor>
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.)
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.
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
+
+=item kill SIGNAL
X<kill> X<signal>
Sends a signal to a list of processes. Returns the number of
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 limitations related to kill that have to be considered,
+for code running on Windows and in code intended to be portable.
+
+See L<perlfork> for more details.
+
+If there is no I<LIST> of processes, no signal is sent, and the return
+value is 0. This form is sometimes used, however, because it causes
+tainting checks to be run. But see
+L<perlsec/Laundering and Detecting Tainted Data>.
+
+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
=back
-=item Otherwise, If EXPR has the UTF8 flag set
+=item Otherwise, if C<use locale> is in effect
-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>.)
-Otherwise Unicode semantics are used for the case change.
+Respects current LC_CTYPE locale for code points < 256; and uses Unicode
+semantics for the remaining code points (this last can only happen if
+the UTF8 flag is also set). See L<perllocale>.
-=item Otherwise, if C<use locale> is in effect
+A deficiency in this is that case changes that cross the 255/256
+boundary are not well-defined. For example, the lower case of LATIN CAPITAL
+LETTER SHARP S (U+1E9E) in Unicode semantics is U+00DF (on ASCII
+platforms). But under C<use locale>, the lower case of U+1E9E is
+itself, because 0xDF may not be LATIN SMALL LETTER SHARP S in the
+current locale, and Perl has no way of knowing if that character even
+exists in the locale, much less what code point it is. Perl returns
+the input character unchanged, for all instances (and there aren't
+many) where the 255/256 boundary would otherwise be crossed.
+
+=item Otherwise, If EXPR has the UTF8 flag set
-Respects current LC_CTYPE locale. See L<perllocale>.
+Unicode semantics are used for the case change.
=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.
+Unicode semantics are used for the case change.
=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<$_>. If EXPR is undefined, returns C<undef>.
+omitted, returns the length of C<$_>. 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
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 __LINE__
+X<__LINE__>
+
+A special token that compiles to the current line number.
+
=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>
($sec,$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.
+The value returned is the scalar itself, if the argument is a scalar, or a
+reference, if the argument is a hash, array or subroutine.
+
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. If you are not under C<use threads::shared> this does nothing.
=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<$_>. To get the
+log of another base, use basic algebra:
The base-N log of a number is equal to the natural log of that number
divided by the natural log of N. For example:
See also L</exp> for the inverse operation.
-=item lstat EXPR
+=item lstat FILEHANDLE
X<lstat>
+=item lstat EXPR
+
+=item lstat DIRHANDLE
+
=item lstat
Does the same thing as the C<stat> function (including setting the
If EXPR is omitted, stats C<$_>.
+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
-translates a list of numbers to the corresponding characters. And
+ my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;
+
+which makes the intention more clear.
+
+Map always returns a list, which can be
+assigned to a hash such that the elements
+become key/value pairs. See L<perldata> for more details.
%hash = map { get_a_key_for($_) => $_ } @array;
Creates the directory specified by FILENAME, with permissions
specified by MASK (as modified by C<umask>). If it succeeds it
-returns true, otherwise it returns false and sets C<$!> (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>,
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
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 filename 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 can't 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 two-argument (and one-argument) form of the call, the mode and
-filename should be concatenated (in that order), possibly separated by
-spaces. You may omit the mode in these forms when that 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
+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.
+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 form of pipe opens taking three or more arguments, if LIST is specified
(extra arguments after the command name) then LIST becomes arguments
defined, but experimental "layers" may give extra LIST arguments
meaning.
-In the two-argument (and one-argument) form, opening C<< '<-' >>
-or 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 I/O layers
-(sometimes referred to as "disciplines") to apply 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:
open(my $fh, "<:encoding(UTF-8)", "filename")
|| die "can't open UTF-8 encoded filename: $!";
-opens 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-argument form, then default layers stored in ${^OPEN} (see L<perlvar>;
usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
+Those layers will also be ignored if you specifying a colon with no name
+following it. In that case the default layer for the operating system
+(:raw on Unix, :crlf on Windows) is used.
Open returns nonzero on success, the undefined value otherwise. If
the C<open> involved a pipe, the return value happens to be the pid of
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 $tmp, "+>", undef) or die ...
