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
-argument, while a list operator may provide either scalar and list
+argument, while a list operator may provide either scalar or list
contexts for its arguments. If it does both, the scalar arguments will
be first, and the list argument will follow. (Note that there can ever
-be only one list argument.) For instance, splice() has three scalar
-arguments followed by a list.
+be only one such list argument.) For instance, splice() has three scalar
+arguments followed by a list, whereas gethostbyname() has four scalar
+arguments.
In the syntax descriptions that follow, list operators that expect a
list (and provide list context for the elements of the list) are shown
Any function in the list below may be used either with or without
parentheses around its arguments. (The syntax descriptions omit the
parentheses.) If you use the parentheses, the simple (but occasionally
-surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a
+surprising) rule is this: It I<looks> like a function, therefore it I<is> a
function, and precedence doesn't matter. Otherwise it's a list
operator or unary operator, and precedence does matter. And whitespace
between the function and left parenthesis doesn't count--so you need to
print (...) interpreted as function at - line 1.
Useless use of integer addition in void context at - line 1.
+A few functions take no arguments at all, and therefore work as neither
+unary nor list operators. These include such functions as C<time>
+and C<endpwent>. For example, C<time+86_400> always means
+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
the behavior of an expression in list context to its behavior in scalar
context, or vice versa. It might do two totally different things.
Each operator and function decides which sort of value it would be most
-appropriate to return in a scalar context. Some operators return the
+appropriate to return in scalar context. Some operators return the
length of the list that would have been returned in list context. Some
operators return the first value in the list. Some operators return the
last value in the list. Some operators return a count of successful
of the same name (like chown(2), fork(2), closedir(2), etc.) all 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()>,
-C<waitpid()>, and C<syscall()>. System calls also set the special C<$!>
+which return C<-1> on failure. Exceptions to this rule are C<wait>,
+C<waitpid>, and C<syscall>. System calls also set the special C<$!>
variable on failure. Other functions do not, except accidentally.
=head2 Perl Functions by Category
=head2 Portability
-Perl was born in UNIX and therefore it can access all the common UNIX
-system calls. In non-UNIX environments the functionality of many
-UNIX system calls may not be available or the details of the available
-functionality may be slightly different. The Perl functions affected
+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
+functionality may differ slightly. The Perl functions affected
by this are:
C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
-C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<select>, C<semctl>,
+C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
-C<shmwrite>, C<socketpair>, C<stat>, C<symlink>, C<syscall>,
-C<sysopen>, C<system>, C<times>, C<truncate>, C<umask>, C<utime>,
-C<wait>, C<waitpid>
+C<shmwrite>, C<socket>, C<socketpair>, C<stat>, C<symlink>, C<syscall>,
+C<sysopen>, C<system>, C<times>, C<truncate>, C<umask>, C<unlink>,
+C<utime>, C<wait>, C<waitpid>
For more information about the portability of these functions, see
L<perlport> and other available platform-specific documentation.
operator takes one argument, either a filename or a filehandle, and
tests the associated file to see if something is true about it. If the
argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
-Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
+Unless otherwise documented, it returns C<1> for true and C<''> for false, or
the undefined value if the file doesn't exist. Despite the funny
names, precedence is the same as any other named unary operator, and
the argument may be parenthesized like any other unary operator. The
-g File has setgid bit set.
-k File has sticky bit set.
- -T File is a text file.
- -B File is a binary file (opposite of -T).
+ -T File is an ASCII text file.
+ -B File is a "binary" file (opposite of -T).
-M Age of file in days when script started.
-A Same for access time.
(access control lists), read-only filesystems, and unrecognized
executable formats.
-Also note that, for the superuser on the local filesystems, C<-r>,
-C<-R>, C<-w>, and C<-W> always return 1, and C<-x> and C<-X> return 1
+Also note that, for the superuser on the local filesystems, the C<-r>,
+C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
if any execute bit is set in the mode. Scripts run by the superuser
may thus need to do a stat() to determine the actual mode of the file,
-or temporarily set the uid to something else.
+or temporarily set their effective uid to something else.
If you are using ACLs, there is a pragma called C<filetest> that may
produce more accurate results than the bare stat() mode bits.
-
When under the C<use filetest 'access'> the above-mentioned filetests
will test whether the permission can (not) be granted using the
-access() family of system calls. Also note that the -x and -X may
+access() family of system calls. Also note that the C<-x> and C<-X> may
under this pragma return true even if there are no execute permission
bits set (nor any extra execute permission ACLs). This strangeness is
due to the underlying system calls' definitions. Read the
are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
containing null in the first block is considered a binary file. If C<-T>
or C<-B> is used on a filehandle, the current stdio buffer is examined
-rather than the first block. Both C<-T> and C<-B> return TRUE on a null
+rather than the first block. Both C<-T> and C<-B> return true on a null
file, or a file at EOF when testing a filehandle. Because you have to
read a file to do the C<-T> test, on most occasions you want to use a C<-f>
against the file first, as in C<next unless -f $file && -T $file>.
-If any of the file tests (or either the C<stat()> or C<lstat()> operators) are given
+If any of the file tests (or either the C<stat> or C<lstat> operators) are given
the special filehandle consisting of a solitary underline, then the stat
structure of the previous file test (or stat operator) is used, saving
a system call. (This doesn't work with C<-t>, and you need to remember
=item accept NEWSOCKET,GENERICSOCKET
Accepts an incoming socket connect, just as the accept(2) system call
-does. Returns the packed address if it succeeded, FALSE otherwise.
-See example in L<perlipc/"Sockets: Client/Server Communication">.
+does. Returns the packed address if it succeeded, false otherwise.
+See
the example in L<perlipc/"Sockets: Client/Server Communication">.
=item alarm SECONDS
on the previous timer.
For delays of finer granularity than one second, you may use Perl's
-C<syscall()> interface to access setitimer(2) if your system supports it,
-or else see L</select()>. It is usually a mistake to intermix C<alarm()>
-and C<sleep()> calls.
+four-argument version of select() leaving the first three arguments
+undefined, or you might be able to use the C<syscall> interface to
+access setitimer(2) if your system supports it. The Time::HiRes module
+from CPAN may also prove useful.
-If you want to use C<alarm()> to time out a system call you need to use an
-C<eval()>/C<die()> pair. You can't rely on the alarm causing the system call to
+It is usually a mistake to intermix C<alarm> and C<sleep> calls.
+(C<sleep> may be internally implemented in your system with C<alarm>)
+
+If you want to use C<alarm> to time out a system call you need to use an
+C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
-restart system calls on some systems. Using C<eval()>/C<die()> always works,
+restart system calls on some systems. Using C<eval>/C<die> always works,
modulo the caveats given in L<perlipc/"Signals">.
eval {
=item bind SOCKET,NAME
Binds a network address to a socket, just as the bind system call
-does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
+does. Returns true if it succeeded, false otherwise. NAME should be a
packed address of the appropriate type for the socket. See the examples in
L<perlipc/"Sockets: Client/Server Communication">.
=item binmode FILEHANDLE
-Arranges for the file to be read or written in "binary" mode in operating
-systems that distinguish between binary and text files. Files that are
-not in binary mode have CR LF sequences translated to LF on input and LF
-translated to CR LF on output. Binmode has no effect under Unix; in MS-DOS
-and similarly archaic systems, it may be imperative--otherwise your
-MS-DOS-damaged C library may mangle your file. The key distinction between
-systems that need C<binmode()> and those that don't is their text file
-formats. Systems like Unix, MacOS, and Plan9 that delimit lines with a single
-character, and that encode that character in C as C<"\n">, do not need
-C<binmode()>. The rest need it. If FILEHANDLE is an expression, the value
-is taken as the name of the filehandle.
+Arranges for FILEHANDLE to be read or written in "binary" mode on
+systems whose run-time libraries force the programmer to guess
+between binary and text files. If FILEHANDLE is an expression, the
+value is taken as the name of the filehandle. binmode() should be
+called after the C<open> but before any I/O is done on the filehandle.
+The only way to reset binary mode on a filehandle is to reopen the
+file.
+
+The operating system, device drivers, C libraries, and Perl run-time
+system all conspire to let the programmer conveniently treat a
+simple, one-byte C<\n> as the line terminator, irrespective of its
+external representation. On Unix and its brethren, the native file
+representation exactly matches the internal representation, making
+everyone's lives unbelievably simpler. Consequently, L<binmode>
+has no effect under Unix, Plan9, or Mac OS, all of which use C<\n>
+to end each line. (Unix and Plan9 think C<\n> means C<\cJ> and
+C<\r> means C<\cM>, whereas the Mac goes the other way--it uses
+C<\cM> for c<\n> and C<\cJ> to mean C<\r>. But that's ok, because
+it's only one byte, and the internal and external representations
+match.)
+
+In legacy systems like MS-DOS and its embellishments, your program
+sees a C<\n> as a simple C<\cJ> (just as in Unix), but oddly enough,
+that's not what's physically stored on disk. What's worse, these
+systems refuse to help you with this; it's up to you to remember
+what to do. And you mustn't go applying binmode() with wild abandon,
+either, because if your system does care about binmode(), then using
+it when you shouldn't is just as perilous as failing to use it when
+you should.
+
+That means that on any version of Microsoft WinXX that you might
+care to name (or not), binmode() causes C<\cM\cJ> sequences on disk
+to be converted to C<\n> when read into your program, and causes
+any C<\n> in your program to be converted back to C<\cM\cJ> on
+output to disk. This sad discrepancy leads to no end of
+problems in not just the readline operator, but also when using
+seek(), tell(), and read() calls. See L<perlport> for other painful
+details. See the C<$/> and C<$\> variables in L<perlvar> for how
+to manually set your input and output line-termination sequences.
=item bless REF,CLASSNAME
=item bless REF
-This function tells the thingy referenced by REF that it is now
-an object in the CLASSNAME package--or the current package if no CLASSNAME
-is specified, which is often the case. It returns the reference for
-convenience, because a C<bless()> is often the last thing in a constructor.
-Always use the two-argument version if the function doing the blessing
-might be inherited by a derived class. See L<perltoot> and L<perlobj>
-for more about the blessing (and blessings) of objects.
+This function tells the thingy referenced by REF that it is now an object
+in the CLASSNAME package. If CLASSNAME is omitted, the current package
+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 the function doing the blessing might be inherited by a
+derived class. See L<perltoot> and L<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
+Perl pragmata. Builtin types have all uppercase names, so to prevent
+confusion, you may wish to avoid such package names as well. Make sure
+that CLASSNAME is a true value.
+
+See L<perlmod/"Perl Modules">.
=item caller EXPR
Returns the context of the current subroutine call. In scalar context,
returns the caller's package name if there is a caller, that is, if
-we're in a subroutine or C<eval()> or C<require()>, and the undefined value
+we're in a subroutine or C<eval> or C<require>, and the undefined value
otherwise. In list context, returns
($package, $filename, $line) = caller;
print a stack trace. The value of EXPR indicates how many call frames
to go back before the current one.
- ($package, $filename, $line, $subroutine,
- $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
+ ($package, $filename, $line, $subroutine, $hasargs,
+ $wantarray, $evaltext, $is_require, $hints) = caller($i);
-Here C<$subroutine> may be C<"(eval)"> if the frame is not a subroutine
-call, but an C<eval()>. In such a case additional elements C<$evaltext> and
+Here $subroutine may be C<"(eval)"> if the frame is not a subroutine
+call, but an C<eval>. In such a case additional elements $evaltext and
C<$is_require> are set: C<$is_require> is true if the frame is created by a
-C<require> or C<use> statement, C<$evaltext> contains the text of the
+C<require> or C<use> statement, $evaltext contains the text of the
C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
-C<$filename> is C<"(eval)">, but C<$evaltext> is undefined. (Note also that
+$filename is C<"(eval)">, but $evaltext is undefined. (Note also that
each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
-frame.
+frame. C<$hints> contains pragmatic hints that the caller was
+compiled with. It currently only reflects the hint corresponding to
+C<use utf8>.
Furthermore, when called from within the DB package, caller returns more
detailed information: it sets the list variable C<@DB::args> to be the
arguments with which the subroutine was invoked.
Be aware that the optimizer might have optimized call frames away before
-C<caller()> had a chance to get the information. That means that C<caller(N)>
+C<caller> had a chance to get the information. That means that C<caller(N)>
might not return information about the call frame you expect it do, for
-C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
-previous time C<caller()> was called.
+C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
+previous time C<caller> was called.
=item chdir EXPR
-Changes the working directory to EXPR, if possible. If EXPR is
-omitted, changes to home directory. Returns TRUE upon success, FALSE
-otherwise. See example under C<die()>.
+Changes the working directory to EXPR, if possible. If EXPR is omitted,
+changes to the user's home directory. Returns true upon success,
+false otherwise. See the example under C<die>.
=item chmod LIST
=item chomp
-This is a slightly safer version of L</chop>. It removes any
-line ending that corresponds to the current value of C<$/> (also known as
+This safer version of L</chop> removes any trailing string
+that corresponds to the current value of C<$/> (also known as
$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
number of characters removed from all its arguments. It's often used to
remove the newline from the end of an input record when you're worried
-that the final record may be missing its newline. When in paragraph mode
-(C<$/ = "">), it removes all trailing newlines from the string. If
-VARIABLE is omitted, it chomps C<$_>. Example:
+that the final record may be missing its newline. When in paragraph
+mode (C<$/ = "">), 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
+remove anything.
+If VARIABLE is omitted, it chomps C<$_>. Example:
while (<>) {
chomp; # avoid \n on last field
chop($answer = <STDIN>);
If you chop a list, each element is chopped. Only the value of the
-last C<chop()> is returned.
+last C<chop> is returned.
-Note that C<chop()> returns the last character. To return all but the last
+Note that C<chop> returns the last character. To return all but the last
character, use C<substr($string, 0, -1)>.
=item chown LIST
Changes the owner (and group) of a list of files. The first two
-elements of the list must be the I<NUMERICAL> uid and gid, in that order.
-Returns the number of files successfully changed.
+elements of the list must be the I<numeric> uid and gid, in that
+order. A value of -1 in either position is interpreted by most
+systems to leave that value unchanged. Returns the number of files
+successfully changed.
$cnt = chown $uid, $gid, 'foo', 'bar';
chown $uid, $gid, @filenames;
Here's an example that looks up nonnumeric uids in the passwd file:
print "User: ";
- chop($user = <STDIN>);
+ chomp($user = <STDIN>);
print "Files: ";
- chop($pattern = <STDIN>);
+ chomp($pattern = <STDIN>);
($login,$pass,$uid,$gid) = getpwnam($user)
or die "$user not in passwd file";
file unless you're the superuser, although you should be able to change
the group to any of your secondary groups. On insecure systems, these
restrictions may be relaxed, but this is not a portable assumption.
+On POSIX systems, you can detect this condition this way:
+
+ use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
+ $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
=item chr NUMBER
Returns the character represented by that NUMBER in the character set.
For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
-chr(0x263a) is a Unicode smiley face (but only within the scope of a
-C<use utf8>). For the reverse, use L</ord>.
+chr(0x263a) is a Unicode smiley face (but only within the scope of
+a C<use utf8>). For the reverse, use L</ord>.
+See L<utf8> for more about Unicode.
If NUMBER is omitted, uses C<$_>.
begin with a C<"/"> by your process and all its children. (It doesn't
change your current working directory, which is unaffected.) For security
reasons, this call is restricted to the superuser. If FILENAME is
-omitted, does a C<chroot()> to C<$_>.
+omitted, does a C<chroot> to C<$_>.
=item close FILEHANDLE
=item close
-Closes the file or pipe associated with the file handle, returning TRUE
+Closes the file or pipe associated with the file handle, returning true
only if stdio successfully flushes buffers and closes the system file
-descriptor. Closes the currently selected filehandle if the argument
+descriptor. Closes the currently selected filehandle if the argument
is omitted.
You don't have to close FILEHANDLE if you are immediately going to do
-another C<open()> on it, because C<open()> will close it for you. (See
-C<open()>.) However, an explicit C<close()> on an input file resets the line
-counter (C<$.>), while the implicit close done by C<open()> does not.
+another C<open> on it, because C<open> will close it for you. (See
+C<open>.) However, an explicit C<close> on an input file resets the line
+counter (C<$.>), while the implicit close done by C<open> does not.
-If the file handle came from a piped open C<close()> will additionally
-return FALSE if one of the other system calls involved fails or if the
+If the file handle came from a piped open C<close> will additionally
+return false if one of the other system calls involved fails or if the
program exits with non-zero status. (If the only problem was that the
-program exited non-zero C<$!> will be set to C<0>.) Also, closing a pipe
-waits for the process executing on the pipe to complete, in case you
-want to look at the output of the pipe afterwards. Closing a pipe
-explicitly also puts the exit status value of the command into C<$?>.
+program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
+also waits for the process executing on the pipe to complete, in case you
+want to look at the output of the pipe afterwards, and
+implicitly puts the exit status value of that command into C<$?>.
+
+Prematurely closing the read end of a pipe (i.e. before the process
+writing to it at the other end has closed it) will result in a
+SIGPIPE being delivered to the writer. If the other end can't
+handle that, be sure to read all the data before closing the pipe.
Example:
=item closedir DIRHANDLE
-Closes a directory opened by C<opendir()> and returns the success of that
+Closes a directory opened by C<opendir> and returns the success of that
system call.
DIRHANDLE may be an expression whose value can be used as an indirect
=item connect SOCKET,NAME
Attempts to connect to a remote socket, just as the connect system call
-does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
+does. Returns true if it succeeded, false otherwise. NAME should be a
packed address of the appropriate type for the socket. See the examples in
L<perlipc/"Sockets: Client/Server Communication">.
statement).
C<last>, C<next>, or C<redo> may appear within a C<continue>
-block. C<last> and C<redo> will behave as if they had been executed within
-the main block. So will C<next>, but since it will execute a C<continue>
+block. C<last> and C<redo> will behave as if they had been executed within
+the main block. So will C<next>, but since it will execute a C<continue>
block, it may be more entertaining.
while (EXPR) {
### last always comes here
Omitting the C<continue> section is semantically equivalent to using an
-empty one, logically enough. In that case, C<next> goes directly back
+empty one, logically enough. In that case, C<next> goes directly back
to check the condition at the top of the loop.
=item cos EXPR
the password file for lousy passwords, amongst other things. Only the
guys wearing white hats should do this.
-Note that C<crypt()> is intended to be a one-way function, much like breaking
+Note that C<crypt> is intended to be a one-way function, much like breaking
eggs to make an omelette. There is no (known) corresponding decrypt
function. As a result, this function isn't all that useful for
cryptography. (For that, see your nearby CPAN mirror.)
When verifying an existing encrypted string you should use the encrypted
text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
-allows your code to work with the standard C<crypt()> and with more
+allows your code to work with the standard C<crypt> and with more
exotic implementations. When choosing a new salt create a random two
character string whose characters come from the set C<[./0-9A-Za-z]>
(like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
Of course, typing in your own password to whoever asks you
for it is unwise.
+The L<crypt> function is unsuitable for encrypting large quantities
+of data, not least of all because you can't get the information
+back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
+on your favorite CPAN mirror for a slew of potentially useful
+modules.
+
=item dbmclose HASH
-[This function has been superseded by the C<untie()> function.]
+[This function has been largely superseded by the C<untie> function.]
Breaks the binding between a DBM file and a hash.
-=item dbmopen HASH,DBNAME,MODE
+=item dbmopen HASH,DBNAME,MASK
-[This function has been superseded by the C<tie()> function.]
