r1 - 26 May 2008 - TWikiGuest

NAME

perlfunc - Perl builtin functions

DESCRIPTION

The functions in this section can serve as terms in an expression. They fall into two major categories: list operators and named unary operators. These differ in their precedence relationship with a following comma. (See the precedence table in the perlop manpage.) List operators take more than one argument, while unary operators can never take more than one argument. Thus, a comma terminates the argument of a unary operator, but merely separates the arguments of a list operator. A unary operator generally provides a scalar context to its argument, while a list operator may provide either scalar or list contexts for its arguments. If it does both, the scalar arguments will be first, and the list argument will follow. (Note that there can ever be only one such list argument.) For instance, splice() has three scalar 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 with LIST as an argument. Such a list may consist of any combination of scalar arguments or list values; the list values will be included in the list as if each individual element were interpolated at that point in the list, forming a longer single-dimensional list value. Commas should separate elements of the LIST. 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 looks like a function, therefore it is a function, and precedence doesn't matter. Otherwise it's a list operator or unary operator, and precedence does matter. And whitespace between the function and left parenthesis doesn't count--so you need to be careful sometimes:

    print 1+2+4;        # Prints 7.

    print(1+2) + 4;     # Prints 3.

    print (1+2)+4;      # Also prints 3!

    print +(1+2)+4;     # Prints 7.

    print ((1+2)+4);    # Prints 7.

If you run Perl with the -w switch it can warn you about this. For example, the third line above produces:

    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 time and endpwent. For example, time+86_400 always means time() + 86_400. For functions that can be used in either a scalar or list context, nonabortive failure is generally indicated in a scalar context by returning the undefined value, and in a list context by returning the null list. Remember the following important rule: There is no rule that relates 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 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 operations. In general, they do what you want, unless you want consistency. A named array in scalar context is quite different from what would at first glance appear to be a list in scalar context. You can't get a list like (1,2,3) into being in scalar context, because the compiler knows the context at compile time. It would generate the scalar comma operator there, not the list construction version of the comma. That means it was never a list to start with. In general, functions in Perl that serve as wrappers for system calls of the same name (like chown(2), fork(2), closedir(2), etc.) all return true when they succeed and undef otherwise, as is usually mentioned in the descriptions below. This is different from the C interfaces, which return -1 on failure. Exceptions to this rule are wait, waitpid, and syscall. System calls also set the special $! variable on failure. Other functions do not, except accidentally.

Perl Functions by Category

Here are Perl's functions (including things that look like functions, like some keywords and named operators) arranged by category. Some functions appear in more than one place.

Functions for SCALARs or strings: chomp, chop, chr, crypt, hex, index, lc, lcfirst,; length, oct, ord, pack, q/STRING/, qq/STRING/, reverse,; rindex, sprintf, substr, tr///, uc, ucfirst, y///
Regular expressions and pattern matching: m//, pos, quotemeta, s///, split, study, qr//
Numeric functions: abs, atan2, cos, exp, hex, int, log, oct, rand,; sin, sqrt, srand
Functions for real @ARRAYs: pop, push, shift, splice, unshift
Functions for list data: grep, join, map, qw/STRING/, reverse, sort, unpack
Functions for real %HASHes: delete, each, exists, keys, values
Input and output functions: binmode, close, closedir, dbmclose, dbmopen, die, eof,; fileno, flock, format, getc, print, printf, read,; readdir, rewinddir, seek, seekdir, select, syscall,; sysread, sysseek, syswrite, tell, telldir, truncate,; warn, write
Functions for fixed length data or records: pack, read, syscall, sysread, syswrite, unpack, vec
Functions for filehandles, files, or directories: -X, chdir, chmod, chown, chroot, fcntl, glob,; ioctl, link, lstat, mkdir, open, opendir,; readlink, rename, rmdir, stat, symlink, sysopen,; umask, unlink, utime
Keywords related to the control flow of your Perl program: caller, continue, die, do, dump, eval, exit,; goto, last, next, redo, return, sub, wantarray
Keywords related to scoping: caller, import, local, my, our, package, use
Miscellaneous functions: defined, dump, eval, formline, local, my, our, reset,; scalar, undef, wantarray
Functions for processes and process groups: alarm, exec, fork, getpgrp, getppid, getpriority, kill,; pipe, qx/STRING/, setpgrp, setpriority, sleep, system,; times, wait, waitpid
Keywords related to perl modules: do, import, no, package, require, use
Keywords related to classes and object-orientedness: bless, dbmclose, dbmopen, package, ref, tie, tied,; untie, use
Low-level socket functions: accept, bind, connect, getpeername, getsockname,; getsockopt, listen, recv, send, setsockopt, shutdown,; socket, socketpair
System V interprocess communication functions: msgctl, msgget, msgrcv, msgsnd, semctl, semget, semop,; shmctl, shmget, shmread, shmwrite
Fetching user and group info: endgrent, endhostent, endnetent, endpwent, getgrent,; getgrgid, getgrnam, getlogin, getpwent, getpwnam,; getpwuid, setgrent, setpwent
Fetching network info: endprotoent, endservent, gethostbyaddr, gethostbyname,; gethostent, getnetbyaddr, getnetbyname, getnetent,; getprotobyname, getprotobynumber, getprotoent,; getservbyname, getservbyport, getservent, sethostent,; setnetent, setprotoent, setservent
Time-related functions: gmtime, localtime, time, times
Functions new in perl5: abs, bless, chomp, chr, exists, formline, glob,; import, lc, lcfirst, map, my, no, our, prototype,; qx, qw, readline, readpipe, ref, sub*, sysopen, tie,; tied, uc, ucfirst, untie, use; * - sub was a keyword in perl4, but in perl5 it is an; operator, which can be used in expressions.
Functions obsoleted in perl5: dbmclose, dbmopen