-opens a filehandle to an anonymous temporary file. Also using "+<"
+opens a filehandle to an anonymous temporary file. Also using C<< +< >>
works for symmetry, but you really should consider writing something
to the temporary file first. You will need to seek() to do the
reading.
Since v5.8.0, Perl has built using PerlIO by default. Unless you've
-changed this (i.e., Configure -Uuseperlio), you can open filehandles
-directly to Perl scalars via:
+changed this (such as building Perl with C<Configure -Uuseperlio>), you can
+open filehandles directly to Perl scalars via:
- open($fh, '>', \$variable) || ..
+ open($fh, ">", \$variable) || ..
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: $!";
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
or die "Can't start sort: $!";
# in-memory files
- open(MEMORY,'>', \$var)
+ 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.)
+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 $child_pid;
+ } else {
+ # am the child; use STDIN/STDOUT normally
+ ...
+ exit;
+ }
+
The filehandle behaves normally for the parent, but I/O to that
filehandle is piped from/to the STDOUT/STDIN of the child process.
In the child process, the filehandle isn't opened--I/O happens from/to
pipe command gets executed, such as when running setuid and
you don't want to have to scan shell commands for metacharacters.
-The following triples are more or less equivalent:
+The following blocks are more or less equivalent:
open(FOO, "|tr '[a-z]' '[A-Z]'");
- open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
- open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
- open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
+ open(FOO, "|-", "tr '[a-z]' '[A-Z]'");
+ open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
+ open(FOO, "|-", "tr", '[a-z]', '[A-Z]');
open(FOO, "cat -n '$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:
+If you want a "real" C C<open> (see L<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:
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: $!";
+
See L</seek> for some details about mixing reading and writing.
+Portability issues: L<perlport/open>.
+
=item opendir DIRHANDLE,EXPR
X<opendir>
DIRHANDLE may be an expression whose value can be used as an indirect
dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
scalar variable (or array or hash element), the variable is assigned a
-reference to a new anonymous dirhandle.
+reference to a new anonymous dirhandle; that is, it's autovivified.
DIRHANDLEs have their own namespace separate from FILEHANDLEs.
-See example at C<readdir>.
+See the example at C<readdir>.
=item ord EXPR
X<ord> X<encoding>
=item ord
-Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
-or Unicode) value of the first character of EXPR. If EXPR is omitted,
-uses C<$_>.
+Returns the numeric 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.
A A text (ASCII) string, will be space 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 (ascending bit order inside each byte,
+ like vec()).
B A bit string (descending bit order inside each byte).
h A hex string (low nybble first).
H A hex string (high nybble first).
q A signed quad (64-bit) 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.
- Raises an exception otherwise.)
+ (Quads are available only if your system supports 64-bit
+ integer values _and_ if Perl has been compiled to support
+ those. Raises an exception otherwise.)
i A signed integer value.
I A unsigned integer value.
- (This 'integer' is _at_least_ 32 bits wide. Its exact
- size depends on what a local C compiler calls 'int'.)
+ (This 'integer' is _at_least_ 32 bits wide. Its exact
+ size depends on what a local C compiler calls 'int'.)
n An unsigned short (16-bit) in "network" (big-endian) order.
N An unsigned long (32-bit) in "network" (big-endian) order.
v An unsigned short (16-bit) in "VAX" (little-endian) order.
V An unsigned long (32-bit) in "VAX" (little-endian) order.
- j A Perl internal signed integer value (IV).
- J A Perl internal unsigned integer value (UV).
+ j A Perl internal signed integer value (IV).
+ J A Perl internal unsigned integer value (UV).
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 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.
- Raises an exception otherwise.)
+ (Long doubles are available only if your system supports
+ long double values _and_ if Perl has been compiled to
+ support those. Raises an exception otherwise.)
p A pointer to a null-terminated string.
P A pointer to a structure (fixed-length string).
u A uuencoded string.
- U A Unicode character number. Encodes to a character in character mode
- and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in byte mode.
+ U A Unicode character number. Encodes to a character in char-
+ acter mode 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.
+ 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.
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 the value.
+ . Null-fill or truncate to absolute position specified by
+ the value.
( Start of a ()-group.