+[This function has been largely superseded by the C<tie> 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
-argument is I<NOT> a filehandle, even though it looks like one). DBNAME
+hash. HASH is the name of the hash. (Unlike normal C<open>, the first
+argument is I<not> a filehandle, even though it looks like one). DBNAME
is the name of the database (without the F<.dir> or F<.pag> extension if
any). If the database does not exist, it is created with protection
-specified by MODE (as modified by the C<umask()>). If your system supports
-only the older DBM functions, you may perform only one C<dbmopen()> in your
+specified by MASK (as modified by the C<umask>). If your system supports
+only the older DBM functions, you may perform only one C<dbmopen> in your
program. In older versions of Perl, if your system had neither DBM nor
-ndbm, calling C<dbmopen()> produced a fatal error; it now falls back to
+ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
sdbm(3).
If you don't have write access to the DBM file, you can only read hash
variables, not set them. If you want to test whether you can write,
-either use file tests or try setting a dummy hash entry inside an C<eval()>,
+either use file tests or try setting a dummy hash entry inside an C<eval>,
which will trap the error.
-Note that functions such as C<keys()> and C<values()> may return huge lists
-when used on large DBM files. You may prefer to use the C<each()>
+Note that functions such as C<keys> and C<values> may return huge lists
+when used on large DBM files. You may prefer to use the C<each>
function to iterate over large DBM files. Example:
# print out history file offsets
cons of the various dbm approaches, as well as L<DB_File> for a particularly
rich implementation.
+You can control which DBM library you use by loading that library
+before you call dbmopen():
+
+ use DB_File;
+ dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
+ or die "Can't open netscape history file: $!";
+
=item defined EXPR
=item defined
other values. (A simple Boolean test will not distinguish among
C<undef>, zero, the empty string, and C<"0">, which are all equally
false.) Note that since C<undef> is a valid scalar, its presence
-doesn't I<necessarily> indicate an exceptional condition: C<pop()>
+doesn't I<necessarily> indicate an exceptional condition: C<pop>
returns C<undef> when its argument is an empty array, I<or> when the
element to return happens to be C<undef>.
-You may also use C<defined()> to check whether a subroutine exists, by
-saying C<defined &func> without parentheses. On the other hand, use
-of C<defined()> upon aggregates (hashes and arrays) is not guaranteed to
-produce intuitive results, and should probably be avoided.
+You may also use C<defined(&func)> to check whether subroutine C<&func>
+has ever been defined. The return value is unaffected by any forward
+declarations of C<&foo>.
+
+Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
+used to report whether memory for that aggregate has ever been
+allocated. This behavior may disappear in future versions of Perl.
+You should instead use a simple test for size:
+
+ if (@an_array) { print "has array elements\n" }
+ if (%a_hash) { print "has hash members\n" }
When used on a hash element, it tells you whether the value is defined,
not whether the key exists in the hash. Use L</exists> for the latter
sub foo { defined &$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 then are surprised to
discover that the number C<0> and C<""> (the zero-length string) are, in fact,
defined values. For example, if you say
The pattern match succeeds, and C<$1> is defined, despite the fact that it
matched "nothing". But it didn't really match nothing--rather, it
-matched something that happened to be C<0> characters long. This is all
+matched something that happened to be zero characters long. This is all
very above-board and honest. When a function returns an undefined value,
it's an admission that it couldn't give you an honest answer. So you
-should use C<defined()> only when you're questioning the integrity of what
+should use C<defined> only when you're questioning the integrity of what
you're trying to do. At other times, a simple comparison to C<0> or C<""> is
what you want.
-Currently, using C<defined()> on an entire array or hash reports whether
-memory for that aggregate has ever been allocated. So an array you set
-to the empty list appears undefined initially, and one that once was full
-and that you then set to the empty list still appears defined. You
-should instead use a simple test for size:
-
- if (@an_array) { print "has array elements\n" }
- if (%a_hash) { print "has hash members\n" }
-
-Using C<undef()> on these, however, does clear their memory and then report
-them as not defined anymore, but you shouldn't do that unless you don't
-plan to use them again, because it saves time when you load them up
-again to have memory already ready to be filled. The normal way to
-free up space used by an aggregate is to assign the empty list.
-
-This counterintuitive behavior of C<defined()> on aggregates may be
-changed, fixed, or broken in a future release of Perl.
-
See also L</undef>, L</exists>, L</ref>.
=item delete EXPR
For each key, returns the deleted value associated with that key, or
the undefined value if there was no such key. Deleting from C<$ENV{}>
modifies the environment. Deleting from a hash tied to a DBM file
-deletes the entry from the DBM file. (But deleting from a C<tie()>d hash
+deletes the entry from the DBM file. (But deleting from a C<tie>d hash
doesn't necessarily return anything.)
The following deletes all the values of a hash:
delete @HASH{keys %HASH}
-(But both of these are slower than just assigning the empty list, or
-using C<undef()>.) Note that the EXPR can be arbitrarily complicated as
-long as the final operation is a hash element lookup or hash slice:
+But both of these are slower than just assigning the empty list
+or undefining it:
+
+ %hash = (); # completely empty %hash
+ undef %hash; # forget %hash every existed
+
+Note that the EXPR can be arbitrarily complicated as long as the final
+operation is a hash element lookup or hash slice:
delete $ref->[$x][$y]{$key};
delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
=item die LIST
-Outside an C<eval()>, prints the value of LIST to C<STDERR> and exits with
-the current value of C<$!> (errno). If C<$!> is C<0>, exits with the value of
-C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
-is C<0>, exits with C<255>. Inside an C<eval(),> the error message is stuffed into
-C<$@> and the C<eval()> is terminated with the undefined value. This makes
-C<die()> the way to raise an exception.
+Outside an C<eval>, prints the value of LIST to C<STDERR> and
+exits with the current value of C<$!> (errno). If C<$!> is C<0>,
+exits with the value of C<($? E<gt>E<gt> 8)> (backtick `command`
+status). If C<($? E<gt>E<gt> 8)> is C<0>, exits with C<255>. Inside
+an C<eval(),> the error message is stuffed into C<$@> and the
+C<eval> is terminated with the undefined value. This makes
+C<die> the way to raise an exception.
Equivalent examples:
If the value of EXPR does not end in a newline, the current script line
number and input line number (if any) are also printed, and a newline
-is supplied. Hint: sometimes appending C<", stopped"> to your message
+is supplied. Note that the "input line number" (also known as "chunk")
+is subject to whatever notion of "line" happens to be currently in
+effect, and is also available as the special variable C<$.>.
+See L<perlvar/"$/"> and L<perlvar/"$.">.
+
+Hint: sometimes appending C<", stopped"> to your message
will cause it to make better sense when the string C<"at foo line 123"> is
appended. Suppose you are running script "canasta".
/etc/games is no good at canasta line 123.
/etc/games is no good, stopped at canasta line 123.
-See also C<exit()> and C<warn()>.
+See also exit(), warn(), and the Carp module.
If LIST is empty and C<$@> already contains a value (typically from a
previous eval) that value is reused after appending C<"\t...propagated">.
If C<$@> is empty then the string C<"Died"> is used.
-You can arrange for a callback to be run just before the C<die()> does
-its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
-will be called with the error text and can change the error message, if
-it sees fit, by calling C<die()> again. See L<perlvar/$SIG{expr}> for details on
-setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
+die() can also be called with a reference argument. If this happens to be
+trapped within an eval(), $@ contains the reference. This behavior permits
+a more elaborate exception handling implementation using objects that
+maintain arbitary state about the nature of the exception. Such a scheme
+is sometimes preferable to matching particular string values of $@ using
+regular expressions. Here's an example:
+
+ eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
+ if ($@) {
+ if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
+ # handle Some::Module::Exception
+ }
+ else {
+ # handle all other possible exceptions
+ }
+ }
-Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
-blocks/strings. If one wants the hook to do nothing in such
-situations, put
+Because perl will stringify uncaught exception messages before displaying
+them, you may want to overload stringification operations on such custom
+exception objects. See L<overload> for details about that.
+
+You can arrange for a callback to be run just before the C<die>
+does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
+handler will be called with the error text and can change the error
+message, if it sees fit, by calling C<die> again. See
+L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
+L<"eval BLOCK"> for some examples. Although this feature was meant
+to be run only right before your program was to exit, this is not
+currently the case--the C<$SIG{__DIE__}> hook is currently called
+even inside eval()ed blocks/strings! If one wants the hook to do
+nothing in such situations, put
die @_ if $^S;
-as the first line of the handler (see L<perlvar/$^S>).
+as the first line of the handler (see L<perlvar/$^S>). Because
+this promotes strange action at a distance, this counterintuitive
+behavior may be fixed in a future release.
=item do BLOCK
(On other statements the loop modifiers test the conditional first.)
C<do BLOCK> does I<not> count as a loop, so the loop control statements
-C<next>, C<last> or C<redo> cannot be used to leave or restart the block.
+C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
+See L<perlsyn> for alternative strategies.
=item do SUBROUTINE(LIST)
scalar eval `cat stat.pl`;
-except that it's more efficient and concise, keeps track of the
-current filename for error messages, and searches all the B<-I>
-libraries if the file isn't in the current directory (see also the @INC
-array in L<perlvar/Predefined Names>). It is also different in how
-code evaluated with C<do FILENAME> doesn't see lexicals in the enclosing
-scope like C<eval STRING> does. It's the same, however, in that it does
-reparse the file every time you call it, so you probably don't want to
-do this inside a loop.
+except that it's more efficient and concise, keeps track of the current
+filename for error messages, searches the @INC libraries, and updates
+C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
+variables. It also differs in that code evaluated with C<do FILENAME>
+cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
+same, however, in that it does reparse the file every time you call it,
+so you probably don't want to do this inside a loop.
If C<do> cannot read the file, it returns undef and sets C<$!> to the
-error. If C<do> can read the file but cannot compile it, it
+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.
Note that inclusion of library modules is better done with the
-C<use()> and C<require()> operators, which also do automatic error checking
+C<use> and C<require> operators, which also do automatic error checking
and raise an exception if there's a problem.
You might like to use C<do> to read in a program configuration
# read in config files: system first, then user
for $file ("/share/prog/defaults.rc",
- "$ENV{HOME}/.someprogrc") {
+ "$ENV{HOME}/.someprogrc")
+ {
unless ($return = do $file) {
warn "couldn't parse $file: $@" if $@;
warn "couldn't do $file: $!" unless defined $return;
=item dump LABEL
-This causes an immediate core dump. Primarily this is so that you can
-use the B<undump> program to turn your core dump into an executable binary
-after having initialized all your variables at the beginning of the
-program. When the new binary is executed it will begin by executing a
-C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
-it as a goto with an intervening core dump and reincarnation. If C<LABEL>
-is omitted, restarts the program from the top. WARNING: Any files
-opened at the time of the dump will NOT be open any more when the
-program is reincarnated, with possible resulting confusion on the part
-of Perl. See also B<-u> option in L<perlrun>.
-
-Example:
-
- #!/usr/bin/perl
- require 'getopt.pl';
- require 'stat.pl';
- %days = (
- 'Sun' => 1,
- 'Mon' => 2,
- 'Tue' => 3,
- 'Wed' => 4,
- 'Thu' => 5,
- 'Fri' => 6,
- 'Sat' => 7,
- );
-
- dump QUICKSTART if $ARGV[0] eq '-d';
-
- QUICKSTART:
- Getopt('f');
-
-This operator is largely obsolete, partly because it's very hard to
-convert a core file into an executable, and because the real perl-to-C
-compiler has superseded it.
+=item dump
+
+This function causes an immediate core dump. See also the B<-u>
+command-line switch in L<perlrun>, which does the same thing.
+Primarily this is so that you can use the B<undump> program (not
+supplied) to turn your core dump into an executable binary after
+having initialized all your variables at the beginning of the
+program. When the new binary is executed it will begin by executing
+a C<goto LABEL> (with all the restrictions that C<goto> suffers).
+Think of it as a goto with an intervening core dump and reincarnation.
+If C<LABEL> is omitted, restarts the program from the top.
+
+B<WARNING>: Any files opened at the time of the dump will I<not>
+be open any more when the program is reincarnated, with possible
+resulting confusion on the part of Perl.
+
+This function is now largely obsolete, partly because it's very
+hard to convert a core file into an executable, and because the
+real compiler backends for generating portable bytecode and compilable
+C code have superseded it.
+
+If you're looking to use L<dump> to speed up your program, consider
+generating bytecode or native C code as described in L<perlcc>. If
+you're just trying to accelerate a CGI script, consider using the
+C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
+You might also consider autoloading or selfloading, which at least
+make your program I<appear> to run faster.
=item each HASH
false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
for this reason.)
-Entries are returned in an apparently random order. When the hash is
-entirely read, a null array is returned in list context (which when
-assigned produces a FALSE (C<0>) value), and C<undef> in
-scalar context. The next call to C<each()> after that will start iterating
-again. There is a single iterator for each hash, shared by all C<each()>,
-C<keys()>, and C<values()> function calls in the program; it can be reset by
+Entries are returned in an apparently random order. The actual random
+order is subject to change in future versions of perl, but it is guaranteed
+to be in the same order as either the C<keys> or C<values> function
+would produce on the same (unmodified) hash.
+
+When the hash is entirely read, a null array is returned in list context
+(which when assigned produces a false (C<0>) value), and C<undef> in
+scalar context. The next call to C<each> after that will start iterating
+again. There is a single iterator for each hash, shared by all C<each>,
+C<keys>, and C<values> function calls in the program; it can be reset by
reading all the elements from the hash, or by evaluating C<keys HASH> or
C<values HASH>. If you add or delete elements of a hash while you're
iterating over it, you may get entries skipped or duplicated, so don't.
print "$key=$value\n";
}
-See also C<keys()> and C<values()>.
+See also C<keys>, C<values> and C<sort>.
=item eof FILEHANDLE
Returns 1 if the next read on FILEHANDLE will return end of file, 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 very useful in an
+reads a character and then C<ungetc>s it, so isn't very useful in an
interactive context.) Do not read from a terminal file (or call
-C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
+C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
as terminals may lose the end-of-file condition if you do.
An C<eof> without an argument uses the last file read as argument.
-Using C<eof()> with empty parentheses is very different. It indicates the pseudo file formed of
-the files listed on the command line, i.e., C<eof()> is reasonable to
-use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
-last file. Use C<eof(ARGV)> or eof without the parentheses to test
-I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
+Using C<eof()> with empty parentheses is very different. It indicates
+the pseudo file formed of the files listed on the command line,
+i.e., C<eof()> is reasonable to use inside a C<while (E<lt>E<gt>)>
+loop to detect the end of only the last file. Use C<eof(ARGV)> or
+C<eof> without the parentheses to test I<each> file in a while
+(E<lt>E<gt>) loop. Examples:
# reset line numbering on each input file
while (<>) {
while (<>) {
if (eof()) { # check for end of current file
print "--------------\n";
- close(ARGV); # close or break; is needed if we
+ close(ARGV); # close or last; is needed if we
# are reading from the terminal
}
print;
in void, scalar, or list context, depending on the context of the eval itself.
See L</wantarray> for more on how the evaluation context can be determined.
-If there is a syntax error or runtime error, or a C<die()> statement is
-executed, an undefined value is returned by C<eval()>, and C<$@> is set to the
+If there is a syntax error or runtime error, or a C<die> statement is
+executed, an undefined value is returned by C<eval>, and C<$@> is set to the
error message. If there was no error, C<$@> is guaranteed to be a null
-string. Beware that using C<eval()> neither silences perl from printing
+string. Beware that using C<eval> neither silences perl from printing
warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
L</warn> and L<perlvar>.
-Note that, because C<eval()> traps otherwise-fatal errors, it is useful for
-determining whether a particular feature (such as C<socket()> or C<symlink()>)
+Note that, because C<eval> traps otherwise-fatal errors, it is useful for
+determining whether a particular feature (such as C<socket> or C<symlink>)
is implemented. It is also Perl's exception trapping mechanism, where
the die operator is used to raise exceptions.
# a run-time error
eval '$answer ='; # sets $@
-When using the C<eval{}> form as an exception trap in libraries, you may
-wish not to trigger any C<__DIE__> hooks that user code may have
-installed. You can use the C<local $SIG{__DIE__}> construct for this
-purpose, as shown in this example:
+Due to the current arguably broken state of C<__DIE__> hooks, when using
+the C<eval{}> form as an exception trap in libraries, you may wish not
+to trigger any C<__DIE__> hooks that user code may have installed.
+You can use the C<local $SIG{__DIE__}> construct for this purpose,
+as shown in this example:
# a very private exception trap for divide-by-zero
eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
warn $@ if $@;
This is especially significant, given that C<__DIE__> hooks can call
-C<die()> again, which has the effect of changing their error messages:
+C<die> again, which has the effect of changing their error messages:
# __DIE__ hooks may modify error messages
{
print $@ if $@; # prints "bar lives here"
}
-With an C<eval()>, you should be especially careful to remember what's
+Because this promotes action at a distance, this counterintuitive behavior
+may be fixed in a future release.
+
+With an C<eval>, you should be especially careful to remember what's
being looked at when:
eval $x; # CASE 1
$$x++; # CASE 6
Cases 1 and 2 above behave identically: they run the code contained in
-the variable C<$x>. (Although case 2 has misleading double quotes making
+the variable $x. (Although case 2 has misleading double quotes making
the reader wonder what else might be happening (nothing is).) Cases 3
and 4 likewise behave in the same way: they run the code C<'$x'>, which
-does nothing but return the value of C<$x>. (Case 4 is preferred for
+does nothing but return the value of $x. (Case 4 is preferred for
purely visual reasons, but it also has the advantage of compiling at
compile-time instead of at run-time.) Case 5 is a place where
-normally you I<WOULD> like to use double quotes, except that in this
+normally you I<would> like to use double quotes, except that in this
particular situation, you can just use symbolic references instead, as
in case 6.
C<eval BLOCK> does I<not> count as a loop, so the loop control statements
-C<next>, C<last> or C<redo> cannot be used to leave or restart the block.
-
+C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
=item exec LIST
=item exec PROGRAM LIST
-The C<exec()> function executes a system command I<AND NEVER RETURNS> -
-use C<system()> instead of C<exec()> if you want it to return. It fails and
-returns
FALSE only if the command does not exist I<and> it is executed
+The C<exec> function executes a system command I<and never returns>--
+use C<system> instead of C<exec> if you want it to return. It fails and
+returns
false only if the command does not exist I<and> it is executed
directly instead of via your system's command shell (see below).
-Since it's a common mistake to use C<exec()> instead of C<system()>, Perl
-warns you if there is a following statement which isn't C<die()>, C<warn()>,
-or C<exit()> (if C<-w> is set - but you always do that). If you
-I<really> want to follow an C<exec()> with some other statement, you
+Since it's a common mistake to use C<exec> instead of C<system>, Perl
+warns you if there is a following statement which isn't C<die>, C<warn>,
+or C<exit> (if C<-w> is set - but you always do that). If you
+I<really> want to follow an C<exec> with some other statement, you
can use one of these styles to avoid the warning:
exec ('foo') or print STDERR "couldn't exec foo: $!";
the entire argument is passed to the system's command shell for parsing
(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
If there are no shell metacharacters in the argument, it is split into
-words and passed directly to C<execvp()>, which is more efficient. Note:
-C<exec()> and C<system()> do not flush your output buffer, so you may need to
-set C<$|> to avoid lost output. Examples:
+words and passed directly to C<execvp>, which is more efficient.
+Examples:
exec '/bin/echo', 'Your arguments are: ', @ARGV;
exec "sort $outfile | uniq";
be subject to its quirks and capabilities. See L<perlop/"`STRING`">
for details.
-Using an indirect object with C<exec()> or C<system()> is also more secure.
-This usage forces interpretation of the arguments as a multivalued list,
-even if the list had just one argument. That way you're safe from the
-shell expanding wildcards or splitting up words with whitespace in them.