Portability

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: -X, binmode, chmod, chown, chroot, crypt, dbmclose, dbmopen, dump, endgrent, endhostent, endnetent, endprotoent, endpwent, endservent, exec, fcntl, flock, fork, getgrent, getgrgid, gethostbyname, gethostent, getlogin, getnetbyaddr, getnetbyname, getnetent, getppid, getpgrp, getpriority, getprotobynumber, getprotoent, getpwent, getpwnam, getpwuid, getservbyport, getservent, getsockopt, glob, ioctl, kill, link, lstat, msgctl, msgget, msgrcv, msgsnd, open, pipe, readlink, rename, select, semctl, semget, semop, setgrent, sethostent, setnetent, setpgrp, setpriority, setprotoent, setpwent, setservent, setsockopt, shmctl, shmget, shmread, shmwrite, socket, socketpair, stat, symlink, syscall, sysopen, system, times, truncate, umask, unlink, utime, wait, waitpid For more information about the portability of these functions, see the perlport manpage and other available platform-specific documentation.

Alphabetical Listing of Perl Functions

-X FILEHANDLE

-X EXPR

-X

A file test, where X is one of the letters listed below. This unary

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 $_, except for -t, which tests STDIN.

Unless otherwise documented, it returns 1 for true and '' 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

operator may be any of:

    -r  File is readable by effective uid/gid.

    -w  File is writable by effective uid/gid.

    -x  File is executable by effective uid/gid.

    -o  File is owned by effective uid.

    -R  File is readable by real uid/gid.

    -W  File is writable by real uid/gid.

    -X  File is executable by real uid/gid.

    -O  File is owned by real uid.

    -e  File exists.

    -z  File has zero size (is empty).

    -s  File has nonzero size (returns size in bytes).

    -f  File is a plain file.

    -d  File is a directory.

    -l  File is a symbolic link.

    -p  File is a named pipe (FIFO), or Filehandle is a pipe.

    -S  File is a socket.

    -b  File is a block special file.

    -c  File is a character special file.

    -t  Filehandle is opened to a tty.

    -u  File has setuid bit set.

    -g  File has setgid bit set.

    -k  File has sticky bit set.

    -T  File is an ASCII text file (heuristic guess).

    -B  File is a "binary" file (opposite of -T).

    -M  Script start time minus file modification time, in days.

    -A  Same for access time.

    -C  Same for inode change time (Unix, may differ for other platforms)

Example:

    while (<>) {

        chomp;

        next unless -f $_;      # ignore specials

        #...

The interpretation of the file permission operators -r, -R,

-w, -W, -x, and -X is by default based solely on the mode

of the file and the uids and gids of the user. There may be other

reasons you can't actually read, write, or execute the file. Such

reasons may be for example network filesystem access controls, ACLs

(access control lists), read-only filesystems, and unrecognized

executable formats.

Also note that, for the superuser on the local filesystems, the -r,

-R, -w, and -W tests always return 1, and -x and -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 their effective uid to something else.

If you are using ACLs, there is a pragma called filetest that may

produce more accurate results than the bare stat() mode bits.

When under the 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

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

documentation for the filetest pragma for more information.

Note that -s/a/b/ does not do a negated substitution. Saying

-exp($foo) still works as expected, however--only single letters

following a minus are interpreted as file tests.

The -T and -B switches work as follows. The first block or so of the

file is examined for odd characters such as strange control codes or

characters with the high bit set. If too many strange characters (>30%)

are found, it's a -B file; otherwise it's a -T file. Also, any file

containing null in the first block is considered a binary file. If -T

or -B is used on a filehandle, the current IO buffer is examined

rather than the first block. Both -T and -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 -T test, on most occasions you want to use a -f

against the file first, as in next unless -f $file && -T $file.