One or more modifiers below may optionally follow certain letters in the
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
+something else, described below. Supplying a C<*> for the repeat count
instead of a number means to use however many items are left, except for:
=over
=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
+I<n>th innermost C<( )> group, or to the start of the string if I<n> is
bigger then the group level.
=back
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 needed. When
unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
-after the first null, and C<a> returns data without any sort of trimming.
+after the first null, and C<a> returns data with no stripping at all.
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
-for when the count is 0.
+when the count is 0.
=item *
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.
+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($char)%2>. In particular, characters C<"0">
If the input string is longer than needed, remaining characters are ignored.
A C<*> for the repeat count uses all characters of the input field.
-On unpacking, bits are converted to a string of C<"0">s and C<"1">s.
+On unpacking, bits are converted to a string of C<0>s and C<1>s.
=item *
The C<h> and C<H> formats pack a string that many nybbles (4-bit groups,
representable as hexadecimal digits, C<"0".."9"> C<"a".."f">) long.
-For each such format, pack() generates 4 bits of the result.
+For each such format, pack() generates 4 bits of result.
With non-alphabetical characters, the result is based on the 4 least-significant
bits of the input character, i.e., on C<ord($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>. Do not use any characters
-but these with this format.
+C<"A"> both generate the nybble C<0xA==10>. Use only these specific hex
+characters with this format.
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
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", ".")
+ This code: gives this result:
+
+ unpack("W/a", "\004Gurusamy") ("Guru")
+ unpack("a3/A A*", "007 Bond J ") (" Bond", "J")
+ unpack("a3 x2 /A A*", "007: Bond, J.") ("Bond, J", ".")
- pack("n/a* w/a","hello,","world") gives "\000\006hello,\005world"
- pack("a/W2", ord("a") .. ord("z")) gives "2ab"
+ pack("n/a* w/a","hello,","world") "\000\006hello,\005world"
+ pack("a/W2", ord("a") .. ord("z")) "2ab"
The I<length-item> is not returned explicitly from C<unpack>.
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 used/uses them in
-little-endian mode, but SGI/Cray uses them in big-endian mode.
+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 I<big-endian> and I<little-endian> are comic references to the
egg-eating habits of the little-endian Lilliputians and the big-endian
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,
+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 endianness modifier:
+Here are some concerns to keep in mind when using an endianness modifier:
=over
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,
=item *
-If TEMPLATE requires more arguments that pack() is given, pack()
+If TEMPLATE requires more arguments than pack() is given, pack()
assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
than given, extra arguments are ignored.
$foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
# same thing with Unicode circled letters.
$foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
- # same thing with Unicode circled letters. You don't get the UTF-8
- # bytes because the U at the start of the format caused a switch to
- # U0-mode, so the UTF-8 bytes get joined into characters
+ # same thing with Unicode circled letters. You don't get the
+ # UTF-8 bytes because the U at the start of the format caused
+ # a switch to U0-mode, so the UTF-8 bytes get joined into
+ # characters
$foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
# foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
- # This is the UTF-8 encoding of the string in the previous example
+ # This is the UTF-8 encoding of the string in the
+ # previous example
$foo = pack("ccxxcc",65,66,67,68);
# foo eq "AB\0\0CD"
# $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>
-=item package NAMESPACE BLOCK
-
-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
+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 the enclosing block, file, or eval (the same
-as the C<my> operator). All unqualified dynamic identifiers in this
-scope will be in the given namespace, except where overridden by another
-C<package> 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> (or C<our> (or C<state>)). Typically it would be the first
+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
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, 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 __PACKAGE__
+X<__PACKAGE__>
+
+A special token that returns the name of the package in which it occurs.
+
=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
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 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 standard output by default, or
-to the last selected output channel; see L</select>. If LIST is
-also omitted, prints C<$_> to the currently selected output handle.
-To set the default output handle to something other than STDOUT
-use the select operation. The current value of C<$,> (if any) is
-printed between each LIST item. The current value of C<$\> (if
-any) is printed after the entire LIST has been printed. Because
-print takes a LIST, anything in the LIST is evaluated in list
-context, and any subroutine that you call will have one or more of
-its expressions evaluated in list context. Also be careful not to
-follow the print keyword with a left parenthesis unless you want
-the corresponding right parenthesis to terminate the arguments to
-the print; put parentheses around all the arguments
+FILEHANDLE may be a scalar variable containing the name of or a reference
+to the filehandle, thus introducing one level of indirection. (NOTE: If
+FILEHANDLE is a variable and the next token is a term, it may be
+misinterpreted as an operator unless you interpose a C<+> or put
+parentheses around the arguments.) If FILEHANDLE is omitted, prints to the
+last selected (see L</select>) output handle. If LIST is omitted, prints
+C<$_> 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).