+Using an indirect object with C<exec> or C<system> is also more
+secure. This usage (which also works fine with system()) forces
+interpretation of the arguments as a multivalued list, even if the
+list had just one argument. That way you're safe from the shell
+expanding wildcards or splitting up words with whitespace in them.
@args = ( "echo surprise" );
- system @args; # subject to shell escapes
+ exec @args; # subject to shell escapes
# if @args == 1
- system { $args[0] } @args; # safe even with one-arg list
+ exec { $args[0] } @args; # safe even with one-arg list
The first version, the one without the indirect object, ran the I<echo>
program, passing it C<"surprise"> an argument. The second version
didn't--it tried to run a program literally called I<"echo surprise">,
didn't find it, and set C<$?> to a non-zero value indicating failure.
-Note that C<exec()> will not call your C<END> blocks, nor will it call
+Note that C<exec> will not call your C<END> blocks, nor will it call
any C<DESTROY> methods in your objects.
=item exists EXPR
-Returns TRUE if the specified hash key exists in its hash array, even
+Returns true if the specified hash key exists in its hash, even
if the corresponding value is undefined.
- print "Exists\n" if exists $array{$key};
- print "Defined\n" if defined $array{$key};
- print "True\n" if $array{$key};
+ print "Exists\n" if exists $array{$key};
+ print "Defined\n" if defined $array{$key};
+ print "True\n" if $array{$key};
-A hash element can be TRUE only if it's defined, and defined if
+A hash element can be true only if it's defined, and defined if
it exists, but the reverse doesn't necessarily hold true.
Note that the EXPR can be arbitrarily complicated as long as the final
operation is a hash key lookup:
- if (exists $ref->{"A"}{"B"}{$key}) { ... }
+ if (exists $ref->{A}->{B}->{$key}) { }
+ if (exists $hash{A}{B}{$key}) { }
-Although the last element will not spring into existence just because its
-existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
-C<$ref-E<gt>{"B"}> will spring into existence due to the existence
-test for a $key element. This autovivification may be fixed in a later
+Although the last element will not spring into existence just because
+its existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
+and C<$ref-E<gt>{"A"}-E<gt>{"B"}> will spring into existence due to the
+existence test for a $key element. This happens anywhere the arrow
+operator is used, including even
+
+ undef $ref;
+ if (exists $ref->{"Some key"}) { }
+ print $ref; # prints HASH(0x80d3d5c)
+
+This surprising autovivification in what does not at first--or even
+second--glance appear to be an lvalue context may be fixed in a future
release.
+See L<perlref/"Pseudo-hashes"> for specifics on how exists() acts when
+used on a pseudo-hash.
+
=item exit EXPR
-Evaluates EXPR and exits immediately with that value. (Actually, it
-calls any defined C<END> routines first, but the C<END> routines may not
-abort the exit. Likewise any object destructors that need to be called
-are called before exit.) Example:
+Evaluates EXPR and exits immediately with that value. Example:
$ans = <STDIN>;
exit 0 if $ans =~ /^[Xx]/;
-See also C<die()>. If EXPR is omitted, exits with C<0> status. The only
-universally portable values for EXPR are C<0> for success and C<1> for error;
-all other values are subject to unpredictable interpretation depending
-on the environment in which the Perl program is running.
+See also C<die>. If EXPR is omitted, exits with C<0> status. The only
+universally recognized values for EXPR are C<0> for success and C<1>
+for error; other values are subject to interpretation depending on the
+environment in which the Perl program is running. For example, exiting
+69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
+the mailer to return the item undelivered, but that's not true everywhere.
-You shouldn't use C<exit()> to abort a subroutine if there's any chance that
-someone might want to trap whatever error happened. Use C<die()> instead,
-which can be trapped by an C<eval()>.
+Don't use C<exit> to abort a subroutine if there's any chance that
+someone might want to trap whatever error happened. Use C<die> instead,
+which can be trapped by an C<eval>.
-All C<END{}> blocks are run at exit time. See L<perlsub> for details.
+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
+can call C<POSIX:_exit($status)> to avoid END and destructor processing.
+See L<perlmod> for details.
=item exp EXPR
=item exp
-Returns I<e> (the natural logarithm base) to the power of EXPR.
+Returns I<e> (the natural logarithm base) to the power of EXPR.
If EXPR is omitted, gives C<exp($_)>.
=item fcntl FILEHANDLE,FUNCTION,SCALAR
use Fcntl;
first to get the correct constant definitions. Argument processing and
-value return works just like C<ioctl()> below.
+value return works just like C<ioctl> below.
For example:
use Fcntl;
fcntl($filehandle, F_GETFL, $packed_return_buffer)
or die "can't fcntl F_GETFL: $!";
-You don't have to check for C<defined()> on the return from
-C<fnctl()>. Like C<ioctl()>, it maps a C<0> return from the system
-call into "C<0> but true" in Perl. This string is true in
-boolean context and C<0> in numeric context. It is also
-exempt from the normal B<-w> warnings on improper numeric
-conversions.
+You don't have to check for C<defined> on the return from C<fnctl>.
+Like C<ioctl>, it maps a C<0> return from the system call into C<"0
+but true"> in Perl. This string is true in boolean context and C<0>
+in numeric context. It is also exempt from the normal B<-w> warnings
+on improper numeric conversions.
-Note that C<fcntl()> will produce a fatal error if used on a machine that
-doesn't implement fcntl(2).
+Note that C<fcntl> will produce a fatal error if used on a machine that
+doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
+manpage to learn what functions are available on your system.
=item fileno FILEHANDLE
-Returns the file descriptor for a filehandle. This is 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.
+Returns the file descriptor for a filehandle, or undefined if the
+filehandle is not open. 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.
You can use this to find out whether two handles refer to the
same underlying descriptor:
=item flock FILEHANDLE,OPERATION
-Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
-success, FALSE on failure. Produces a fatal error if used on a machine
-that doesn't implement flock(2), fcntl(2) locking, or lockf(3). C<flock()>
-is Perl's portable file locking interface, although it locks only entire
-files, not records.
-
-On many platforms (including most versions or clones of Unix), locks
-established by C<flock()> are B<merely advisory>. Such discretionary locks
-are more flexible, but offer fewer guarantees. This means that files
-locked with C<flock()> may be modified by programs that do not also use
-C<flock()>. Windows NT and OS/2 are among the platforms which
-enforce mandatory locking. See your local documentation for details.
+Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
+for success, false on failure. Produces a fatal error if used on a
+machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
+C<flock> is Perl's portable file locking interface, although it locks
+only entire files, not records.
+
+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 files locked with C<flock> may be
+modified by programs that do not also use C<flock>. See L<perlport>,
+your port's specific documentation, or 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
+"features"). Slavish adherence to portability concerns shouldn't get
+in the way of your getting your job done.)
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
either individually, or as a group using the ':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 added to LOCK_SH or
-LOCK_EX then C<flock()> will return immediately rather than blocking
+LOCK_EX then C<flock> will return immediately rather than blocking
waiting for the lock (check the return status to see if you got it).
-To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
-before (un)locking it.
+To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
+before locking or unlocking it.
Note that the emulation built with lockf(3) doesn't provide shared
locks, and it requires that FILEHANDLE be open with write intent. These
-are the semantics that lockf(3) implements. Most (all?) systems
+are the semantics that lockf(3) implements. Most if not all systems
implement lockf(3) in terms of fcntl(2) locking, though, so the
differing semantics shouldn't bite too many people.
-Note also that some versions of C<flock()> cannot lock things over the
-network; you would need to use the more system-specific C<fcntl()> for
+Note also that some versions of C<flock> cannot lock things over the
+network; you would need to use the more system-specific C<fcntl> for
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
print MBOX $msg,"\n\n";
unlock();
+On systems that support a real flock(), locks are inherited across fork()
+calls, whereas those that must resort to the more capricious fcntl()
+function lose the locks, making it harder to write servers.
+
See also L<DB_File> for other flock() examples.
=item fork
-Does a fork(2) system call. Returns the child pid to the parent process,
-C<0> to the child process, or C<undef> if the fork is unsuccessful.
-
-Note: unflushed buffers remain unflushed in both processes, which means
-you may need to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()>
-method of C<IO::Handle> to avoid duplicate output.
-
-If you C<fork()> without ever waiting on your children, you will accumulate
-zombies:
-
- $SIG{CHLD} = sub { wait };
-
-There's also the double-fork trick (error checking on
-C<fork()> returns omitted);
+Does a fork(2) system call to create a new process running the
+same program at the same point. It returns the child pid to the
+parent process, C<0> to the child process, or C<undef> if the fork is
+unsuccessful. File descriptors (and sometimes locks on those descriptors)
+are shared, while everything else is copied. On most systems supporting
+fork(), great care has gone into making it extremely efficient (for
+example, using copy-on-write technology on data pages), making it the
+dominant paradigm for multitasking over the last few decades.
- unless ($pid = fork) {
- unless (fork) {
- exec "what you really wanna do";
- die "no exec";
- # ... or ...
- ## (some_perl_code_here)
- exit 0;
- }
- exit 0;
- }
- waitpid($pid,0);
+All files opened for output are flushed before forking the child process.
-See also L<perlipc> for more examples of forking and reaping
-moribund children.
+If you C<fork> without ever waiting on your children, you will
+accumulate zombies. On some systems, you can avoid this by setting
+C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
+forking and reaping moribund children.
Note that if your forked child inherits system file descriptors like
STDIN and STDOUT that are actually connected by a pipe or socket, even
-if you exit, then the remote server (such as, say, httpd or rsh) won't think
-you're done. You should reopen those to F</dev/null> if it's any issue.
+if you exit, then the remote server (such as, say, a CGI script or a
+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.
=item format
-Declare a picture format for use by the C<write()> function. For
+Declare a picture format for use by the C<write> function. For
example:
format Something =
too. It formats (see L<perlform>) a list of values according to the
contents of PICTURE, placing the output into the format output
accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
-Eventually, when a C<write()> is done, the contents of
+Eventually, when a C<write> is done, the contents of
C<$^A> are written to some filehandle, but you could also read C<$^A>
yourself and then set C<$^A> back to C<"">. Note that a format typically
-does one C<formline()> per line of form, but the C<formline()> function itself
+does one C<formline> per line of form, but the C<formline> function itself
doesn't care how many newlines are embedded in the PICTURE. This means
that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
You may therefore need to use multiple formlines to implement a single
record format, just like the format compiler.
-Be careful if you put double quotes around the picture, because an "C<@>"
+Be careful if you put double quotes around the picture, because an C<@>
character may be taken to mean the beginning of an array name.
-C<formline
()> always returns TRUE. See L<perlform> for other examples.
+C<formline
> always returns true. See L<perlform> for other examples.
=item getc FILEHANDLE
=item getc
Returns the next character from the input file attached to FILEHANDLE,
-or the undefined value at end of file, or if there was an error. If
-FILEHANDLE is omitted, reads from STDIN. This is not particularly
-efficient. It cannot be used to get unbuffered single-characters,
-however. For that, try something more like:
+or the undefined value at end of file, or if there was an error.
+If FILEHANDLE is omitted, reads from STDIN. This is not particularly
+efficient. However, it cannot be used by itself to fetch single
+characters without waiting for the user to hit enter. For that, try
+something more like:
if ($BSD_STYLE) {
system "stty cbreak </dev/tty >/dev/tty 2>&1";
Determination of whether $BSD_STYLE should be set
is left as an exercise to the reader.
-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 L<perlmod/CPAN>.
+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
+L<perlmodlib/CPAN>.
=item getlogin
Implements the C library function of the same name, which on most
systems returns the current login from F</etc/utmp>, if any. If null,
-use C<getpwuid()>.
+use C<getpwuid>.
$login = getlogin || getpwuid($<) || "Kilroy";
-Do not consider C<getlogin()> for authentication: it is not as
-secure as C<getpwuid()>.
+Do not consider C<getlogin> for authentication: it is not as
+secure as C<getpwuid>.
=item getpeername SOCKET
use Socket;
$hersockaddr = getpeername(SOCK);
- ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
+ ($port, $iaddr) = sockaddr_in($hersockaddr);
$herhostname = gethostbyaddr($iaddr, AF_INET);
$herstraddr = inet_ntoa($iaddr);
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()>
+group of current process. Note that the POSIX version of C<getpgrp>
does not accept a PID argument, so only C<PID==0> is truly portable.
=item getppid
$name = getgrent();
#etc.
-In I<getpw*()> the fields C<$quota>, C<$comment>, and C<$expire> are special
-cases in the sense that in many systems they are unsupported. If the
-C<$quota> is unsupported, it is an empty scalar. If it is supported, it
-usually encodes the disk quota. If the C<$comment> field is unsupported,
-it is an empty scalar. If it is supported it usually encodes some
-administrative comment about the user. In some systems the $quota
-field may be C<$change> or C<$age>, fields that have to do with password
-aging. In some systems the C<$comment> field may be C<$class>. The C<$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
-F<pwd.h> file. You can also find out from within Perl which meaning
-your C<$quota> and C<$comment> fields have and whether you have the C<$expire>
-field by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
-C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>.
-
-The C<$members> value returned by I<getgr*()> is a space separated list of
+In I<getpw*()> the fields $quota, $comment, and $expire are
+special cases in the sense that in many systems they are unsupported.
+If the $quota is unsupported, it is an empty scalar. If it is
+supported, it usually encodes the disk quota. If the $comment
+field is unsupported, it is an empty scalar. If it is supported it
+usually encodes some administrative comment about the user. In some
+systems the $quota field may be $change or $age, fields that have
+to do with password 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 F<pwd.h> file. You can also find
+out from within Perl what your $quota and $comment fields mean
+and whether you have the $expire field by using the C<Config> module
+and the values C<d_pwquota>, C<d_pwage>, C<d_pwchange>, C<d_pwcomment>,
+and C<d_pwexpire>. Shadow password files are only supported if your
+vendor has implemented them in the intuitive fashion that calling the
+regular C library routines gets the shadow versions if you're running
+under privilege. Those that incorrectly implement a separate library
+call are not supported.
+
+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
($a,$b,$c,$d) = unpack('C4',$addr[0]);
-If you get tired of remembering which element of the return list contains
-which return value, by-name interfaces are also provided in modules:
-C<File::stat>, C<Net::hostent>, C<Net::netent>, C<Net::protoent>, C<Net::servent>,
-C<Time::gmtime>, C<Time::localtime>, and C<User::grent>. These override the
-normal built-in, replacing them with versions that return objects with
-the appropriate names for each field. For example:
+The Socket library makes this slightly easier:
+
+ use Socket;
+ $iaddr = inet_aton("127.1"); # or whatever address
+ $name = gethostbyaddr($iaddr, AF_INET);
+
+ # or going the other way
+ $straddr = inet_ntoa($iaddr);
+
+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>,
+C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
+and C<User::grent>. These override the normal built-ins, supplying
+versions that return objects with the appropriate names
+for each field. For example:
use File::stat;
use User::pwent;
$is_his = (stat($filename)->uid == pwent($whoever)->uid);
Even though it looks like they're the same method calls (uid),
-they aren't, because a C<File::stat> object is different from a C<User::pwent> object.
+they aren't, because a C<File::stat> object is different from
+a C<User::pwent> object.
=item getsockname SOCKET
-Returns the packed sockaddr address of this end of the SOCKET connection.
+Returns the packed sockaddr address of this end of the SOCKET connection,
+in case you don't know the address because you have several different
+IPs that the connection might have come in on.
use Socket;
$mysockaddr = getsockname(SOCK);
- ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
+ ($port, $myaddr) = sockaddr_in($mysockaddr);
+ printf "Connect to %s [%s]\n",
+ scalar gethostbyaddr($myaddr, AF_INET),
+ inet_ntoa($myaddr);
=item getsockopt SOCKET,LEVEL,OPTNAME
=item glob
-Returns the value of EXPR with filename expansions such as the standard Unix shell F</bin/sh> would
-do. This is the internal function implementing the C<E<lt>*.cE<gt>>
-operator, but you can use it directly. If EXPR is omitted, C<$_> is used.
-The C<E<lt>*.cE<gt>> operator is discussed in more detail in
-L<perlop/"I/O Operators">.
+Returns the value of EXPR with filename expansions such as the
+standard Unix shell F</bin/csh> would do. This is the internal function
+implementing the C<E<lt>*.cE<gt>> operator, but you can use it directly.
+If EXPR is omitted, C<$_> is used. The C<E<lt>*.cE<gt>> operator is
+
discussed in more detail in L<perlop/"I/O Operators">.
=item gmtime EXPR
-Converts a time as returned by the time function to a 9-element array
+Converts a time as returned by the time function to a 9-element list
with the time localized for the standard Greenwich time zone.
Typically used as follows:
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
gmtime(time);
-All array elements are numeric, and come straight out of a struct tm.
-In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
-the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
-years since 1900, that is, C<$year> is C<123> in year 2023, I<not> simply the last two digits of the year.
+All list elements are numeric, and come straight out of a struct tm.
+In particular this means that $mon has the range C<0..11> and $wday
+has the range C<0..6> with sunday as day C<0>. Also, $year is the
+number of years since 1900, that is, $year is C<123> in year 2023,
+I<not> simply the last two digits of the year. If you assume it is,
+then you create non-Y2K-compliant programs--and you wouldn't want to do
+that, would you?
If EXPR is omitted, does C<gmtime(time())>.
$now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
-Also see the C<timegm()> function provided by the C<Time::Local> module,
+Also see the C<timegm> function provided by the C<Time::Local> module,
and the strftime(3) function available via the POSIX module.
-This scalar value is B<not> locale dependent, see L<perllocale>, but
-instead a Perl builtin. Also see the C<Time::Local> module, and the
-strftime(3) and mktime(3) function available via the POSIX module. To
+This scalar value is B<not> locale dependent (see L<perllocale>), but
+is instead a Perl builtin. Also see the C<Time::Local> module, and the
+strftime(3) and mktime(3) functions available via the POSIX module. To
get somewhat similar but locale dependent date strings, set up your
locale environment variables appropriately (please see L<perllocale>)
and try for example:
use POSIX qw(strftime);
- $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
+ $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
-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.
+Note that the C<%a> and C<%b> escapes, which represent the short forms
+of the day of the week and the month of the year, may not necessarily
+be three characters wide in all locales.
=item goto LABEL
execution there. It may not be used to go into any construct that
requires initialization, such as a subroutine or a C<foreach> loop. It
also can't be used to go into a construct that is optimized away,
-or to get out of a block or subroutine given to C<sort()>.
+or to get out of a block or subroutine given to C<sort>.
It can be used to go almost anywhere else within the dynamic scope,
including out of subroutines, but it's usually better to use some other
-construct such as C<last> or C<die()>. The author of Perl has never felt the
+construct such as C<last> or C<die>. The author of Perl has never felt the
need to use this form of C<goto> (in Perl, that is--C is another matter).
The C<goto-EXPR> form expects a label name, whose scope will be resolved
C<AUTOLOAD> subroutines that wish to load another subroutine and then
pretend that the other subroutine had been called in the first place
(except that any modifications to C<@_> in the current subroutine are
-propagated to the other subroutine.) After the C<goto>, not even C<caller()>
+propagated to the other subroutine.) After the C<goto>, not even C<caller>
will be able to tell that this routine was called first.
=item grep BLOCK LIST
=item grep EXPR,LIST
-This is similar in spirit to, but not the same as, grep(1)
-and its relatives. In particular, it is not limited to using
-regular expressions.
+This is similar in spirit to, but not the same as, grep(1) and its
+relatives. In particular, it is not limited to using regular expressions.