If any of the file tests (or either the stat or 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 -t, and you need to remember

that lstat() and -l will leave values in the stat structure for the

symbolic link, not the real file.) (Also, if the stat buffer was filled by

an lstat call, -T and -B will reset it with the results of stat _).

Example:

    print "Can do.\n" if -r $a || -w _ || -x _;

    stat($filename);

    print "Readable\n" if -r _;

    print "Writable\n" if -w _;

    print "Executable\n" if -x _;

    print "Setuid\n" if -u _;

    print "Setgid\n" if -g _;

    print "Sticky\n" if -k _;

    print "Text\n" if -T _;

    print "Binary\n" if -B _;

abs VALUE

abs

Returns the absolute value of its argument.

If VALUE is omitted, uses $_.

accept NEWSOCKET,GENERICSOCKET

Accepts an incoming socket connect, just as the the accept(2) manpage system call

does. Returns the packed address if it succeeded, false otherwise.

See the example in Sockets: Client/Server Communication in the perlipc manpage.

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 $^F in the perlvar manpage.

alarm SECONDS

alarm

Arranges to have a SIGALRM delivered to this process after the

specified number of wallclock seconds has elapsed. If SECONDS is not

specified, the value stored in $_ is used. (On some machines,

unfortunately, the elapsed time may be up to one second less or more

than you specified because of how seconds are counted, and process

scheduling may delay the delivery of the signal even further.)

Only one timer may be counting at once. Each call disables the

previous timer, and an argument of 0 may be supplied to cancel the

previous timer without starting a new one. The returned value is the

amount of time remaining on the previous timer.

For delays of finer granularity than one second, you may use Perl's

four-argument version of select() leaving the first three arguments

undefined, or you might be able to use the syscall interface to

access the setitimer(2) manpage if your system supports it. The Time::HiRes

module (from CPAN, and starting from Perl 5.8 part of the standard

distribution) may also prove useful.

It is usually a mistake to intermix alarm and sleep calls.

(sleep may be internally implemented in your system with alarm)

If you want to use alarm to time out a system call you need to use an

eval/die pair. You can't rely on the alarm causing the system call to

fail with $! set to EINTR because Perl sets up signal handlers to

restart system calls on some systems. Using eval/die always works,

modulo the caveats given in Signals in the perlipc manpage.

    eval {

        local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required

        alarm $timeout;

        $nread = sysread SOCKET, $buffer, $size;

        alarm 0;

};

    if ($@) {

        die unless $@ eq "alarm\n";   # propagate unexpected errors

        # timed out

    else {

        # didn't

For more information see the perlipc manpage.

atan2 Y,X

Returns the arctangent of Y/X in the range -PI to PI.

For the tangent operation, you may use the Math::Trig::tan

function, or use the familiar relation:

    sub tan { sin($_[0]) / cos($_[0])  }

Note that atan2(0, 0) is not well-defined.

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

packed address of the appropriate type for the socket. See the examples in

Sockets: Client/Server Communication in the perlipc manpage.

binmode FILEHANDLE, LAYER

binmode FILEHANDLE

Arranges for FILEHANDLE to be read or written in ``binary'' or ``text''

mode on systems where the run-time libraries distinguish between

binary and text files. If FILEHANDLE is an expression, the value is

taken as the name of the filehandle. Returns true on success,

otherwise it returns undef and sets $! (errno).

On some systems (in general, DOS and Windows-based systems) binmode()

is necessary when you're not working with a text file. For the sake

of portability it is a good idea to always use it when appropriate,

and to never use it when it isn't appropriate. Also, people can

set their I/O to be by default UTF-8 encoded Unicode, not bytes.

In other words: regardless of platform, use binmode() on binary data,

like for example images.

If LAYER is present it is a single string, but may contain multiple

directives. The directives alter the behaviour of the file handle.

When LAYER is present using binmode on text file makes sense.

If LAYER is omitted or specified as :raw the filehandle is made

suitable for passing binary data. This includes turning off possible CRLF

translation and marking it as bytes (as opposed to Unicode characters).

Note that, despite what may be implied in ``Programming Perl'' (the

Camel) or elsewhere, :raw is not the simply inverse of :crlf

-- other layers which would affect binary nature of the stream are

also disabled. See the PerlIO manpage, the perlrun manpage and the discussion about the

PERLIO environment variable.

The :bytes, :crlf, and :utf8, and any other directives of the

form :..., are called I/O layers. The open pragma can be used to

establish default I/O layers. See the open manpage.