-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:
+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
+(the output record separator) is not appended. The first argument of the
+list will be interpreted as the C<printf> format. See
+L<sprintf|/sprintf FORMAT, LIST> 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. See L<perllocale> and L<POSIX>.
+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>
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<Data::Entropy>, L<Crypt::Random>, L<Math::Random::Secure>,
+and L<Math::TrulyRandom>.
+
=item read FILEHANDLE,SCALAR,LENGTH,OFFSET
X<read> X<file, read>
bytes before the result of the read is appended.
The call is implemented in terms of either Perl's or your system's native
-fread(3) library function. To get a true read(2) system call, see C<sysread>.
+fread(3) library function. To get a true read(2) system call, see
+L<sysread|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>.
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.
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"
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>
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 Perl code
+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.
+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
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<$_>.
Again, return 1 for each valid line, and 0 after all lines have been
returned.
If an empty list, C<undef>, or nothing that matches the first 3 values above
is returned, then C<require> looks at the remaining elements of @INC.
Note that this filehandle must be a real filehandle (strictly a typeglob
-or reference to a typeglob, blessed or unblessed); tied filehandles will be
-ignored and return value processing will stop there.
+or reference to a typeglob, whether blessed or unblessed); tied filehandles
+will be ignored and processing will stop there.
If the hook is an array reference, its first element must be a subroutine
reference. This subroutine is called as above, but the first parameter is
Returns from a subroutine, C<eval>, or C<do FILE> with the value
given in EXPR. Evaluation of EXPR may be in list, scalar, or void
context, depending on how the return value will be used, and the context
-may vary from one execution to the next (see C<wantarray>). If no EXPR
+may vary from one execution to the next (see L</wantarray>). If no EXPR
is given, returns an empty list in list context, the undefined value in
scalar context, and (of course) nothing at all in void context.
Sets the current position to the beginning of the directory for the
C<readdir> routine on DIRHANDLE.
+Portability issues: L<perlport/rewinddir>.
+
=item rindex STR,SUBSTR,POSITION
X<rindex>
=item rmdir
Deletes the directory specified by FILENAME if that directory is
-empty. If it succeeds it returns true, otherwise it returns false and
+empty. If it succeeds it returns true; otherwise it returns false and
sets C<$!> (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 available only when the "say" feature is
-enabled: see L<feature>.
+This keyword is available only when the C<"say"> feature
+is enabled, or when prefixed with C<CORE::>; 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 a 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
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>
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 | $win;
If you want to select on many filehandles, you may wish to write a
subroutine like this:
sub fhbits {
- my(@fhlist) = split(' ',$_[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');
+ $rin = fhbits(*STDIN, *TTY, *MYSOCK);
The usual idiom is:
is implementation-dependent. See also L<perlport> for notes on the
portability of C<select>.
-On error, C<select> behaves like select(2): it returns
+On error, C<select> behaves just like select(2): it returns
-1 and sets C<$!>.
-On some Unixes, select(2) may report a socket file
-descriptor as "ready for reading" when no data is available, and
-thus a subsequent read blocks. This can be avoided if you always use
-O_NONBLOCK 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>
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(2). Returns the semaphore id, or
-the undefined value if there is an error. See also
+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", $semnum, -1, 0);
L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
documentation.
+Portability issues: L<perlport/semop>.
+
=item send SOCKET,MSG,FLAGS,TO
X<send>
accept any arguments, so only C<setpgrp(0,0)> is portable. See also
C<POSIX::setsid()>.
+Portability issues: L<perlport/setpgrp>.
+
=item setpriority WHICH,WHO,PRIORITY
X<setpriority> X<priority> X<nice> X<renice>
(See setpriority(2).) Raises an exception when used on a machine
that doesn't implement setpriority(2).
+Portability issues: L<perlport/setpriority>.