Evaluates the BLOCK or EXPR for each element of LIST (locally setting
C<$_> to each element) and returns the list value consisting of those
-elements for which the expression evaluated to TRUE. In a scalar
-context, returns the number of times the expression was TRUE.
+elements for which the expression evaluated to true. In scalar
+context, returns the number of times the expression was true.
@foo = grep(!/^#/, @bar); # weed out comments
@foo = grep {!/^#/} @bar; # weed out comments
-Note that, because C<$_> is a reference into the list value, it can be used
-to modify the elements of the array. While this is useful and
-supported, it can cause bizarre results if the LIST is not a named
-array. Similarly, grep returns aliases into the original list,
-much like the way that a for loop's index variable aliases the list
-elements. That is, modifying an element of a list returned by grep
-(for example, in a C<foreach>, C<map()> or another C<grep()>)
-actually modifies the element in the original list.
+Note that, because C<$_> is a reference into the list value, it can
+be used to modify the elements of the array. While this is useful and
+supported, it can cause bizarre results if the LIST is not a named array.
+Similarly, grep returns aliases into the original list, much as a for
+loop's index variable aliases the list elements. That is, modifying an
+element of a list returned by grep (for example, in a C<foreach>, C<map>
+or another C<grep>) actually modifies the element in the original list.
+This is usually something to be avoided when writing clear code.
-See also L</map> for an array composed of the results of the BLOCK or EXPR.
+See also L</map> for a list composed of the results of the BLOCK or EXPR.
=item hex EXPR
=item hex
-Interprets EXPR as a hex string and returns the corresponding
-value. (To convert strings that might start with either 0 or 0x
-see L</oct>.) If EXPR is omitted, uses C<$_>.
+Interprets EXPR as a hex string and returns the corresponding value.
+(To convert strings that might start with either 0, 0x, or 0b, see
+L</oct>.) If EXPR is omitted, uses C<$_>.
print hex '0xAf'; # prints '175'
print hex 'aF'; # same
+Hex strings may only represent integers. Strings that would cause
+integer overflow trigger a mandatory error message.
+
=item import
-There is no builtin C<import()> function. It is just an ordinary
+There is no builtin C<import> function. It is just an ordinary
method (subroutine) defined (or inherited) by modules that wish to export
-names to another module. The C<use()> function calls the C<import()> method
+names to another module. The C<use> function calls the C<import> method
for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
=item index STR,SUBSTR,POSITION
=item index STR,SUBSTR
-Returns the position of the first occurrence of SUBSTR in STR at or after
-POSITION. If POSITION is omitted, starts searching from the beginning of
-the string. The return value is based at C<0> (or whatever you've set the C<$[>
-variable to--but don't do that). If the substring is not found, returns
-one less than the base, ordinarily C<-1>.
+The index function searches for one string within another, but without
+the wildcard-like behavior of a full regular-expression pattern match.
+It returns the position of the first occurrence of SUBSTR in STR at
+or after POSITION. If POSITION is omitted, starts searching from the
+beginning of the string. The return value is based at C<0> (or whatever
+you've set the C<$[> variable to--but don't do that). If the substring
+is not found, returns one less than the base, ordinarily C<-1>.
=item int EXPR
=item int
Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
-You should not use this for rounding, because it truncates
-towards C<0>, and because machine representations of floating point
-numbers can sometimes produce counterintuitive results. Usually C<sprintf()> or C<printf()>,
-or the C<POSIX::floor> or C<POSIX::ceil> functions, would serve you better.
+You should not use this function for rounding: one because it truncates
+towards C<0>, and two because machine representations of floating point
+numbers can sometimes produce counterintuitive results. For example,
+C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
+because it's really more like -268.99999999999994315658 instead. Usually,
+the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
+functions will serve you better than will int().
=item ioctl FILEHANDLE,FUNCTION,SCALAR
-Implements the ioctl(2) function. You'll probably have to say
+Implements the ioctl(2) function. You'll probably first have to say
require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
-first to get the correct function definitions. If F<ioctl.ph> doesn't
+to get the correct function definitions. If F<ioctl.ph> doesn't
exist or doesn't have the correct definitions you'll have to roll your
own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
(There is a Perl script called B<h2ph> that comes with the Perl kit that
may help you in this, but it's nontrivial.) SCALAR will be read and/or
written depending on the FUNCTION--a pointer to the string value of SCALAR
-will be passed as the third argument of the actual C<ioctl()> call. (If SCALAR
+will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
has no string value but does have a numeric value, that value will be
passed rather than a pointer to the string value. To guarantee this to be
-TRUE, add a C<0> to the scalar before using it.) The C<pack()> and C<unpack()>
-functions are useful for manipulating the values of structures used by
-C<ioctl()>. The following example sets the erase character to DEL.
-
- require 'ioctl.ph';
- $getp = &TIOCGETP;
- die "NO TIOCGETP" if $@ || !$getp;
- $sgttyb_t = "ccccs"; # 4 chars and a short
- if (ioctl(STDIN,$getp,$sgttyb)) {
- @ary = unpack($sgttyb_t,$sgttyb);
- $ary[2] = 127;
- $sgttyb = pack($sgttyb_t,@ary);
- ioctl(STDIN,&TIOCSETP,$sgttyb)
- || die "Can't ioctl: $!";
- }
+true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
+functions may be needed to manipulate the values of structures used by
+C<ioctl>.
-The return value of C<ioctl()> (and C<fcntl()>) is as follows:
+The return value of C<ioctl> (and C<fcntl>) is as follows:
if OS returns: then Perl returns:
-1 undefined value
0 string "0 but true"
anything else that number
-Thus Perl returns TRUE on success and FALSE on failure, yet you can
+Thus Perl returns true on success and false on failure, yet you can
still easily determine the actual value returned by the operating
system:
- ($retval = ioctl(...)) || ($retval = -1);
+ $retval = ioctl(...) || -1;
printf "System returned %d\n", $retval;
-The special string "C<0> but true" is excempt from B<-w> complaints
+The special string "C<0> but true" is exempt from B<-w> complaints
about improper numeric conversions.
+Here's an example of setting a filehandle named C<REMOTE> to be
+non-blocking at the system level. You'll have to negotiate C<$|>
+on your own, though.
+
+ use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
+
+ $flags = fcntl(REMOTE, F_GETFL, 0)
+ or die "Can't get flags for the socket: $!\n";
+
+ $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
+ or die "Can't set flags for the socket: $!\n";
+
=item join EXPR,LIST
-Joins the separate strings of LIST into a single string with
-fields separated by the value of EXPR, and returns the string.
-Example:
+Joins the separate strings of LIST into a single string with fields
+separated by the value of EXPR, and returns that new string. Example:
- $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
+ $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
-See L</split>.
+Beware that unlike C<split>, C<join> doesn't take a pattern as its
+first argument. Compare L</split>.
=item keys HASH
-Returns a list consisting of all the keys of the named hash. (In a
+Returns a list consisting of all the keys of the named hash. (In
scalar context, returns the number of keys.) The keys are returned in
-an apparently random order, but it is the same order as either the
-C<values()> or C<each()> function produces (given that the hash has not been
-modified). As a side effect, it resets HASH's iterator.
+an apparently random order. The actual 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). As a side effect, it resets
+HASH's iterator.
Here is yet another way to print your environment:
@keys = keys %ENV;
@values = values %ENV;
- while ($#keys >= 0) {
+ while (@keys) {
print pop(@keys), '=', pop(@values), "\n";
}
print $key, '=', $ENV{$key}, "\n";
}
-To sort a hash by value, you'll need to use a C<sort()> function.
+To sort a hash by value, you'll need to use a C<sort> function.
Here's a descending numeric sort of a hash by its values:
foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
printf "%4d %s\n", $hash{$key}, $key;
}
-As an lvalue C<keys()> allows you to increase the number of hash buckets
+As an lvalue C<keys> allows you to increase the number of hash buckets
allocated for the given hash. This can gain you a measure of efficiency if
you know the hash is going to get big. (This is similar to pre-extending
an array by assigning a larger number to $#array.) If you say
keys %hash = 200;
-then C<%hash> will have at least 200 buckets allocated for it--256 of them, in fact, since
-it rounds up to the next power of two. These
+then C<%hash> will have at least 200 buckets allocated for it--256 of them,
+in fact, since it rounds up to the next power of two. These
buckets will be retained even if you do C<%hash = ()>, use C<undef
%hash> if you want to free the storage while C<%hash> is still in scope.
You can't shrink the number of buckets allocated for the hash using
-C<keys()> in this way (but you needn't worry about doing this by accident,
+C<keys> in this way (but you needn't worry about doing this by accident,
as trying has no effect).
+See also C<each>, C<values> and C<sort>.
+
=item kill LIST
Sends a signal to a list of processes. The first element of
the list must be the signal to send. Returns the number of
-processes successfully signaled.
+processes successfully signaled (which is not necessarily the
+same as the number actually killed).
$cnt = kill 1, $child1, $child2;
kill 9, @goners;
}
C<last> cannot be used to exit a block which returns a value such as
-C<eval {}>, C<sub {}> or C<do {}>.
+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
+that executes once. Thus C<last> can be used to effect an early
+exit out of such a block.
See also L</continue> for an illustration of how C<last>, C<next>, and
C<redo> work.
Returns an lowercased version of EXPR. This is the internal function
implementing the C<\L> escape in double-quoted strings.
-Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
+Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
+and L<utf8>.
If EXPR is omitted, uses C<$_>.
=item length
Returns the length in characters of the value of EXPR. If EXPR is
-omitted, returns length of C<$_>.
+omitted, returns length of C<$_>. Note that this cannot be used on
+an entire array or hash to find out how many elements these have.
+For that, use C<scalar @array> and C<scalar keys %hash> respectively.
=item link OLDFILE,NEWFILE
-Creates a new filename linked to the old filename. Returns TRUE for
-success, FALSE otherwise.
+Creates a new filename linked to the old filename. Returns true for
+success, false otherwise.
=item listen SOCKET,QUEUESIZE
-Does the same thing that the listen system call does. Returns TRUE if
-it succeeded,
FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
+Does the same thing that the listen system call does. Returns true if
+it succeeded,
false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
=item local EXPR
+You really probably want to be using C<my> instead, because C<local> isn't
+what most people think of as "local". See L<perlsub/"Private Variables
+via my()"> for details.
+
A local modifies the listed variables to be local to the enclosing
block, file, or eval. If more than one value is listed, the list must
be placed in parentheses. See L<perlsub/"Temporary Values via local()">
for details, including issues with tied arrays and hashes.
-You really probably want to be using C<my()> instead, because C<local()> isn't
-what most people think of as "local". See L<perlsub/"Private Variables
-via my()"> for details.
-
=item localtime EXPR
-Converts a time as returned by the time function to a 9-element array
+Converts a time as returned by the time function to a 9-element list
with the time analyzed for the local time zone. Typically used as
follows:
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
localtime(time);
-All array elements are numeric, and come straight out of a struct tm.
-In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
-the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
-years since 1900, that is, C<$year> is C<123> in year 2023, and I<not> simply the last two digits of the year.
+All list elements are numeric, and come straight out of a struct tm.
+In particular this means that $mon has the range C<0..11> and $wday
+has the range C<0..6> with sunday as day C<0>. Also, $year is the
+number of years since 1900, that is, $year is C<123> in year 2023,
+and I<not> simply the last two digits of the year. If you assume it is,
+then you create non-Y2K-compliant programs--and you wouldn't want to do
+that, would you?
If EXPR is omitted, uses the current time (C<localtime(time)>).
$now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
This scalar value is B<not> locale dependent, see L<perllocale>, but
-instead a Perl builtin. Also see the C<Time::Local> module, and the
-strftime(3) and mktime(3) function available via the POSIX module. To
-get somewhat similar but locale dependent date strings, set up your
-locale environment variables appropriately (please see L<perllocale>)
-and try for example:
+instead a Perl builtin. Also see the C<Time::Local> module
+(to convert the second, minutes, hours, ... back to seconds since the
+stroke of midnight the 1st of January 1970, the value returned by
+time()), and the strftime(3) and mktime(3) function available via the
+POSIX module. To get somewhat similar but locale dependent date
+strings, set up your locale environment variables appropriately
+(please see L<perllocale>) and try for example:
use POSIX qw(strftime);
- $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
+ $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
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.
+=item lock
+
+ lock I<THING>
+
+This function places an advisory lock on a variable, subroutine,
+or referenced object contained in I<THING> until the lock goes out
+of scope. This is a built-in function only if your version of Perl
+was built with threading enabled, and if you've said C<use Threads>.
+Otherwise a user-defined function by this name will be called. See
+L<Thread>.
+
=item log EXPR
=item log
-Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
-of C<$_>.
+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:
+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:
+
+ sub log10 {
+ my $n = shift;
+ return log($n)/log(10);
+ }
+
+See also L</exp> for the inverse operation.
=item lstat FILEHANDLE
=item lstat
-Does the same thing as the C<stat()> function (including setting the
+Does the same thing as the C<stat> function (including setting the
special C<_> filehandle) but stats a symbolic link instead of the file
the symbolic link points to. If symbolic links are unimplemented on
-your system, a normal C<stat()> is done.
+your system, a normal C<stat> is done.
If EXPR is omitted, stats C<$_>.
=item map EXPR,LIST
-Evaluates the BLOCK or EXPR for each element of LIST (locally setting C<$_> to each
-element) and returns the list value composed of the results of each such
-evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
-may produce zero, one, or more elements in the returned value.
+Evaluates the BLOCK or EXPR for each element of LIST (locally setting
+C<$_> to each element) and returns the list value composed of the
+results of each such evaluation. In scalar context, returns the
+total number of elements so generated. Evaluates BLOCK or EXPR in
+list context, so each element of LIST may produce zero, one, or
+more elements in the returned value.
@chars = map(chr, @nums);
$hash{getkey($_)} = $_;
}
-Note that, because C<$_> is a reference into the list value, it can be used
-to modify the elements of the array. While this is useful and
-supported, it can cause bizarre results if the LIST is not a named
-array. See also L</grep> for an array composed of those items of the
-original list for which the BLOCK or EXPR evaluates to true.
+Note that, because C<$_> is a reference into the list value, it can
+be used to modify the elements of the array. While this is useful and
+supported, it can cause bizarre results if the LIST is not a named array.
+Using a regular C<foreach> loop for this purpose would be clearer in
+most cases. See also L</grep> for an array composed of those items of
+the original list for which the BLOCK or EXPR evaluates to true.
-=item mkdir FILENAME,MODE
+=item mkdir FILENAME,MASK
Creates the directory specified by FILENAME, with permissions
-specified by MODE (as modified by C<umask>). If it succeeds it
-returns TRUE, otherwise it returns FALSE and sets C<$!> (errno).
+specified by MASK (as modified by C<umask>). If it succeeds it
+returns true, otherwise it returns false and sets C<$!> (errno).
-In general, it is better to create directories with permissive MODEs,
+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
-a restrictive MODE and give the user no way to be more permissive.
+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
-C<umask> discusses the choice of MODE in more detail.
+C<umask> discusses the choice of MASK in more detail.
=item msgctl ID,CMD,ARG
first to get the correct constant definitions. If CMD is C<IPC_STAT>,
then ARG must be a variable which 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
-C<IPC::SysV> and C<IPC::Semaphore::Msg> documentation.
+structure. Returns like C<ioctl>: the undefined value for error, C<"0 but
+true"> for zero, or the actual return value otherwise. See also
+C<IPC::SysV> and C<IPC::Semaphore> documentation.
=item msgget KEY,FLAGS
Calls the System V IPC function msgget(2). Returns the message queue
id, or the undefined value if there is an error. See also C<IPC::SysV>
-and C<IPC::SysV::Msg> documentation.
+and C<IPC::Msg> documentation.
=item msgsnd ID,MSG,FLAGS
Calls the System V IPC function msgsnd to send the message MSG to the
message queue ID. MSG must begin with the long integer message type,
-which may be created with C<pack("l", $type)>. Returns TRUE if
-successful, or FALSE if there is an error. See also C<IPC::SysV>
+which may be created with C<pack("l", $type)>. Returns true if
+successful, or false if there is an error. See also C<IPC::SysV>
and C<IPC::SysV::Msg> documentation.
=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
message queue ID into variable VAR with a maximum message size of
SIZE. Note that if a message is received, the message type will be
the first thing in VAR, and the maximum length of VAR is SIZE plus the
-size of the message type. Returns TRUE if successful, or FALSE if
+size of the message type. Returns true if successful, or false if
there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
=item my EXPR
-A C<my()> declares the listed variables to be local (lexically) to the
-enclosing block, file, or C<eval()>. If
+=item my EXPR : ATTRIBUTES
+
+A C<my> declares the listed variables to be local (lexically) to the
+enclosing block, file, or C<eval>. If
more than one value is listed, the list must be placed in parentheses. See
L<perlsub/"Private Variables via my()"> for details.
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 {}>.
+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
+that executes once. Thus C<next> will exit such a block early.
See also L</continue> for an illustration of how C<last>, C<next>, and
C<redo> work.
=item oct
Interprets EXPR as an octal string and returns the corresponding
-value. (If EXPR happens to start off with C<0x>, interprets it as
-a hex string instead.) The following will handle decimal, octal, and
+value. (If EXPR happens to start off with C<0x>, interprets it as a
+hex string. If EXPR starts off with C<0b>, it is interpreted as a
+binary string.) The following will handle decimal, binary, octal, and
hex in the standard Perl or C notation:
$val = oct($val) if $val =~ /^0/;
-If EXPR is omitted, uses C<$_>. This function is commonly used when
-a string such as C<644> needs to be converted into a file mode, for
-example. (Although perl will automatically convert strings into
-numbers as needed, this automatic conversion assumes base 10.)
+If EXPR is omitted, uses C<$_>. To go the other way (produce a number
+in octal), use sprintf() or printf():
+
+ $perms = (stat("filename"))[2] & 07777;
+ $oct_perms = sprintf "%lo", $perms;
+
+The oct() function is commonly used when a string such as C<644> needs
+to be converted into a file mode, for example. (Although perl will
+automatically convert strings into numbers as needed, this automatic
+conversion assumes base 10.)
+
+=item open FILEHANDLE,MODE,EXPR
=item open FILEHANDLE,EXPR
FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
name of the real filehandle wanted. 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 the filename begins with C<'E<lt>'> or nothing, the file is opened for input.
-If the filename begins with C<'E<gt>'>, the file is truncated and opened for
-output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>,
+(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.) See L<perlopentut> for a kinder, gentler explanation of opening
+files.
+
+If MODE is C<'E<lt>'> or nothing, the file is opened for input.
+If MODE is C<'E<gt>'>, the file is truncated and opened for
+output, being created if necessary. If MODE is C<'E<gt>E<gt>'>,
the file is opened for appending, again being created if necessary.
You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
you want both read and write access to the file; thus C<'+E<lt>'> is almost
switch in L<perlrun> for a better approach. The file is created with
permissions of C<0666> modified by the process' C<umask> value.
-The prefix and the filename may be separated with spaces.
These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
C<'w+'>, C<'a'>, and C<'a+'>.
+In the 2-arguments (and 1-argument) form of the call the mode and
+filename should be concatenated (in this order), possibly separated by
+spaces. It is possible to omit the mode if the mode is C<'E<lt>'>.
+
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 See L<perlipc/"Using open() for IPC">
-for more examples of this. (You are not allowed to C<open()> to a command
+C<'|'>, the filename is interpreted as a command which 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"> for alternatives.)
-Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT. Open returns
-nonzero upon success, the undefined value otherwise. If the C<open()>
+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 which pipes output to
+us. In the 2-arguments (and 1-argument) form one should replace dash
+(C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
+for more examples of this. (You are not allowed to C<open> to a command
+that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
+and L<perlipc/"Bidirectional Communication"> for alternatives.)
+
+In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
+and opening C<'E<gt>-'> opens STDOUT.
+
+Open returns
+nonzero upon success, the undefined value otherwise. If the C<open>
involved a pipe, the return value happens to be the pid of the
subprocess.
If you're unfortunate enough to be running Perl on a system that
distinguishes between text files and binary files (modern operating
systems don't care), then you should check out L</binmode> for tips for
-dealing with this. The key distinction between systems that need C<binmode()>
+dealing with this. The key distinction between systems that need C<binmode>
and those that don't is their text file formats. Systems like Unix, MacOS, and
Plan9, which delimit lines with a single character, and which encode that
-character in C as C<"\n">, do not need C<binmode()>. The rest need it.
+character in C as C<"\n">, do not need C<binmode>. The rest need it.
When opening a file, it's usually a bad idea to continue normal execution
-if the request failed, so C<open()> is frequently used in connection with
-C<die()>. Even if C<die()> won't do what you want (say, in a CGI script,
+if the request failed, so C<open> is frequently used in connection with
+C<die>. Even if C<die> won't do what you want (say, in a CGI script,
where you want to make a nicely formatted error message (but there are
modules that can help with that problem)) you should always check
-the return value from opening a file. The infrequent exception is when
+the return value from opening a file. The infrequent exception is when
working with an unopened filehandle is actually what you want to do.
Examples:
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(DBASE, '+<dbase.mine') # open for update
+ open(DBASE, '+<', 'dbase.mine') # open for update
or die "Can't open 'dbase.mine' for update: $!";
- open(ARTICLE, "caesar <$article |") # decrypt article
+ open(DBASE, '+<dbase.mine') # ditto
+ or die "Can't open 'dbase.mine' for update: $!";
+
+ open(ARTICLE, '-|', "caesar <$article") # decrypt article
+ or die "Can't start caesar: $!";
+
+ open(ARTICLE, "caesar <$article |") # ditto
or die "Can't start caesar: $!";
- open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
+ open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
or die "Can't start sort: $!";
# process argument list of files along with any includes
You may also, in the Bourne shell tradition, specify an EXPR beginning
with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
name of a filehandle (or file descriptor, if numeric) to be
-duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>,
-C<+E<gt>E<gt>>, and C<+E<lt>>. The
+duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>,
+C<E<lt>>, C<+E<gt>>, C<+E<gt>E<gt>>, and C<+E<lt>>. The
mode you specify should match the mode of the original filehandle.
(Duping a filehandle does not take into account any existing contents of
-stdio buffers.)
+stdio buffers.) Duping file handles is not yet supported for 3-argument
+open().
+
Here is a script that saves, redirects, and restores STDOUT and
STDERR:
open(OLDOUT, ">&STDOUT");
open(OLDERR, ">&STDERR");
- open(STDOUT, ">foo.out") || die "Can't redirect stdout";
- open(STDERR, ">&STDOUT") || die "Can't dup stdout";
+ open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
+ open(STDERR, ">&STDOUT") || die "Can't dup stdout";
select(STDERR); $| = 1; # make unbuffered
select(STDOUT); $| = 1; # make unbuffered
print STDOUT "stdout 2\n";
print STDERR "stderr 2\n";
-
If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
-equivalent of C's C<fdopen()> of that file descriptor; this is more
+equivalent of C's C<fdopen> of that file descriptor; this is more
parsimonious of file descriptors. For example:
open(FILEHANDLE, "<&=$fd")
-If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then
+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.)
piped open when you want to exercise more control over just how the
pipe command gets executed, such as when you are running setuid, and
don't want to have to scan shell commands for metacharacters.
-The following pairs are more or less equivalent:
+The following triples are more or less equivalent:
open(FOO, "|tr '[a-z]' '[A-Z]'");
- open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
+ open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
+ open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
open(FOO, "cat -n '$file'|");
- open(FOO, "-|") || exec 'cat', '-n', $file;
+ open(FOO, '-|', "cat -n '$file'");
+ open(FOO, '-|') || exec 'cat', '-n', $file;
See L<perlipc/"Safe Pipe Opens"> for more examples of this.
-NOTE: On any operation that may do a fork, any unflushed buffers remain
-unflushed in both processes, which means you may need to set C<$|> to
-avoid duplicate output.
+NOTE: On any operation that may do a fork, all files opened for output
+are flushed before the fork is attempted. 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>.
Closing any piped filehandle causes the parent process to wait for the
child to finish, and returns the status value in C<$?>.
-The filename passed to open will have leading and trailing
+The filename passed to 2-argument (or 1-argument) form of open()
+will have leading and trailing
whitespace deleted, and the normal redirection characters
honored. This property, known as "magic open",
can often be used to good effect. A user could specify a filename of
$filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
open(FH, $filename) or die "Can't open $filename: $!";
-However, to open a file with arbitrary weird characters in it, it's
-necessary to protect any leading and trailing whitespace:
+Use 3-argument form to open a file with arbitrary weird characters in it,
+
+ open(FOO, '<', $file);
+
+otherwise it's necessary to protect any leading and trailing whitespace:
$file =~ s#^(\s)#./$1#;
open(FOO, "< $file\0");
-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. This is
+(this may not work on some bizzare filesystems). One should
+conscientiously choose between the the I<magic> and 3-arguments form
+of open():
+
+ open IN, $ARGV[0];
+
+will allow the user to specify an argument of the form C<"rsh cat file |">,
+but will not work on a filename which happens to have a trailing space, while
+
+ open IN, '<', $ARGV[0];
+
+will have exactly the opposite restrictions.
+
+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;
$first; # Or here.
}
-See L</seek()> for some details about mixing reading and writing.
+See L</seek> for some details about mixing reading and writing.
=item opendir DIRHANDLE,EXPR
-Opens a directory named EXPR for processing by C<readdir()>, C<telldir()>,
-C<seekdir()>, C<rewinddir()>, and C<closedir()>. Returns TRUE if successful.
+Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
+C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
DIRHANDLEs have their own namespace separate from FILEHANDLEs.
=item ord EXPR
Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
+See L<utf8> for more about Unicode.
+
+=item our EXPR
+
+An C<our> declares the listed variables to be valid globals within
+the enclosing block, file, or C<eval>. That is, it has the same
+scoping rules as a "my" declaration, but does not create a local
+variable. If more than one value is listed, the list must be placed
+in parentheses. The C<our> declaration has no semantic effect unless
+"use strict vars" is in effect, in which case it lets you use the
+declared global variable without qualifying it with a package name.
+(But only within the lexical scope of the C<our> declaration. In this
+it differs from "use vars", which is package scoped.)
=item pack TEMPLATE,LIST
-Takes an array or list of values and packs it into a binary structure,
+Takes a list of values and packs it into a binary structure,
returning the string containing the structure. The TEMPLATE is a
sequence of characters that give the order and type of values, as
follows:
+ a A string with arbitrary binary data, will be null padded.
A An ascii string, will be space padded.
- a An ascii string, will be null padded.
+ Z A null terminated (asciz) string, will be null padded.
+
b A bit string (ascending bit order, like vec()).
B A bit string (descending bit order).
h A hex string (low nybble first).
s A signed short value.
S An unsigned short value.
(This 'short' is _exactly_ 16 bits, which may differ from
- what a local C compiler calls 'short'.)
+ what a local C compiler calls 'short'. If you want
+ native-length shorts, use the '!' suffix.)
i A signed integer value.
I An unsigned integer value.
l A signed long value.
L An unsigned long value.
(This 'long' is _exactly_ 32 bits, which may differ from
- what a local C compiler calls 'long'.)
+ what a local C compiler calls 'long'. If you want
+ native-length longs, use the '!' suffix.)
n A short in "network" (big-endian) order.
N A long in "network" (big-endian) order.
(These 'shorts' and 'longs' are _exactly_ 16 bits and
_exactly_ 32 bits, respectively.)
+ 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.
+ Causes a fatal error otherwise.)
+
f A single-precision float in the native format.
d A double-precision float in the native format.
X Back up a byte.
@ Null fill to absolute position.
+The following rules apply:
+
+=over 8
+
+=item *
+
Each letter may optionally be followed by a number giving a repeat
-count. With all types except C<"a">, C<"A">, C<"b">, C<"B">, C<"h">, C<"H">, and C<"P"> the
-pack function will gobble up that many values from the LIST. A C<*> for the
-repeat count means to use however many items are left. The C<"a"> and C<"A">
-types gobble just one value, but pack it as a string of length count,
-padding with nulls or spaces as necessary. (When unpacking, C<"A"> strips
-trailing spaces and nulls, but C<"a"> does not.) Likewise, the C<"b"> and C<"B">
-fields pack a string that many bits long. The C<"h"> and C<"H"> fields pack a
-string that many nybbles long. The C<"p"> type packs a pointer to a null-
-terminated string. You are responsible for ensuring the string is not a
-temporary value (which can potentially get deallocated before you get
-around to using the packed result). The C<"P"> packs a pointer to a structure
-of the size indicated by the length. A NULL pointer is created if the
-corresponding value for C<"p"> or C<"P"> is C<undef>.
-Real numbers (floats and doubles) are
-in the native machine format only; due to the multiplicity of floating
-formats around, and the lack of a standard "network" representation, no
-facility for interchange has been made. This means that packed floating
-point data written on one machine may not be readable on another - even if
-both use IEEE floating point arithmetic (as the endian-ness of the memory
-representation is not part of the IEEE spec). Note that Perl uses doubles
-internally for all numeric calculation, and converting from double into
-float and thence back to double again will lose precision (i.e.,
-C<unpack("f", pack("f", $foo)>) will not in general equal C<$foo>).
+count. With all types except C<"a">, C<"A">, C<"Z">, C<"b">, C<"B">, C<"h">,
+C<"H">, and C<"P"> the pack function will gobble up that many values from
+the LIST. A C<*> for the repeat count means to use however many items are
+left.
+
+=item *
+
+The C<"a">, C<"A">, and C<"Z"> types gobble just one value, but pack it as a
+string of length count, padding with nulls or spaces as necessary. When
+unpacking, C<"A"> strips trailing spaces and nulls, C<"Z"> strips everything
+after the first null, and C<"a"> returns data verbatim.
+
+=item *
+
+Likewise, the C<"b"> and C<"B"> fields pack a string that many bits long.
+
+=item *
+
+The C<"h"> and C<"H"> fields pack a string that many nybbles (4-bit groups,
+representable as hexadecimal digits, 0-9a-f) long.
+
+=item *
+
+The C<"p"> type packs a pointer to a null-terminated string. You are
+responsible for ensuring the string is not a temporary value (which can
+potentially get deallocated before you get around to using the packed result).
+The C<"P"> type packs a pointer to a structure of the size indicated by the
+length. A NULL pointer is created if the corresponding value for C<"p"> or
+C<"P"> is C<undef>.
+
+=item *
+
+The C<"/"> character allows packing and unpacking of strings where the
+packed structure contains a byte count followed by the string itself.
+You write I<length-item>C</>I<string-item>.
+
+The I<length-item> can be any C<pack> template letter,
+and describes how the length value is packed.
+The ones likely to be of most use are integer-packing ones like
+C<"n"> (for Java strings), C<"w"> (for ASN.1 or SNMP)
+and C<"N"> (for Sun XDR).
+
+The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
+For C<unpack> the length of the string is obtained from the I<length-item>,
+but if you put in the '*' it will be ignored.
+
+ unpack 'C/a', "\04Gurusamy"; gives 'Guru'
+ unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
+ pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
+
+The I<length-item> is not returned explicitly from C<unpack>.
+
+Adding a count to the I<length-item> letter
+is unlikely to do anything useful,
+unless that letter is C<"A">, C<"a"> or C<"Z">.
+Packing with a I<length-item> of C<"a"> or C<"Z">
+may introduce C<"\000"> characters,
+which Perl does not regard as legal in numeric strings.
+
+=item *
+
+The integer types C<"s">, C<"S">, C<"l">, and C<"L"> may be
+immediately followed by a C<"!"> suffix to signify native shorts or
+longs--as you can see from above for example a bare C<"l"> does mean
+exactly 32 bits, the native C<long> (as seen by the local C compiler)
+may be larger. This is an issue mainly in 64-bit platforms. You can
+see whether using C<"!"> makes any difference by
+
+ print length(pack("s")), " ", length(pack("s!")), "\n";
+ print length(pack("l")), " ", length(pack("l!")), "\n";
+
+C<"i!"> and C<"I!"> also work but only because of completeness;
+they are identical to C<"i"> and C<"I">.
+
+The actual sizes (in bytes) of native shorts, ints, longs, and long
+longs on the platform where Perl was built are also available via
+L<Config>:
+
+ use Config;
+ print $Config{shortsize}, "\n";
+ print $Config{intsize}, "\n";
+ print $Config{longsize}, "\n";
+ print $Config{longlongsize}, "\n";
+
+(The C<$Config{longlongsize}> will be undefine if your system does
+not support long longs.)
+
+=item *
+
+The integer formats C<"s">, C<"S">, C<"i">, C<"I">, C<"l">, and C<"L">
+are inherently non-portable between processors and operating systems
+because they obey the native byteorder and endianness. For example a
+4-byte integer 0x12345678 (305419896 decimal) be ordered natively
+(arranged in and handled by the CPU registers) into bytes as
+
+ 0x12 0x34 0x56 0x78 # little-endian
+ 0x78 0x56 0x34 0x12 # big-endian
+
+Basically, the Intel, Alpha, and VAX CPUs and little-endian, while
+everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
+Power, and Cray are big-endian. MIPS can be either: Digital used it
+in little-endian mode; SGI uses it in big-endian mode.
+
+The names `big-endian' and `little-endian' are comic references to
+the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
+Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
+the egg-eating habits of the Lilliputians.
+
+Some systems may have even weirder byte orders such as
+
+ 0x56 0x78 0x12 0x34
+ 0x34 0x12 0x78 0x56
+
+You can see your system's preference with
+
+ print join(" ", map { sprintf "%#02x", $_ }
+ unpack("C*",pack("L",0x12345678))), "\n";
+
+The byteorder on the platform where Perl was built is also available
+via L<Config>:
+
+ use Config;
+ print $Config{byteorder}, "\n";
+
+Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
+and C<'87654321'> are big-endian.
+
+If you want portable packed integers use the formats C<"n">, C<"N">,
+C<"v">, and C<"V">, their byte endianness and size is known.
+See also L<perlport>.
+
+=item *
+
+Real numbers (floats and doubles) are in the native machine format only;
+due to the multiplicity of floating formats around, and the lack of a
+standard "network" representation, no facility for interchange has been
+made. This means that packed floating point data written on one machine
+may not be readable on another - even if both use IEEE floating point
+arithmetic (as the endian-ness of the memory representation is not part
+of the IEEE spec). See also L<perlport>.
+
+Note that Perl uses doubles internally for all numeric calculation, and
+converting from double into float and thence back to double again will
+lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
+equal $foo).
+
+=item *
+
+You must yourself do any alignment or padding by inserting for example
+enough C<'x'>es while packing. There is no way to pack() and unpack()
+could know where the bytes are going to or coming from. Therefore
+C<pack> (and C<unpack>) handle their output and input as flat
+sequences of bytes.
+
+=item *
+
+A comment in a TEMPLATE starts with C<#> and goes to the end of line.
+
+=back
Examples:
$foo = pack("ccxxcc",65,66,67,68);
# foo eq "AB\0\0CD"
+ # note: the above examples featuring "C" and "c" are true
+ # only on ASCII and ASCII-derived systems such as ISO Latin 1
+ # and UTF-8. In EBCDIC the first example would be
+ # $foo = pack("CCCC",193,194,195,196);
+
$foo = pack("s2",1,2);
# "\1\0\2\0" on little-endian
# "\0\1\0\2" on big-endian
$foo = pack("i9pl", gmtime);
# a real struct tm (on my system anyway)
+ $utmp_template = "Z8 Z8 Z16 L";
+ $utmp = pack($utmp_template, @utmp1);
+ # a struct utmp (BSDish)
+
+ @utmp2 = unpack($utmp_template, $utmp);
+ # "@utmp1" eq "@utmp2"
+
sub bintodec {
unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
}
-The same template may generally also be used in the unpack function.
+ $foo = pack('sx2l', 12, 34);
+ # short 12, two zero bytes padding, long 34
+ $bar = pack('s@4l', 12, 34);
+ # short 12, zero fill to position 4, long 34
+ # $foo eq $bar
+
+The same template may generally also be used in unpack().
=item package
=item package NAMESPACE
Declares the compilation unit as being in the given namespace. The scope
-of the package declaration is from the declaration itself through the end of
-the enclosing block (the same scope as the C<local()> operator). All further
-unqualified dynamic identifiers will be in this namespace. A package
-statement affects only dynamic variables--including those you've used
-C<local()> on--but I<not> lexical variables created with C<my()>. Typically it
-would be the first declaration in a file to be included by the C<require>
-or C<use> operator. You can switch into a package in more than one place;
-it merely influences which symbol table is used by the compiler for the
-rest of that block. You can refer to variables and filehandles in other
-packages by prefixing the identifier with the package name and a double
-colon: C<$Package::Variable>. If the package name is null, the C<main>
-package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
+of the package declaration is from the declaration itself through the end
+of the enclosing block, file, or eval (the same as the C<my> operator).
+All further unqualified dynamic identifiers will be in this namespace.
+A package statement affects only dynamic variables--including those
+you've used C<local> on--but I<not> lexical variables, which are created
+with C<my>. Typically it would be the first declaration in a file to
+be included by the C<require> or C<use> operator. You can switch into a
+package in more than one place; it merely influences which symbol table
+is used by the compiler for the rest of that block. You can refer to
+variables and filehandles in other packages by prefixing the identifier
+with the package name and a double colon: C<$Package::Variable>.
+If the package name is null, the C<main> package as assumed. That is,
+C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
+still seen in older code).
If NAMESPACE is omitted, then there is no current package, and all
identifiers must be fully qualified or lexicals. This is stricter
See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
for examples of such things.
+On systems that support a close-on-exec flag on files, the flag will be set
+for the newly opened file descriptors as determined by the value of $^F.
+See L<perlvar/$^F>.
+
=item pop ARRAY
=item pop
Pops and returns the last value of the array, shortening the array by
-1. Has a similar effect to
+one element. Has an effect similar to
- $tmp = $ARRAY[$#ARRAY--];
+ $ARRAY[$#ARRAY--]
-If there are no elements in the array, returns the undefined value.
-If ARRAY is omitted, pops the
-C<@ARGV> array in the main program, and the C<@_> array in subroutines, just
-like C<shift()>.
+If there are no elements in the array, returns the undefined value
+(although this may happen at other times as well). If ARRAY is
+omitted, pops the C<@ARGV> array in the main program, and the C<@_>
+array in subroutines, just like C<shift>.
=item pos SCALAR
=item print
-Prints a string or a comma-separated list of strings. Returns TRUE
-if successful. FILEHANDLE may be a scalar variable name, in which case
-the variable contains the name of or a reference to the filehandle, thus introducing one
-level of indirection. (NOTE: If FILEHANDLE is a variable and the next
-token is a term, it may be misinterpreted as an operator unless you
-interpose a C<+> or put parentheses around the arguments.) If FILEHANDLE is
-omitted, prints by default to standard output (or to the last selected
-output channel--see L</select>). If LIST is also omitted, prints C<$_> to
-the currently selected output channel. To set the default output channel to something other than
-STDOUT use the select operation. Note that, because print takes a
-LIST, anything in the LIST is evaluated in list context, and any
-subroutine that you call will have one or more of its expressions
-evaluated in list context. Also be careful not to follow the print
-keyword with a left parenthesis unless you want the corresponding right
-parenthesis to terminate the arguments to the print--interpose a C<+> or
-put parentheses around all the arguments.