+
=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
X<setsockopt>
-Sets the socket option requested. Returns undefined if there is an
-error. Use integer constants provided by the C<Socket> module for
+Sets the socket option requested. Returns C<undef> on error.
+Use integer constants provided by the C<Socket> module for
LEVEL and OPNAME. Values for LEVEL can also be obtained from
getprotobyname. OPTVAL might either be a packed string or an integer.
An integer OPTVAL is shorthand for pack("i", OPTVAL).
use Socket qw(IPPROTO_TCP TCP_NODELAY);
setsockopt($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>
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>
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.
+
+If the subroutine is an XSUB, the elements to be compared are pushed on to
+the stack, the way arguments are usually passed to XSUBs. $a and $b are
+not set.
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
# sort using explicit subroutine name
sub byage {
- $age{$a} <=> $age{$b}; # presuming numeric
+ $age{$a} <=> $age{$b}; # presuming numeric
}
@sortedclass = sort byage @class;
my @new = sort {
($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
- ||
- uc($a) cmp uc($b)
+ ||
+ uc($a) cmp uc($b)
} @old;
# same thing, but much more efficiently;
}
my @new = @old[ sort {
- $nums[$b] <=> $nums[$a]
- ||
- $caps[$a] cmp $caps[$b]
- } 0..$#old
- ];
+ $nums[$b] <=> $nums[$a]
+ ||
+ $caps[$a] cmp $caps[$b]
+ } 0..$#old
+ ];
# same thing, but without any temps
@new = map { $_->[0] }
sort { $b->[1] <=> $a->[1]
- ||
- $a->[2] cmp $b->[2]
- } map { [$_, /=(\d+)/, uc($_)] } @old;
+ ||
+ $a->[2] cmp $b->[2]
+ } map { [$_, /=(\d+)/, uc($_)] } @old;
# using a prototype allows you to use any comparison subroutine
# as a sort subroutine (including other package's subroutines)
well-defined.
Because C<< <=> >> returns C<undef> when either operand is C<NaN>
-(not-a-number), and because C<sort> raises an exception 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
+(not-a-number), 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,
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 >> )
+The following equivalences hold (assuming C<< $#a >= $i >> )
push(@a,$x,$y) splice(@a,@a,0,$x,$y)
pop(@a) splice(@a,-1)
}
if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
+Starting with Perl 5.14, C<splice> can take scalar EXPR, which must hold a
+reference to an unblessed array. The argument will be dereferenced
+automatically. This aspect of C<splice> is considered highly experimental.
+The exact behaviour may change in a future version of Perl.
+
=item split /PATTERN/,EXPR,LIMIT
X<split>
A pattern matching the empty string (not to be confused with
an empty pattern C<//>, which is just one member of the set of patterns
-matching the epmty string), splits EXPR into individual
+matching the empty string), splits EXPR into individual
characters. For example:
print join(':', split(/ */, 'hi there')), "\n";
Returns a string formatted by the usual C<printf> conventions of the C
library function C<sprintf>. See below for more details
-and see
C<sprintf(3)> or C<printf(3)> on your system for an explanation of
+and see
L<sprintf(3)> or L<printf(3)> on your system for an explanation of
the general principles.
For example:
%B like %b, but using an upper-case "B" with the # flag
%p a pointer (outputs the Perl value's address in hexadecimal)
%n special: *stores* the number of characters output so far
- into the next variable in the parameter list
+ into the next argument in the parameter list
Finally, for backward (and we do mean "backward") compatibility, Perl
permits these unnecessary but widely-supported conversions:
a number here, or get the width from the next argument (with C<*>)
or from a specified argument (e.g., with C<*2$>):
- 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
+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:
printf '<%.1e>', 10; # prints "<1.0e+01>"
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
+including those prior to the decimal point and those after it; for
example:
# These examples are subject to system-specific variation.
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
+
+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";
-The last will raise an exception if Perl does not understand "quads" in your
-installation. (This requires either that the platform natively support quads,
-or that Perl were specifically compiled to support quads.) You can find out
-whether your Perl supports quads via L<Config>:
+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;
if ($Config{use64bitint} eq "define" || $Config{longsize} >= 8) {
=item srand
-Sets the random number seed for the C<rand> operator.
+Sets and returns the random number seed for the C<rand> operator.