+Prints a string or a list of strings. Returns true if successful.
+FILEHANDLE may be a scalar variable name, in which case the variable
+contains the name of or a reference to the filehandle, thus introducing
+one level of indirection. (NOTE: If FILEHANDLE is a variable and
+the next token is a term, it may be misinterpreted as an operator
+unless you interpose a C<+> or put parentheses around the arguments.)
+If FILEHANDLE is omitted, prints by default to standard output (or
+to the last selected output channel--see L</select>). If LIST is
+also omitted, prints C<$_> to the currently selected output channel.
+To set the default output channel to something other than STDOUT
+use the select operation. The current value of C<$,> (if any) is
+printed between each LIST item. The current value of C<$\> (if
+any) is printed after the entire LIST has been printed. Because
+print takes a LIST, anything in the LIST is evaluated in list
+context, and any subroutine that you call will have one or more of
+its expressions evaluated in list context. Also be careful not to
+follow the print keyword with a left parenthesis unless you want
+the corresponding right parenthesis to terminate the arguments to
+the print--interpose a C<+> or put parentheses around all the
+arguments.
Note that if you're storing FILEHANDLES in an array or other expression,
you will have to use a block returning its value instead:
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. If C<use locale> is
+of the list will be interpreted as the C<printf> format. If C<use locale> is
in effect, the character used for the decimal point in formatted real numbers
is affected by the LC_NUMERIC locale. See L<perllocale>.
-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
+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
error prone.
=item prototype FUNCTION
function has no prototype). FUNCTION is a reference to, or the name of,
the function whose prototype you want to retrieve.
-If FUNCTION is a string starting with C<CORE::>, the rest is taken as
-a name for Perl builtin. If builtin is not I<overridable> (such as
+If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
+name for Perl builtin. If the builtin is not I<overridable> (such as
C<qw//>) or its arguments cannot be expressed by a prototype (such as
-C<system()>) - in other words, the builtin does not behave like a Perl
-function - returns C<undef>. Otherwise, the string describing the
-equivalent prototype is returned.
+C<system>) returns C<undef> because the builtin does not really behave
+like a Perl function. Otherwise, the string describing the equivalent
+prototype is returned.
=item push ARRAY,LIST
=item qw/STRING/
-Generalized quotes. See L<perlop>.
+Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
=item quotemeta EXPR
Returns a random fractional number greater than or equal to C<0> and less
than the value of EXPR. (EXPR should be positive.) If EXPR is
-omitted, the value C<1> is used. Automatically calls C<srand()> unless
-C<srand()> has already been called. See also C<srand()>.
+omitted, the value C<1> is used. Automatically calls C<srand> unless
+C<srand> has already been called. See also C<srand>.
(Note: If your rand function consistently returns numbers that are too
large or too small, then your version of Perl was probably compiled
or shrunk to the length actually read. An OFFSET may be specified to
place the read data at some other place than the beginning of the
string. This call is actually implemented in terms of stdio's fread(3)
-call. To get a true read(2) system call, see C<sysread()>.
+call. To get a true read(2) system call, see C<sysread>.
=item readdir DIRHANDLE
-Returns the next directory entry for a directory opened by C<opendir()>.
+Returns the next directory entry for a directory opened by C<opendir>.
If used in list context, returns all the rest of the entries in the
directory. If there are no more entries, returns an undefined value in
scalar context or a null list in list context.
-If you're planning to filetest the return values out of a C<readdir()>, you'd
+If you're planning to filetest the return values out of a C<readdir>, you'd
better prepend the directory in question. Otherwise, because we didn't
-C<chdir()> there, it would have been testing the wrong file.
+C<chdir> there, it would have been testing the wrong file.
opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
@dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
=item readline EXPR
-Reads from the filehandle whose typeglob is contained in EXPR. In scalar context, a single line
-is read and returned. In list context, reads until end-of-file is
-reached and returns a list of lines (however you've defined lines
-with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
+Reads from the filehandle whose typeglob is contained in EXPR. In scalar
+context, each call reads and returns the next line, until end-of-file is
+reached, whereupon the subsequent call returns undef. In list context,
+reads until end-of-file is reached and returns a list of lines. Note that
+the notion of "line" used here is however you may have defined it
+with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
+
+When C<$/> is set to C<undef>, when readline() is in scalar
+context (i.e. file slurp mode), and when an empty file is read, it
+returns C<''> the first time, followed by C<undef> subsequently.
+
This is the internal function implementing the C<E<lt>EXPRE<gt>>
operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
operator is discussed in more detail in L<perlop/"I/O Operators">.
operator, but you can use it directly. The C<qx/EXPR/>
operator is discussed in more detail in L<perlop/"I/O Operators">.
-=item recv SOCKET,SCALAR,LEN,FLAGS
+=item recv SOCKET,SCALAR,LENGTH,FLAGS
Receives a message on a socket. Attempts to receive LENGTH bytes of
data into variable SCALAR from the specified SOCKET filehandle.
-Actually does a C C<recvfrom()>, so that it can return the address of the
+Actually does a C C<recvfrom>, so that it can return the address of the
sender. Returns the undefined value if there's an error. SCALAR will
be grown or shrunk to the length actually read. Takes the same flags
as the system call of the same name.
}
C<redo> cannot be used to retry a block which returns a value such as
-C<eval {}>, C<sub {}> or C<do {}>.
+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
+that executes once. Thus C<redo> inside such a block will effectively
+turn it into a looping construct.
See also L</continue> for an illustration of how C<last>, C<next>, and
C<redo> work.
=item ref
-Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
+Returns a true value if EXPR is a reference, false otherwise. If EXPR
is not specified, C<$_> will be used. The value returned depends on the
type of thing the reference is a reference to.
Builtin types include:
- REF
SCALAR
ARRAY
HASH
CODE
+ REF
GLOB
+ LVALUE
If the referenced object has been blessed into a package, then that package
-name is returned instead. You can think of C<ref()> as a C<typeof()> operator.
+name is returned instead. You can think of C<ref> as a C<typeof> operator.
if (ref($r) eq "HASH") {
print "r is a reference to a hash.\n";
}
- if (!ref($r)) {
+ unless (ref($r)) {
print "r is not a reference at all.\n";
}
+ if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
+ print "r is a reference to something that isa hash.\n";
+ }
See also L<perlref>.
=item rename OLDNAME,NEWNAME
-Changes the name of a file. Returns C<1> for success, C<0> otherwise. Will
-not work across file system boundaries.
+Changes the name of a file; an existing file NEWNAME will be
+clobbered. Returns true for success, false otherwise.
+
+Behavior of this function varies wildly depending on your system
+implementation. For example, it will usually not work across file system
+boundaries, even though the system I<mv> command sometimes compensates
+for this. Other restrictions include whether it works on directories,
+open files, or pre-existing files. Check L<perlport> and either the
+rename(2) manpage or equivalent system documentation for details.
=item require EXPR
Otherwise, demands that a library file be included if it hasn't already
been included. The file is included via the do-FILE mechanism, which is
-essentially just a variety of C<eval()>. Has semantics similar to the following
+essentially just a variety of C<eval>. Has semantics similar to the following
subroutine:
sub require {
}
Note that the file will not be included twice under the same specified
-name. The file must return TRUE as the last statement to indicate
+name. The file must return true as the last statement to indicate
successful execution of any initialization code, so it's customary to
-end such a file with "C<1;>" unless you're sure it'll return TRUE
-otherwise. But it's better just to put the "C<1;>", in case you add more
+end such a file with C<1;> unless you're sure it'll return true
+otherwise. But it's better just to put the C<1;>, in case you add more
statements.
If EXPR is a bareword, the require assumes a "F<.pm>" extension and
require "Foo::Bar"; # not a bareword because of the ""
The require function will look for the "F<Foo::Bar>" file in the @INC array and
-will complain about not finding "F<Foo::Bar>" there. In this case you can do:
+will complain about not finding "F<Foo::Bar>" there. In this case you can do:
eval "require $class";
reset 'X'; # reset all X variables
reset 'a-z'; # reset lower case variables
- reset; # just reset ?? searches
+ reset; # just reset ?one-time? searches
Resetting C<"A-Z"> is not recommended because you'll wipe out your
-C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package variables--lexical variables
-are unaffected, but they clean themselves up on scope exit anyway,
-so you'll probably want to use them instead. See L</my>.
+C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
+variables--lexical variables are unaffected, but they clean themselves
+up on scope exit anyway, so you'll probably want to use them instead.
+See L</my>.
=item return EXPR
=item return
-Returns from a subroutine, C<eval()>, or C<do FILE> with the value
+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
-is given, returns an empty list in list context, an undefined value in
-scalar context, or nothing in a void context.
+may vary from one execution to the next (see C<wantarray>). If no EXPR
+is given, returns an empty list in list context, the undefined value in
+scalar context, and (of course) nothing at all in a void context.
-(Note that in the absence of a return, a subroutine, eval, or do FILE
-will automatically return the value of the last expression evaluated.)
+(Note that in the absence of a explicit C<return>, a subroutine, eval,
+or do FILE will automatically return the value of the last expression
+evaluated.)
=item reverse LIST
In list context, returns a list value consisting of the elements
of LIST in the opposite order. In scalar context, concatenates the
-elements of LIST, and returns a string value with all the characters
+elements of LIST and returns a string value with all characters
in the opposite order.
print reverse <>; # line tac, last line first
caveats. If a value is duplicated in the original hash, only one of those
can be represented as a key in the inverted hash. Also, this has to
unwind one hash and build a whole new one, which may take some time
-on a large hash.
+on a large hash, such as from a DBM file.
%by_name = reverse %by_address; # Invert the hash
=item rewinddir DIRHANDLE
Sets the current position to the beginning of the directory for the
-C<readdir()> routine on DIRHANDLE.
+C<readdir> routine on DIRHANDLE.
=item rindex STR,SUBSTR,POSITION
=item rindex STR,SUBSTR
-Works just like index except that it returns the position of the LAST
+Works just like index() except that it returns the position of the LAST
occurrence of SUBSTR in STR. If POSITION is specified, returns the
last occurrence at or before that position.
=item rmdir
Deletes the directory specified by FILENAME if that directory is empty. If it
-succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). If
+succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
FILENAME is omitted, uses C<$_>.
=item s///
@counts = ( scalar @a, scalar @b, scalar @c );
There is no equivalent operator to force an expression to
-be interpolated in list context because it's in practice never
+be interpolated in list context because in practice, this is never
needed. If you really wanted to do so, however, you could use
the construction C<@{[ (some expression) ]}>, but usually a simple
C<(some expression)> suffices.
+Because C<scalar> is unary operator, if you accidentally use for EXPR a
+parenthesized list, this behaves as a scalar comma expression, evaluating
+all but the last element in void context and returning the final element
+evaluated in scalar context. This is seldom what you want.
+
+The following single statement:
+
+ print uc(scalar(&foo,$bar)),$baz;
+
+is the moral equivalent of these two:
+
+ &foo;
+ print(uc($bar),$baz);
+
+See L<perlop> for more details on unary operators and the comma operator.
+
=item seek FILEHANDLE,POSITION,WHENCE
-Sets FILEHANDLE's position, just like the C<fseek()> call of C<stdio()>.
+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 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> from either the
-C<IO::Seekable> or the POSIX module. Returns C<1> upon success, C<0> otherwise.
-
-If you want to position file for C<sysread()> or C<syswrite()>, don't use
-C<seek()> -- buffering makes its effect on the file's system position
-unpredictable and non-portable. Use C<sysseek()> instead.
-
-On some systems you have to do a seek whenever you switch between reading
-and writing. Amongst other things, this may have the effect of calling
-stdio's clearerr(3). A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving
-the file position:
+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 any of the
+modules Fcntl, C<IO::Seekable>, or POSIX. Returns C<1> upon success,
+C<0> otherwise.
+
+If you want to position file for C<sysread> or C<syswrite>, don't use
+C<seek>--buffering makes its effect on the file's system position
+unpredictable and non-portable. Use C<sysseek> instead.
+
+Due to the rules and rigors of ANSI C, on some systems you have to do a
+seek whenever you switch between reading and writing. Amongst other
+things, this may have the effect of calling stdio's clearerr(3).
+A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
seek(TEST,0,1);
This is also useful for applications emulating C<tail -f>. Once you hit
EOF on your read, and then sleep for a while, you might have to stick in a
-seek() to reset things. The C<seek()> doesn't change the current position,
+seek() to reset things. The C<seek> doesn't change the current position,
but it I<does> clear the end-of-file condition on the handle, so that the
next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
=item seekdir DIRHANDLE,POS
-Sets the current position for the C<readdir()> routine on DIRHANDLE. POS
-must be a value returned by C<telldir()>. Has the same caveats about
+Sets the current position for the C<readdir> routine on DIRHANDLE. POS
+must be a value returned by C<telldir>. Has the same caveats about
possible directory compaction as the corresponding system library
routine.
Returns the currently selected filehandle. Sets the current default
filehandle for output, if FILEHANDLE is supplied. 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 will
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
=item select RBITS,WBITS,EBITS,TIMEOUT
This calls the select(2) system call with the bit masks specified, which
-can be constructed using C<fileno()> and C<vec()>, along these lines:
+can be constructed using C<fileno> and C<vec>, along these lines:
$rin = $win = $ein = '';
vec($rin,fileno(STDIN),1) = 1;
$nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
-Most systems do not bother to return anything useful in C<$timeleft>, so
-calling select() in scalar context just returns C<$nfound>.
+Most systems do not bother to return anything useful in $timeleft, so
+calling select() in scalar context just returns $nfound.
Any of the bit masks can also be undef. The timeout, if specified, is
in seconds, which may be fractional. Note: not all implementations are
-capable of returning theC<$timeleft>. If not, they always return
-C<$timeleft> equal to the supplied C<$timeout>.
+capable of returning the$timeleft. If not, they always return
+$timeleft equal to the supplied $timeout.
You can effect a sleep of 250 milliseconds this way:
select(undef, undef, undef, 0.25);
-B<WARNING>: One should not attempt to mix buffered I/O (like C<read()>
-or E<lt>FHE<gt>) with C<select()>, except as permitted by POSIX, and even
-then only on POSIX systems. You have to use C<sysread()> instead.
+B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
+or E<lt>FHE<gt>) with C<select>, except as permitted by POSIX, and even
+then only on POSIX systems. You have to use C<sysread> instead.
=item semctl ID,SEMNUM,CMD,ARG
-Calls the System V IPC function C<semctl()>. You'll probably have to say
+Calls the System V IPC function C<semctl>. You'll probably have to say
use IPC::SysV;
first to get the correct constant definitions. If CMD is IPC_STAT or
GETALL, then ARG must be a variable which will hold the returned
-semid_ds structure or semaphore value array. Returns like C<ioctl()>: the
-undefined value for error, "C<0> but true" for zero, or the actual return
+semid_ds structure or semaphore value array. Returns like C<ioctl>: the
+undefined value for error, "C<0 but true>" for zero, or the actual return
value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
=item semget KEY,NSEMS,FLAGS
such as signaling and waiting. OPSTRING must be a packed array of
semop structures. Each semop structure can be generated with
C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
-operations is implied by the length of OPSTRING. Returns TRUE if
-successful, or FALSE if there is an error. As an example, the
-following code waits on semaphore C<$semnum> of semaphore id C<$semid>:
+operations is implied by the length of OPSTRING. Returns true if
+successful, or false if there is an error. As an example, the
+following code waits on semaphore $semnum of semaphore id $semid:
$semop = pack("sss", $semnum, -1, 0);
die "Semaphore trouble: $!\n" unless semop($semid, $semop);
Sends a message on a socket. Takes the same flags as the system call
of the same name. On unconnected sockets you must specify a
-destination to send TO, in which case it does a C C<sendto()>. Returns
+destination to send TO, in which case it does a C C<sendto>. Returns
the number of characters sent, or the undefined value if there is an
-error.
+error. The C system call sendmsg(2) is currently unimplemented.
See L<perlipc/"UDP: Message Passing"> for examples.
=item setpgrp PID,PGRP
Sets the current process group for the specified PID, C<0> for the current
process. Will produce a fatal error if used on a machine that doesn't
implement setpgrp(2). If the arguments are omitted, it defaults to
-C<0,0>. Note that the POSIX version of C<setpgrp()> does not accept any
-arguments, so only setpgrp C<0,0> is portable.
+C<0,0>. Note that the POSIX version of C<setpgrp> does not accept any
+arguments, so only C<setpgrp(0,0)> is portable. See also C<POSIX::setsid()>.
=item setpriority WHICH,WHO,PRIORITY
C<@_> array within the lexical scope of subroutines and formats, and the
C<@ARGV> array at file scopes or within the lexical scopes established by
the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
-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
+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
right end.
=item shmctl ID,CMD,ARG
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.
-See also C<IPC::SysV> documentation.
+SIZE bytes. Return true if successful, or false if there is an error.
+See also C<IPC::SysV> documentation and the C<IPC::Shareable> module
+from CPAN.
=item shutdown SOCKET,HOW
This is useful with sockets when you want to tell the other
side you're done writing but not done reading, or vice versa.
It's also a more insistent form of close because it also
-disables the filedescriptor in any forked copies in other
+disables the file descriptor in any forked copies in other
processes.
=item sin EXPR
Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
returns sine of C<$_>.
-For the inverse sine operation, you may use the C<POSIX::asin()>
+For the inverse sine operation, you may use the C<POSIX::asin>
function, or use this relation:
sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
Causes the script to sleep for EXPR seconds, or forever if no EXPR.
May be interrupted if the process receives a signal such as C<SIGALRM>.
Returns the number of seconds actually slept. You probably cannot
-mix C<alarm()> and C<sleep()> calls, because C<sleep()> is often implemented
-using C<alarm()>.
+mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
+using C<alarm>.
On some older systems, it may sleep up to a full second less than what
you requested, depending on how it counts seconds. Most modern systems
busy multitasking system.
For delays of finer granularity than one second, you may use Perl's
-C<syscall()> interface to access setitimer(2) if your system supports it,
-or else see L</select()> above.
+C<syscall> interface to access setitimer(2) if your system supports
+it, or else see L</select> above. The Time::HiRes module from CPAN
+may also help.
-See also the POSIX module's C<sigpause()> function.
+See also the POSIX module's C<sigpause> function.
=item socket SOCKET,DOMAIN,TYPE,PROTOCOL
Opens a socket of the specified kind and attaches it to filehandle
-SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
-system call of the same name. You should "C<use Socket;>" first to get
-the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
+SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
+the system call of the same name. You should C<use Socket> first
+to get the proper definitions imported. See the examples in
+L<perlipc/"Sockets: Client/Server Communication">.
=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
Creates an unnamed pair of sockets in the specified domain, of the
specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
for the system call of the same name. If unimplemented, yields a fatal
-error. Returns TRUE if successful.
+error. Returns true if successful.
-Some systems defined C<pipe()> in terms of C<socketpair()>, in which a call
+Some systems defined C<pipe> in terms of C<socketpair>, in which a call
to C<pipe(Rdr, Wtr)> is essentially:
use Socket;
=item sort LIST
Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
-is omitted, C<sort()>s in standard string comparison order. If SUBNAME is
+is omitted, C<sort>s in standard string comparison order. If SUBNAME is
specified, it gives the name of a subroutine that returns an integer
less than, equal to, or greater than C<0>, depending on how the elements
-of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
+of the list are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
operators are extremely useful in such routines.) SUBNAME may be a
scalar variable name (unsubscripted), in which case the value provides
the name of (or a reference to) the actual subroutine to use. In place
In the interests of efficiency the normal calling code for subroutines is
bypassed, with the following effects: the subroutine may not be a
recursive subroutine, and the two elements to be compared are passed into
-the subroutine not via C<@_> but as the package global variables C<$a> and
-C<$b> (see example below). They are passed by reference, so don't
-modify C<$a> and C<$b>. And don't try to declare them as lexicals either.
+the subroutine not via C<@_> but as the package global variables $a and
+$b (see example below). They are passed by reference, so don't
+modify $a and $b. And don't try to declare them as lexicals either.
You also cannot exit out of the sort block or subroutine using any of the
-loop control operators described in L<perlsyn> or with C<goto()>.
+loop control operators described in L<perlsyn> or with C<goto>.
When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
current collation locale. See L<perllocale>.
# sort numerically descending
@articles = sort {$b <=> $a} @files;
+ # this sorts the %age hash by value instead of key
+ # using an in-line function
+ @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
+
# sort using explicit subroutine name
sub byage {
$age{$a} <=> $age{$b}; # presuming numeric
}
@sortedclass = sort byage @class;
- # this sorts the %age hash by value instead of key
- # using an in-line function
- @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
-
- sub backwards { $b cmp $a; }
- @harry = ('dog','cat','x','Cain','Abel');
- @george = ('gone','chased','yz','Punished','Axed');
+ sub backwards { $b cmp $a }
+ @harry = qw(dog cat x Cain Abel);
+ @george = qw(gone chased yz Punished Axed);
print sort @harry;
# prints AbelCaincatdogx
print sort backwards @harry;
} 0..$#old
];
- # same thing using a Schwartzian Transform (no temps)
+ # same thing, but without any temps
@new = map { $_->[0] }
- sort { $b->[1] <=> $a->[1]
- ||
- $a->[2] cmp $b->[2]
- } map { [$_, /=(\d+)/, uc($_)] } @old;
+ sort { $b->[1] <=> $a->[1]
+ ||
+ $a->[2] cmp $b->[2]
+ } map { [$_, /=(\d+)/, uc($_)] } @old;
-If you're using strict, you I<MUST NOT> declare C<$a>
-and C<$b> as lexicals. They are package globals. That means
+If you're using strict, you I<must not> declare $a
+and $b as lexicals. They are package globals. That means
if you're in the C<main> package, it's
@articles = sort {$main::b <=> $main::a} @files;
returns the elements removed from the array. In scalar context,
returns the last element removed, or C<undef> if no elements are
removed. The array grows or shrinks as necessary.
-If OFFSET is negative then it start that far from the end of the array.
+If OFFSET is negative then it starts that far from the end of the array.
If LENGTH is omitted, removes everything from OFFSET onward.
If LENGTH is negative, leave that many elements off the end of the array.
The following equivalences hold (assuming C<$[ == 0>):
=item split
-Splits a string into an array of strings, and returns it. By default,
+Splits a string into a list of strings and returns that list. By default,
empty leading fields are preserved, and empty trailing ones are deleted.
If not in list context, returns the number of fields found and splits into
If LIMIT is specified and positive, splits into no more than that
many fields (though it may split into fewer). If LIMIT is unspecified
or zero, trailing null fields are stripped (which potential users
-of C<pop()> would do well to remember). If LIMIT is negative, it is
+of C<pop> would do well to remember). If LIMIT is negative, it is
treated as if an arbitrarily large LIMIT had been specified.
A pattern matching the null string (not to be confused with
default. In time critical applications it behooves you not to split
into more fields than you really need.
-If the PATTERN contains parentheses, additional array elements are
+If the PATTERN contains parentheses, additional list elements are
created from each matching substring in the delimiter.
split(/([,-])/, "1-10,20", 3);
(1, '-', 10, ',', 20)
-If you had the entire header of a normal Unix email message in C<$header>,
+If you had the entire header of a normal Unix email message in $header,
you could split it up into fields and their values this way:
$header =~ s/\n\s+/ /g; # fix continuation lines
use C</$variable/o>.)
As a special case, specifying a PATTERN of space (C<' '>) will split on
-white space just as C<split()> with no arguments does. Thus, C<split(' ')> can
+white space just as C<split> with no arguments does. Thus, C<split(' ')> can
be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
will give you as many null initial fields as there are leading spaces.
-A C<split()> on C</\s+/> is like a C<split(' ')> except that any leading
-whitespace produces a null first field. A C<split()> with no arguments
+A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
+whitespace produces a null first field. A C<split> with no arguments
really does a C<split(' ', $_)> internally.
Example:
#...
}
-(Note that C<$shell> above will still have a newline on it. See L</chop>,
+(Note that $shell above will still have a newline on it. See L</chop>,
L</chomp>, and L</join>.)
=item sprintf FORMAT, LIST
-Returns a string formatted by the usual C<printf()> conventions of the
-C library function C<sprintf
()>. See L<sprintf(3)> or L<printf(3)>
+Returns a string formatted by the usual C<printf> conventions of the
+C library function C<sprintf>. See L<sprintf(3)> or L<printf(3)>
on your system for an explanation of the general principles.
-Perl does its own C<sprintf()> formatting -- it emulates the C
-function C<sprintf()>, but it doesn't use it (except for floating-point
+Perl does its own C<sprintf> formatting--it emulates the C
+function C<sprintf>, but it doesn't use it (except for floating-point
numbers, and even then only the standard modifiers are allowed). As a
-result, any non-standard extensions in your local C<sprintf()> are not
+result, any non-standard extensions in your local C<sprintf> are not
available from Perl.
-Perl's C<sprintf()> permits the following universally-known conversions:
+Perl's C<sprintf> permits the following universally-known conversions:
%% a percent sign
%c a character with the given number
%X like %x, but using upper-case letters
%E like %e, but using an upper-case "E"
%G like %g, but with an upper-case "E" (if applicable)
+ %b an unsigned integer, in binary
%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
for integer
l interpret integer as C type "long" or "unsigned long"
h interpret integer as C type "short" or "unsigned short"
+ If no flags, interpret integer as C type "int" or "unsigned"
There is also one Perl-specific flag:
V interpret integer as Perl's standard integer type
-Where a number would appear in the flags, an asterisk ("C<*>") may be
+Where a number would appear in the flags, an asterisk (C<*>) may be
used instead, in which case Perl uses the next item in the parameter
list as the given number (that is, as the field width or precision).
-If a field width obtained through "C<*>" is negative, it has the same
-effect as the "C<->" flag: left-justification.
+If a field width obtained through C<*> is negative, it has the same
+effect as the C<-> flag: left-justification.
If C<use locale> is in effect, the character used for the decimal
point in formatted real numbers is affected by the LC_NUMERIC locale.
See L<perllocale>.
+To cope with broken systems that allow the standard locales to be
+overridden by malicious users, the return value may be tainted
+if any of the floating point formats are used and the conversion
+yields something that doesn't look like a normal C-locale floating
+point number. This happens regardless of whether C<use locale> is
+in effect or not.
+
+If Perl understands "quads" (64-bit integers) (this requires
+either that the platform natively supports quads or that Perl
+has been specifically compiled to support quads), the characters
+
+ d u o x X b i D U O
+
+print quads, and they may optionally be preceded by
+
+ ll L q
+
+For example
+
+ %lld %16LX %qo
+
+You can find out whether your Perl supports quads via L<Config>:
+
+ use Config;
+ ($Config{use64bits} eq 'define' || $Config{longsize} == 8) &&
+ print "quads\n";
+
+If Perl understands "long doubles" (this requires that the platform
+supports long doubles), the flags
+
+ e f g E F G
+
+may optionally be preceded by
+
+ ll L
+
+For example
+
+ %llf %Lg
+
+You can find out whether your Perl supports long doubles via L<Config>:
+
+ use Config;
+ $Config{d_longdbl} eq 'define' && print "long doubles\n";
+
=item sqrt EXPR
=item sqrt
Return the square root of EXPR. If EXPR is omitted, returns square
-root of C<$_>.
+root of C<$_>. Only works on non-negative operands, unless you've
+loaded the standard Math::Complex module.
+
+ use Math::Complex;
+ print sqrt(-2); # prints 1.4142135623731i
=item srand EXPR
=item srand
-Sets the random number seed for the C<rand()> operator. If EXPR is
+Sets the random number seed for the C<rand> operator. If EXPR is
omitted, uses a semi-random value supplied by the kernel (if it supports
the F</dev/urandom> device) or based on the current time and process
ID, among other things. In versions of Perl prior to 5.004 the default
-seed was just the current C<time()>. This isn't a particularly good seed,
+seed was just the current C<time>. This isn't a particularly good seed,
so many old programs supply their own seed value (often C<time ^ $$> or
C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.
-In fact, it's usually not necessary to call C<srand()> at all, because if
+In fact, it's usually not necessary to call C<srand> at all, because if
it is not called explicitly, it is called implicitly at the first use of
-the C<rand()> operator. However, this was not the case in version of Perl
+the C<rand> operator. However, this was not the case in version of Perl
before 5.004, so if your script will run under older Perl versions, it
-should call C<srand()>.
+should call C<srand>.
Note that you need something much more random than the default seed for
cryptographic purposes. Checksumming the compressed output of one or more
If you're particularly concerned with this, see the C<Math::TrulyRandom>
module in CPAN.
-Do I<not> call C<srand()> multiple times in your program unless you know
+Do I<not> call C<srand> multiple times in your program unless you know
exactly what you're doing and why you're doing it. The point of the
-function is to "seed" the C<rand()> function so that C<rand()> can produce
+function is to "seed" the C<rand> function so that C<rand> can produce
a different sequence each time you run your program. Just do it once at the
-top of your program, or you I<won't> get random numbers out of C<rand()>!
+top of your program, or you I<won't> get random numbers out of C<rand>!
Frequently called programs (like CGI scripts) that simply use
(This works on machines only for which the device number is negative under NFS.)
-In scalar context, C<stat()> returns a boolean value indicating success
+Because the mode contains both the file type and its permissions, you
+should mask off the file type portion and (s)printf using a C<"%o">
+if you want to see the real permissions.
+
+ $mode = (stat($filename))[2];
+ printf "Permissions are %04o\n", $mode & 07777;
+
+In scalar context, C<stat> returns a boolean value indicating success
or failure, and, if successful, sets the information associated with
the special filehandle C<_>.
+The File::stat module provides a convenient, by-name access mechanism:
+
+ use File::stat;
+ $sb = stat($filename);
+ printf "File is %s, size is %s, perm %04o, mtime %s\n",
+ $filename, $sb->size, $sb->mode & 07777,
+ scalar localtime $sb->mtime;
+
=item study SCALAR
=item study
doing many pattern matches on the string before it is next modified.
This may or may not save time, depending on the nature and number of
patterns you are searching on, and on the distribution of character
-frequencies in the string to be searched -- you probably want to compare
+frequencies in the string to be searched--you probably want to compare
run times with and without it to see which runs faster. Those loops
which scan for many short constant strings (including the constant
parts of more complex patterns) will benefit most. You may have only
-one C<study()> active at a time -- if you study a different scalar the first
-is "unstudied". (The way C<study()> works is this: a linked list of every
+one C<study> active at a time--if you study a different scalar the first
+is "unstudied". (The way C<study> works is this: a linked list of every
character in the string to be searched is made, so we know, for
example, where all the C<'k'> characters are. From each search string,
the rarest character is selected, based on some static frequency tables
while (<>) {
study;
- print ".IX foo\n" if /\bfoo\b/;
- print ".IX bar\n" if /\bbar\b/;
- print ".IX blurfl\n" if /\bblurfl\b/;
+ print ".IX foo\n" if /\bfoo\b/;
+ print ".IX bar\n" if /\bbar\b/;
+ print ".IX blurfl\n" if /\bblurfl\b/;
# ...
print;
}
first place.
Note that if you have to look for strings that you don't know till
-runtime, you can build an entire loop as a string and C<eval()> that to
+runtime, you can build an entire loop as a string and C<eval> that to
avoid recompiling all your patterns all the time. Together with
undefining C<$/> to input entire files as one record, this can be very
fast, often faster than specialized programs like fgrep(1). The following
=item sub NAME BLOCK
This is subroutine definition, not a real function I<per se>. With just a
-NAME (and possibly prototypes), 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. See L<perlsub> and
-L<perlref> for details.
+NAME (and possibly prototypes or attributes), 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. See L<perlsub>
+
and L<perlref> for details.
-=item substr EXPR,OFFSET,LEN,REPLACEMENT
+=item substr EXPR,OFFSET,LENGTH,REPLACEMENT
-=item substr EXPR,OFFSET,LEN
+=item substr EXPR,OFFSET,LENGTH
=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 LEN is omitted, returns
-everything to the end of the string. If LEN is negative, leaves that
+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
many characters off the end of the string.
-If you specify a substring that is partly outside the string, the part
-within the string is returned. If the substring is totally outside
-the string a warning is produced.
-
-You can use the C<substr()> function
-as an lvalue, in which case EXPR must be an lvalue. If you assign
-something shorter than LEN, the string will shrink, and if you assign
-something longer than LEN, the string will grow to accommodate it. To
-keep the string the same length you may need to pad or chop your value
-using C<sprintf()>.
-
-An alternative to using C<substr()> as an lvalue is to specify the
+You can use the substr() function as an lvalue, in which case EXPR
+must itself be an lvalue. If you assign something shorter than LENGTH,
+the string will shrink, and if you assign something longer than LENGTH,
+the string will grow to accommodate it. To keep the string the same
+length you may need to pad or chop your value using C<sprintf>.
+
+If OFFSET and LENGTH specify a substring that is partly outside the
+string, only the part within the string is returned. If the substring
+is beyond either end of the string, substr() returns the undefined
+value and produces a warning. When used as an lvalue, specifying a
+substring that is entirely outside the string is a fatal error.
+Here's an example showing the behavior for boundary cases:
+
+ my $name = 'fred';
+ substr($name, 4) = 'dy'; # $name is now 'freddy'
+ my $null = substr $name, 6, 2; # returns '' (no warning)
+ my $oops = substr $name, 7; # returns undef, with warning
+ substr($name, 7) = 'gap'; # fatal error
+
+An alternative to using substr() as an lvalue is to specify the
replacement string as the 4th argument. This allows you to replace
-parts of the EXPR and return what was there before in one operation.
+parts of the EXPR and return what was there before in one operation,
+just as you can with splice().
=item symlink OLDFILE,NEWFILE
symbolic links, produces a fatal error at run time. To check for that,
use eval:
- $symlink_exists = eval { symlink("",""); 1 };
+ $symlink_exists = eval { symlink("",""); 1 };
=item syscall LIST
an int. If not, the pointer to the string value is passed. You are
responsible to make sure a string is pre-extended long enough to
receive any result that might be written into a string. You can't use a
-string literal (or other read-only string) as an argument to C<syscall()>
+string literal (or other read-only string) as an argument to C<syscall>
because Perl has to assume that any string pointer might be written
through. If your
integer arguments are not literals and have never been interpreted in a
numeric context, you may need to add C<0> to them to force them to look
-like numbers. This emulates the C<syswrite()> function (or vice versa):
+like numbers. This emulates the C<syswrite> function (or vice versa):
require 'syscall.ph'; # may need to run h2ph
$s = "hi there\n";
which in practice should usually suffice.
Syscall returns whatever value returned by the system call it calls.
-If the system call fails, C<syscall()> returns C<-1> and sets C<$!> (errno).
+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>.
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
to retrieve the file number of the other end. You can avoid this
-problem by using C<pipe()> instead.
+problem by using C<pipe> instead.
=item sysopen FILEHANDLE,FILENAME,MODE
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
+underlying operating system's C<open> function with the parameters
FILENAME, MODE, 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
+using the C<|>-operator.
+
+Some of the most common values are C<O_RDONLY> for opening the file in
+read-only mode, C<O_WRONLY> for opening the file in write-only mode,
+and C<O_RDWR> for opening the file in read-write mode, and.
+
For historical reasons, some values work on almost every system
supported by perl: zero means read-only, one means write-only, and two
means read/write. We know that these values do I<not> work under
-OS/390 Unix and on the Macintosh; you probably don't want to use them
-in new code.
+OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
+se them in new code, use thhe constants discussed in the preceding
+paragraph.
-If the file named by FILENAME does not exist and the C<open()> call creates
+If the file named by FILENAME does not exist and the C<open> call creates
it (typically because MODE includes the C<O_CREAT> flag), then the value of
PERMS specifies the permissions of the newly created file. If you omit
-the PERMS argument to C<sysopen()>, Perl uses the octal value C<0666>.
+the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
These permission values need to be in octal, and are modified by your
process's current C<umask>.
-Seldom if ever use C<0644> as argument to C<sysopen()> because that
-takes away the user's option to have a more permissive umask. Better
-to omit it. See the perlfunc(1) entry on C<umask> for more on this.
+In many systems the C<O_EXCL> flag is available for opening files in
+exclusive mode. This is B<not> locking: exclusiveness means here that
+if the file already exists, sysopen() fails. The C<O_EXCL> wins
+C<O_TRUNC>.
+
+Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
+
+You should seldom if ever use C<0644> as argument to C<sysopen>, because
+that takes away the user's option to have a more permissive umask.
+Better to omit it. See the perlfunc(1) entry on C<umask> for more
+on this.
-The C<IO::File> module provides a more object-oriented approach, if you're
-into that kind of thing.
+See L<perlopentut> for a kinder, gentler explanation of opening files.
=item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
Attempts to read LENGTH bytes of data into variable SCALAR from the
specified FILEHANDLE, using the system call read(2). It bypasses stdio,
-so mixing this with other kinds of reads, C<print()>, C<write()>,
-C<seek()>, C<tell()>, or C<eof()> can cause confusion because stdio
+so mixing this with other kinds of reads, C<print>, C<write>,
+C<seek>, C<tell>, or C<eof> can cause confusion because stdio
usually buffers data. Returns the number of bytes actually read, C<0>
at end of file, or undef if there was an error. SCALAR will be grown or
shrunk so that the last byte actually read is the last byte of the
in the string being padded to the required size with C<"\0"> bytes before
the result of the read is appended.
+There is no syseof() function, which is ok, since eof() doesn't work
+very well on device files (like ttys) anyway. Use sysread() and check
+for a return value for 0 to decide whether you're done.
+
=item sysseek FILEHANDLE,POSITION,WHENCE
Sets FILEHANDLE's system position using the system call lseek(2). It
-bypasses stdio, so mixing this with reads (other than C<sysread()>),
-C<print()>, C<write()>, C<seek()>, C<tell()>, or C<eof()> may cause
-confusion. 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).
-For WHENCE, you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and
-C<SEEK_END> from either the C<IO::Seekable> or the POSIX module.
+bypasses stdio, so mixing this with reads (other than C<sysread>),
+C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause confusion.
+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). For
+WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, and
+C<SEEK_END> (start of the file, current position, end of the file)
+from any of the modules Fcntl, C<IO::Seekable>, or POSIX.
Returns the new position, or the undefined value on failure. A position
-of zero is returned as the string "C<0> but true"; thus C<sysseek()> returns
-TRUE on success and FALSE on failure, yet you can still easily determine
+of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
+true on success and false on failure, yet you can still easily determine
the new position.
=item system LIST
=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 complete.
-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 given by the
-first element of the list with arguments given by the rest of the list.
-If there is only one scalar argument, the argument is
-checked for shell metacharacters, and if there are any, the entire
-argument is passed to the system's command shell for parsing (this is
-C</bin/sh -c> on Unix platforms, but varies on other platforms). If
-there are no shell metacharacters in the argument, it is split into
-words and passed directly to C<execvp()>, which is more efficient.