The point of the function is to "seed" the C<rand> function so that
C<rand> can produce a different sequence each time you run your
-program.
-
-If srand() is not called explicitly, it is called implicitly at the
-first use of the C<rand> operator. However, this was not true of
-versions of Perl before 5.004, so if your script will run under older
-Perl versions, it should call C<srand>.
-
-Most programs won't even call srand() at all, except those that
-need a cryptographically-strong starting point rather than the
-generally acceptable default, which is based on time of day,
-process ID, and memory allocation, or the F</dev/urandom> device
-if available.
-
-You can call srand($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 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
programs supply their own seed value (often C<time ^ $$> or C<time ^
($$ + ($$ << 15))>), but that isn't necessary any more.
-For cryptographic purposes, however, you need something much more random
-than the default seed. Checksumming the compressed output of one or more
-rapidly changing operating system status programs is the usual method. For
-example:
-
- srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip -f`);
-
-If you're particularly concerned with this, see the C<Math::TrulyRandom>
-module in CPAN.
-
Frequently called programs (like CGI scripts) that simply use
time ^ $$
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.
+
+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<Data::Entropy>, L<Crypt::Random>, L<Math::Random::Secure>,
+and L<Math::TrulyRandom>.
+
=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<$_>. Returns the empty list if C<stat> fails. Typically
+omitted, it stats C<$_> (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
S_IRWXO S_IROTH S_IWOTH S_IXOTH
# Setuid/Setgid/Stickiness/SaveText.
- # Note that the exact meaning of these is system dependent.
+ # Note that the exact meaning of these is system-dependent.
S_ISUID S_ISGID S_ISVTX S_ISTXT
- # File types. Not necessarily all are available on your system.
+ # File types. Not all are necessarily available on
+ # your system.
- S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
+ S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR
+ S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
- # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
+ # The following are compatibility aliases for S_IRUSR,
+ # S_IWUSR, and S_IXUSR.
S_IREAD S_IWRITE S_IEXEC
and the C<S_IF*> functions are
- S_IMODE($mode) the part of $mode containing the permission bits
- and the setuid/setgid/sticky bits
+ S_IMODE($mode) the part of $mode containing the permission
+ bits and the setuid/setgid/sticky bits
- S_IFMT($mode) the part of $mode containing the file type
- which can be bit-anded with (for example) S_IFREG
- or with the following functions
+ S_IFMT($mode) the part of $mode containing the file type
+ which can be bit-anded with (for example)
+ S_IFREG or with the following functions
# The operators -f, -d, -l, -b, -c, -p, and -S.
about the C<S_*> constants. To get status info for a symbolic link
instead of the target file behind the link, use the C<lstat> function.
+Portability issues: L<perlport/stat>.
+
=item state EXPR
X<state>
=item state TYPE EXPR : ATTRS
-C<state> declares a lexically scoped variable, just like C<my> does.
+C<state> declares a lexically scoped variable, just like C<my>.
However, those variables will never be reinitialized, contrary to
lexical variables that are reinitialized each time their enclosing block
is entered.
+See L<perlsub/"Persistent Private Variables"> for details.
C<state> variables are enabled only when the C<use feature "state"> pragma
-is in effect. See L<feature>.
+is in effect, unless the keyword is written as C<CORE::state>.
+See also L<feature>.
=item study SCALAR
X<study>
Takes extra time to study SCALAR (C<$_> 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
+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 runs faster. Those loops
+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
-is "unstudied". (The way C<study> works is this: a linked list of every
+parts of more complex patterns) will benefit most.
+(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,
the rarest character is selected, based on some static frequency tables
=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 __SUB__
+X<__SUB__>
+
+A special token that returns the a reference to the current subroutine, or
+C<undef> outside of a subroutine.
+
+This token is only available under C<use v5.16> or the "current_sub"
+feature. See L<feature>.
+
=item substr EXPR,OFFSET,LENGTH,REPLACEMENT
X<substr> X<substring> X<mid> X<left> X<right>
=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<$[> to (but don't do that).