+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
+complete. 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
+given by the first element of the list with arguments given by the
+rest of the list. If there is only one scalar argument, the argument
+is checked for shell metacharacters, and if there are any, the
+entire argument is passed to the system's command shell for parsing
+(this is C</bin/sh -c> on Unix platforms, but varies on other
+platforms). If there are no shell metacharacters in the argument,
+it is split into words and passed directly to C<execvp>, which is
+more efficient.
+
+All files opened for output are flushed before attempting the exec().
The return value is the exit status of the program as
-returned by the C<wait()> call. To get the actual exit value divide by
-256. See also L</exec>. This is I<NOT> what you want to use to capture
+returned by the C<wait> call. To get the actual exit value divide by
+256. 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
-C<qx//>, as described in L<perlop/"`STRING`">.
+C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
+indicates a failure to start the program (inspect $! for the reason).
-Like C<exec()>, C<system()> allows you to lie to a program about its name if
-you use the "C<system PROGRAM LIST>" syntax. Again, see L</exec>.
+Like C<exec>, C<system> allows you to lie to a program about its name if
+you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
-Because C<system()> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
+Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
program they're running doesn't actually interrupt your program.
@args = ("command", "arg1", "arg2");
=item syswrite FILEHANDLE,SCALAR
Attempts to write LENGTH bytes of data from variable SCALAR to the
-specified FILEHANDLE, using the system call write(2). If LENGTH is
-not specified, writes whole SCALAR. It bypasses
-stdio, so mixing this with reads (other than C<sysread())>, C<print()>,
-C<write()>, C<seek()>, C<tell()>, or C<eof()> may cause confusion
-because stdio usually buffers data. Returns the number of bytes
-actually written, or C<undef> if there was an error. If the LENGTH is
-greater than the available data in the SCALAR after the OFFSET, only as
-much data as is available will be written.
+specified FILEHANDLE, using the system call write(2). If LENGTH
+is not specified, writes whole SCALAR. It bypasses stdio, so mixing
+this with reads (other than C<sysread())>, C<print>, C<write>,
+C<seek>, C<tell>, or C<eof> may cause confusion because stdio
+usually buffers data. Returns the number of bytes actually written,
+or C<undef> if there was an error. If the LENGTH is greater than
+the available data in the SCALAR after the OFFSET, only as much
+data as is available will be written.
An OFFSET may be specified to write the data from some part of the
string other than the beginning. A negative OFFSET specifies writing
Returns the current position for FILEHANDLE. FILEHANDLE may be an
expression whose value gives the name of the actual filehandle. If
-FILEHANDLE is omitted, assumes the file last read.
+FILEHANDLE is omitted, assumes the file last read.
+
+There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
=item telldir DIRHANDLE
-Returns the current position of the C<readdir()> routines on DIRHANDLE.
-Value may be given to C<seekdir()> to access a particular location in a
+Returns the current position of the C<readdir> routines on DIRHANDLE.
+Value may be given to C<seekdir> to access a particular location in a
directory. Has the same caveats about possible directory compaction as
the corresponding system library routine.
This function binds a variable to a package class that will provide the
implementation for the variable. VARIABLE is the name of the variable
to be enchanted. CLASSNAME is the name of a class implementing objects
-of correct type. Any additional arguments are passed to the "C<new()>"
-method of the class (meaning C<TIESCALAR>, C<TIEARRAY>, or C<TIEHASH>).
-Typically these are arguments such as might be passed to the C<dbm_open()>
-function of C. The object returned by the "C<new()>" method is also
-returned by the C<tie()> function, which would be useful if you want to
-access other methods in CLASSNAME.
-
-Note that functions such as C<keys()> and C<values()> may return huge lists
+of correct type. Any additional arguments are passed to the C<new>
+method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
+or C<TIEHASH>). Typically these are arguments such as might be passed
+to the C<dbm_open()> function of C. The object returned by the C<new>
+method is also returned by the C<tie> function, which would be useful
+if you want to access other methods in CLASSNAME.
+
+Note that functions such as C<keys> and C<values> may return huge lists
when used on large objects, like DBM files. You may prefer to use the
-C<each()> function to iterate over such. Example:
+C<each> function to iterate over such. Example:
# print out history file offsets
use NDBM_File;
A class implementing a hash should have the following methods:
TIEHASH classname, LIST
- DESTROY this
FETCH this, key
STORE this, key, value
DELETE this, key
+ CLEAR this
EXISTS this, key
FIRSTKEY this
NEXTKEY this, lastkey
+ DESTROY this
A class implementing an ordinary array should have the following methods:
TIEARRAY classname, LIST
- DESTROY this
FETCH this, key
STORE this, key, value
- [others TBD]
+ FETCHSIZE this
+ STORESIZE this, count
+ CLEAR this
+ PUSH this, LIST
+ POP this
+ SHIFT this
+ UNSHIFT this, LIST
+ SPLICE this, offset, length, LIST
+ EXTEND this, count
+ DESTROY this
+
+A class implementing a file handle should have the following methods:
+
+ TIEHANDLE classname, LIST
+ READ this, scalar, length, offset
+ READLINE this
+ GETC this
+ WRITE this, scalar, length, offset
+ PRINT this, LIST
+ PRINTF this, format, LIST
+ CLOSE this
+ DESTROY this
A class implementing a scalar should have the following methods:
TIESCALAR classname, LIST
- DESTROY this
FETCH this,
STORE this, value
+ DESTROY this
-Unlike C<dbmopen()>, the C<tie()> function will not use or require a module
+Not all methods indicated above need be implemented. See L<perltie>,
+L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
+
+Unlike C<dbmopen>, the C<tie> function will not use or require a module
for you--you need to do that explicitly yourself. See L<DB_File>
-or the F<Config> module for interesting C<tie()> implementations.
+or the F<Config> module for interesting C<tie> implementations.
-For further details see L<perltie>, L<tied VARIABLE>.
+For further details see L<perltie>, L<"tied VARIABLE">.
=item tied VARIABLE
Returns a reference to the object underlying VARIABLE (the same value
-that was originally returned by the C<tie()> call that bound the variable
+that was originally returned by the C<tie> call that bound the variable
to a package.) Returns the undefined value if VARIABLE isn't tied to a
package.
Returns the number of non-leap seconds since whatever time the system
considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
and 00:00:00 UTC, January 1, 1970 for most other systems).
-Suitable for feeding to C<gmtime()> and C<localtime()>.
+Suitable for feeding to C<gmtime> and C<localtime>.
+
+For measuring time in better granularity than one second,
+you may use either the Time::HiRes module from CPAN, or
+if you have gettimeofday(2), you may be able to use the
+C<syscall> interface of Perl, see L<perlfaq8> for details.
=item times
=item tr///
-The transliteration operator. Same as C<y///>. See L<perlop>.
+The transliteration operator. Same as C<y///>.
See L<perlop>.
=item truncate FILEHANDLE,LENGTH
Truncates the file opened on FILEHANDLE, or named by EXPR, to the
specified length. Produces a fatal error if truncate isn't implemented
-on your system. Returns TRUE if successful, the undefined value
+on your system. Returns true if successful, the undefined value
otherwise.
=item uc EXPR
implementing the C<\U> escape in double-quoted strings.
Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
-does not attempt to do titlecase mapping on initial letters. See C<ucfirst()> for that.)
+does not attempt to do titlecase mapping on initial letters. See C<ucfirst> for that.)
If EXPR is omitted, uses C<$_>.
Returns the value of EXPR with the first character
in uppercase (titlecase in Unicode). This is
the internal function implementing the C<\u> escape in double-quoted strings.
-Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
+Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
+and L<utf8>.
If EXPR is omitted, uses C<$_>.
The Unix permission C<rwxr-x---> is represented as three sets of three
bits, or three octal digits: C<0750> (the leading 0 indicates octal
-and isn't one of the the digits). The C<umask> value is such a number
+and isn't one of the digits). The C<umask> value is such a number
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
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 &~
+C<sysopen> C<0666> would create a file with mode C<0640> (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
-C<mkdir()>) and executable files. This gives users the freedom of
+files (in C<sysopen>) and one of C<0777> for directories (in
+C<mkdir>) and executable files. This gives users the freedom of
choice: if they want protected files, they might choose process umasks
of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
Programs should rarely if ever make policy decisions better left to
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.
-
-
=item undef EXPR
=item undef
Undefines the value of EXPR, which must be an lvalue. Use only on a
-scalar value, an array (using "C<@>"), a hash (using "C<%>"), a subroutine
-(using "C<&>"), or a typeglob (using "<*>"). (Saying C<undef $hash{$key}>
+scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
+(using C<&>), or a typeglob (using <*>). (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
undefined value. You can omit the EXPR, in which case nothing is
unlink @goners;
unlink <*.bak>;
-Note: C<unlink()> will not delete directories unless you are superuser and
+Note: C<unlink> will not delete directories unless you are superuser and
the B<-U> flag is supplied to Perl. Even if these conditions are
met, be warned that unlinking a directory can inflict damage on your
-filesystem. Use C<rmdir()> instead.
+filesystem. Use C<rmdir> instead.
If LIST is omitted, uses C<$_>.
=item unpack TEMPLATE,EXPR
-C<Unpack()> does the reverse of C<pack()>: it takes a string representing a
-structure and expands it out into a list value, returning the array
-value. (In scalar context, it returns merely the first value
-produced.) The TEMPLATE has the same format as in the C<pack()> function.
+C<unpack> does the reverse of C<pack>: it takes a string
+representing a structure and expands it out into a list of values.
+(In scalar context, it returns merely the first value produced.)
+The TEMPLATE has the same format as in the C<pack> function.
Here's a subroutine that does substring:
sub substr {
themselves. Default is a 16-bit checksum. For example, the following
computes the same number as the System V sum program:
- while (<>) {
- $checksum += unpack("%16C*", $_);
- }
- $checksum %= 65536;
+ $checksum = do {
+ local $/; # slurp!
+ unpack("%32C*",<>) % 65535;
+ };
The following efficiently counts the number of set bits in a bit vector:
$setbits = unpack("%32b*", $selectmask);
+The C<"p"> and C<"P"> formats should be used with care. Since Perl
+has no way of checking whether the value passed to C<unpack()>
+corresponds to a valid memory location, passing a pointer value that's
+not known to be valid is likely to have disastrous consequences.
+
+See L</pack> for more examples and notes.
+
=item untie VARIABLE
-Breaks the binding between a variable and a package. (See C<tie()>.)
+Breaks the binding between a variable and a package. (See C<tie>.)
=item unshift ARRAY,LIST
-Does the opposite of a C<shift()>. Or the opposite of a C<push()>,
+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.
unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
Note the LIST is prepended whole, not one element at a time, so the
-prepended elements stay in the same order. Use C<reverse()> to do the
+prepended elements stay in the same order. Use C<reverse> to do the
reverse.
=item use Module LIST
incompatible ways from older versions of Perl. (We try not to do
this more than we have to.)
-The C<BEGIN> forces the C<require> and C<import()> to happen at compile time. The
+The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
C<require> makes sure the module is loaded into memory if it hasn't been
-yet. The C<import()> is not a builtin--it's just an ordinary static method
-call into the "C<Module>" package to tell the module to import the list of
+yet. The C<import> is not a builtin--it's just an ordinary static method
+call into the C<Module> package to tell the module to import the list of
features back into the current package. The module can implement its
-C<import()> method any way it likes, though most modules just choose to
-derive their C<import()> method via inheritance from the C<Exporter> class that
-is defined in the C<Exporter> module. See L<Exporter>. If no C<import()>
+C<import> method any way it likes, though most modules just choose to
+derive their C<import> method via inheritance from the C<Exporter> class that
+is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
method can be found then the error is currently silently ignored. This
may change to a fatal error in a future version.
use integer;
use diagnostics;
- use sigtrap qw(SEGV BUS);
- use strict qw(subs vars refs);
- use subs qw(afunc blurfl);
+ use sigtrap qw(SEGV BUS);
+ use strict qw(subs vars refs);
+ use subs qw(afunc blurfl);
+ use warnings qw(all);
-Some of these these pseudo-modules import semantics into the current
+Some of these pseudo-modules import semantics into the current
block scope (like C<strict> or C<integer>, unlike ordinary modules,
which import symbols into the current package (which are effective
through the end of the file).
-There's a corresponding "C<no>" command that unimports meanings imported
-by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import()>.
+There's a corresponding C<no> command that unimports meanings imported
+by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
no integer;
no strict 'refs';
+ no warnings;
-If no C<unimport()> method can be found the call fails with a fatal error.
+If no C<unimport> method can be found the call fails with a fatal error.
See L<perlmod> for a list of standard modules and pragmas.
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
and modification times, in that order. Returns the number of files
-successfully changed. The inode modification time of each file is set
-to the current time. This code has the same effect as the "C<touch>"
+successfully changed. The inode change time of each file is set
+to the current time. This code has the same effect as the C<touch>
command if the files already exist:
#!/usr/bin/perl
Returns a list consisting of all the values of the named hash. (In a
scalar context, returns the number of values.) The values are
-returned in an apparently random order, but it is the same order as
-either the C<keys()> or C<each()> function would produce on the same hash.
-As a side effect, it resets HASH's iterator. See also C<keys()>, C<each()>,
-and C<sort()>.
+returned in an apparently random order. The actual random order is
+subject to change in future versions of perl, but it is guaranteed to
+be the same order as either the C<keys> or C<each> function would
+produce on the same (unmodified) hash.
+
+Note that you cannot modify the values of a hash this way, because the
+returned list is just a copy. You need to use a hash slice for that,
+since it's lvaluable in a way that values() is not.
+
+ for (values %hash) { s/foo/bar/g } # FAILS!
+ for (@hash{keys %hash}) { s/foo/bar/g } # ok
+
+As a side effect, calling values() resets the HASH's internal iterator.
+See also C<keys>, C<each>, and C<sort>.
=item vec EXPR,OFFSET,BITS
Treats the string in EXPR as a vector of unsigned integers, and
-returns the value of the bit field specified by OFFSET. BITS specifies
-the number of bits that are reserved for each entry in the bit
-vector. This must be a power of two from 1 to 32. C<vec()> may also be
-assigned to, in which case parentheses are needed to give the expression
-the correct precedence as in
+returns the value of the bit field specified by OFFSET. BITS
+specifies the number of bits that are reserved for each entry in the
+bit vector. This must be a power of two from 1 to 32 (or 64, if your
+platform supports that).
+
+C<vec> may also be assigned to, in which case parentheses are needed
+to give the expression the correct precedence as in
vec($image, $max_x * $x + $y, 8) = 3;
-Vectors created with C<vec()> can also be manipulated with the logical
-operators C<|>, C<&>, and C<^>, which will assume a bit vector operation is
-desired when both operands are strings.
+Vectors created with C<vec> can also be manipulated with the logical
+operators C<|>, C<&>, and C<^>, which will assume a bit vector
+operation is desired when both operands are strings.
+See L<perlop/"Bitwise String Operators">.
The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
-The comments show the string after each step. Note that this code works
+The comments show the string after each step. Note that this code works
in the same way on big-endian or little-endian machines.
my $foo = '';
vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
# 'l' is "\x6c"
-To transform a bit vector into a string or array of 0's and 1's, use these:
+To transform a bit vector into a string or list of 0's and 1's, use these:
$bits = unpack("b*", $vector);
@bits = split(//, unpack("b*", $vector));
=item wait
-Waits for a child process to terminate and returns the pid of the
-deceased process, or C<-1> if there are no child processes. The status is
-returned in C<$?>.
+Behaves like the wait(2) system call on your system: it waits for a child
+process to terminate and returns the pid of the deceased process, or
+C<-1> if there are no child processes. The status is returned in C<$?>.
+Note that a return value of C<-1> could mean that child processes are
+being automatically reaped, as described in L<perlipc>.
=item waitpid PID,FLAGS
-Waits for a particular child process to terminate and returns the pid
-of the deceased process, or C<-1> if there is no such child process. The
-status is returned in C<$?>. If you say
+Waits for a particular child process to terminate and returns the pid of
+the deceased process, or C<-1> if there is no such child process. On some
+systems, a value of 0 indicates that there are processes still running.
+The status is returned in C<$?>. If you say
use POSIX ":sys_wait_h";
#...
- waitpid(-1,&WNOHANG);
+ do {
+ $kid = waitpid(-1,&WNOHANG);
+ } until $kid == -1;
-then you can do a non-blocking wait for any process. Non-blocking wait
-is available on machines supporting either the waitpid(2) or
-wait4(2) system calls. However, waiting for a particular pid with
-FLAGS of C<0> is implemented everywhere. (Perl emulates the system call
-by remembering the status values of processes that have exited but have
-not been harvested by the Perl script yet.)
+then you can do a non-blocking wait for all pending zombie processes.
+Non-blocking wait is available on machines supporting either the
+waitpid(2) or wait4(2) system calls. However, waiting for a particular
+pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
+system call by remembering the status values of processes that have
+exited but have not been harvested by the Perl script yet.)
-See L<perlipc> for other examples.
+Note that on some systems, a return value of C<-1> could mean that child
+processes are being automatically reaped. See L<perlipc> for details,
+and for other examples.
=item wantarray
-Returns TRUE if the context of the currently executing subroutine is
-looking for a list value. Returns FALSE if the context is looking
+Returns true if the context of the currently executing subroutine is
+looking for a list value. Returns false if the context is looking
for a scalar. Returns the undefined value if the context is looking
for no value (void context).
my @a = complex_calculation();
return wantarray ? @a : "@a";
+This function should have been named wantlist() instead.
+
=item warn LIST
-Produces a message on STDERR just like C<die()>, but doesn't exit or throw
+Produces a message on STDERR just like C<die>, but doesn't exit or throw
an exception.
If LIST is empty and C<$@> already contains a value (typically from a
previous eval) that value is used after appending C<"\t...caught">
-to C<$@>. This is useful for staying almost, but not entirely similar to
-C<die()>.
+to C<$@>. This is useful for staying almost, but not entirely similar to
+C<die>.
If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
No message is printed if there is a C<$SIG{__WARN__}> handler
installed. It is the handler's responsibility to deal with the message
-as it sees fit (like, for instance, converting it into a C<die()>). Most
+as it sees fit (like, for instance, converting it into a C<die>). Most
handlers must therefore make arrangements to actually display the
-warnings that they are not prepared to deal with, by calling C<warn()>
+warnings that they are not prepared to deal with, by calling C<warn>
again in the handler. Note that this is quite safe and will not
produce an endless loop, since C<__WARN__> hooks are not called from
inside one.
You will find this behavior is slightly different from that of
C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
-instead call C<die()> again to change it).
+instead call C<die> again to change it).
Using a C<__WARN__> handler provides a powerful way to silence all
warnings (even the so-called mandatory ones). An example:
warn "\$foo is alive and $foo!"; # does show up
See L<perlvar> for details on setting C<%SIG> entries, and for more
-examples.
+examples. See the Carp module for other kinds of warnings using its
+carp() and cluck() functions.
=item write FILEHANDLE
Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
using the format associated with that file. By default the format for
a file is the one having the same name as the filehandle, but the
-format for the current output channel (see the C<select()> function) may be set
+format for the current output channel (see the C<select> function) may be set
explicitly by assigning the name of the format to the C<$~> variable.
Top of form processing is handled automatically: if there is
If FILEHANDLE is unspecified, output goes to the current default output
channel, which starts out as STDOUT but may be changed by the
-C<select()> operator. If the FILEHANDLE is an EXPR, then the expression
+C<select> operator. If the FILEHANDLE is an EXPR, then the expression
is evaluated and the resulting string is used to look up the name of
the FILEHANDLE at run time. For more on formats, see L<perlform>.
-Note that write is I<NOT> the opposite of C<read()>. Unfortunately.
+Note that write is I<not> the opposite of C<read>. Unfortunately.
=item y///
https://perl5.git.perl.org / perl5.git / blobdiff