-If OFFSET is negative (or more precisely, less than C<$[>), 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
+offset zero. If OFFSET is negative, starts
+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";
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:
$_ = 'pq'; print $x,"\n"; # prints 5pq9
}
-Prior to Perl version 5.9.1, the result of using an lvalue multiple times was
+With negative offsets, it remembers its position from the end of the string
+when the target string is modified:
+
+ $x = '1234';
+ for (substr($x, -3, 2)) {
+ $_ = 'a'; print $x,"\n"; # prints 1a4, as above
+ $x = 'abcdefg';
+ print $_,"\n"; # prints f
+ }
+
+Prior to Perl version 5.10, the result of using an lvalue multiple times was
+unspecified. Prior to 5.16, the result with negative offsets was
unspecified.
=item symlink OLDFILE,NEWFILE
$symlink_exists = eval { symlink("",""); 1 };
+Portability issues: L<perlport/symlink>.
+
=item syscall NUMBER, LIST
X<syscall> X<system call>
Syscall returns whatever value returned by the system call it calls.
If the system call fails, C<syscall> returns C<-1> and sets C<$!> (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
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 sysseek FILEHANDLE,POSITION,WHENCE
X<sysseek> X<lseek>
-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).
+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 $! for the reason).
results and return codes are subject to its quirks.
See L<perlop/"`STRING`"> and L</exec> for details.
+Since C<system> does a C<fork> and C<wait> it may affect a C<SIGCHLD>
+handler. See L<perlipc> for details.
+
+Portability issues: L<perlport/system>.
+
=item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
X<syswrite>
not specified, writes whole SCALAR. It bypasses buffered IO, so
mixing this with reads (other than C<sysread())>, C<print>, C<write>,
C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
-stdio layers usually buffers data. Returns the number of bytes
+stdio layers usually buffer data. Returns the number of bytes
actually written, or C<undef> if there was an error (in this case the
errno variable C<$!> is also set). If the LENGTH is greater than the
data available in the SCALAR after the OFFSET, only as much data as is
that many characters counting backwards from the end of the string.
If SCALAR is of length zero, you can only use an OFFSET of 0.
-B<Warning>: If the filehandle is marked C<:utf8>, 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 (UTF-8 encoded Unicode) characters.
+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 filehandle
-that has been manipulated by sysread(), syswrite() or sysseek().
+that has been manipulated by sysread(), syswrite(), or sysseek().
Those functions ignore the buffering, while tell() does not.
=item telldir DIRHANDLE
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;
Children's times are only included for terminated children.
+Portability issues: L<perlport/times>.
+
=item tr///
The transliteration operator. Same as C<y///>. See
Truncates the file opened on FILEHANDLE, or named by EXPR, to the
specified length. Raises an exception if truncate isn't implemented
-on your system. Returns true if successful, the undefined value
-otherwise.
+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>
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
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>
scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
(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
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:
=item untie VARIABLE
X<untie>
-Breaks the binding between a variable and a package. (See C<tie>.)
+Breaks the binding between a variable and a package.
+(See L<tie|/tie VARIABLE,CLASSNAME,LIST>.)
Has no effect if the variable is not tied.
=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 $ARGV[0] =~ /^-/;
prepended elements stay in the same order. Use C<reverse> to do the
reverse.
+Starting with Perl 5.14, C<unshift> can take a scalar EXPR, which must hold
+a reference to an unblessed array. The argument will be dereferenced
+automatically. This aspect of C<unshift> is considered highly
+experimental. The exact behaviour may change in a future version of Perl.
+
=item use Module VERSION LIST
X<use> X<module> X<import>
BEGIN { require Module; Module->import( LIST ); }
except that Module I<must> be a bareword.
+The importation can be made conditional; see L<if>.
In the peculiar C<use VERSION> form, VERSION may be either a positive
decimal fraction such as 5.006, which will be compared to C<$]>, or a v-string
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,
-C<use VERSION> will also load the C<feature> pragma and enable all
-features available in the requested version. See L<feature>.
+C<use VERSION> also enables all features available in the requested
+version as defined by the C<feature> pragma, disabling any features
+not in the current version's feature bundle. See L<feature>.
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).
+5.11.0, strictures are enabled lexically as
+with C<use strict>. Any explicit use of
+C<use strict> or C<no strict> overrides C<use VERSION>, even if it comes
+before it. In both cases, the F<feature.pm> and F<strict.pm> files are
+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
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>
functionality from the command-line.
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
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
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
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>
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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