The LAYER parameter of the binmode() function is described as ``DISCIPLINE''

in ``Programming Perl, 3rd Edition''. However, since the publishing of this

book, by many known as ``Camel III'', the consensus of the naming of this

functionality has moved from ``discipline'' to ``layer''. All documentation

of this version of Perl therefore refers to ``layers'' rather than to

``disciplines''. Now back to the regularly scheduled documentation...>

To mark FILEHANDLE as UTF-8, use :utf8.

In general, binmode() should be called after open() but before any I/O

is done on the filehandle. Calling binmode() will normally flush any

pending buffered output data (and perhaps pending input data) on the

handle. An exception to this is the :encoding layer that

changes the default character encoding of the handle, see the open manpage.

The :encoding layer sometimes needs to be called in

mid-stream, and it doesn't flush the stream. The :encoding

also implicitly pushes on top of itself the :utf8 layer because

internally Perl will operate on UTF-8 encoded Unicode characters.

The operating system, device drivers, C libraries, and Perl run-time

system all work together to let the programmer treat a single

character (\n) as the line terminator, irrespective of the external

representation. On many operating systems, the native text file

representation matches the internal representation, but on some

platforms the external representation of \n is made up of more than

one character.

Mac OS, all variants of Unix, and Stream_LF files on VMS use a single

character to end each line in the external representation of text (even

though that single character is CARRIAGE RETURN on Mac OS and LINE FEED

on Unix and most VMS files). In other systems like OS/2, DOS and the

various flavors of MS-Windows your program sees a \n as a simple \cJ,

but what's stored in text files are the two characters \cM\cJ. That

means that, if you don't use binmode() on these systems, \cM\cJ

sequences on disk will be converted to \n on input, and any \n in

your program will be converted back to \cM\cJ on output. This is what

you want for text files, but it can be disastrous for binary files.

Another consequence of using binmode() (on some systems) is that

special end-of-file markers will be seen as part of the data stream.

For systems from the Microsoft family this means that if your binary

data contains \cZ, the I/O subsystem will regard it as the end of

the file, unless you use binmode().

binmode() is not only important for readline() and print() operations,

but also when using read(), seek(), sysread(), syswrite() and tell()

(see the perlport manpage for more details). See the $/ and $\ variables

in the perlvar manpage for how to manually set your input and output

line-termination sequences.

bless REF,CLASSNAME

bless REF

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 bless is often the last thing in a constructor,

it returns the reference for convenience. Always use the two-argument

version if a derived class might inherit the function doing the blessing.

See the perltoot manpage and the perlobj manpage 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. To prevent

confusion, you may wish to avoid such package names as well. Make sure

that CLASSNAME is a true value.

See Perl Modules in the perlmod manpage.

caller EXPR

caller

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 eval or require, and the undefined value

otherwise. In list context, returns

    ($package, $filename, $line) = caller;

With EXPR, it returns some extra information that the debugger uses to

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, $hints, $bitmask) = caller($i);

Here $subroutine may be (eval) if the frame is not a subroutine

call, but an eval. In such a case additional elements $evaltext and

$is_require are set: $is_require is true if the frame is created by a

require or use statement, $evaltext contains the text of the

eval EXPR statement. In particular, for an eval BLOCK statement,

$filename is (eval), but $evaltext is undefined. (Note also that

each use statement creates a require frame inside an eval EXPR

frame.) $subroutine may also be (unknown) if this particular

subroutine happens to have been deleted from the symbol table.

$hasargs is true if a new instance of @_ was set up for the frame.

$hints and $bitmask contain pragmatic hints that the caller was

compiled with. The $hints and $bitmask values are subject to change

between versions of Perl, and are not meant for external use.

Furthermore, when called from within the DB package, caller returns more

detailed information: it sets the list variable @DB::args to be the

arguments with which the subroutine was invoked.

Be aware that the optimizer might have optimized call frames away before

caller had a chance to get the information. That means that caller(N)

might not return information about the call frame you expect it do, for

N > 1. In particular, @DB::args might have information from the

previous time caller was called.

chdir EXPR

chdir FILEHANDLE

chdir DIRHANDLE

chdir

Changes the working directory to EXPR, if possible. If EXPR is omitted,

changes to the directory specified by $ENV{HOME}, if set; if not,

changes to the directory specified by $ENV{LOGDIR}. (Under VMS, the

variable $ENV{SYS$LOGIN} is also checked, and used if it is set.) If

neither is set, chdir does nothing. It returns true upon success,

false otherwise. See the example under die.

On systems that support fchdir, you might pass a file handle or

directory handle as argument. On systems that don't support fchdir,

passing handles produces a fatal error at run time.

chmod LIST

Changes the permissions of a list of files. The first element of the

list must be the numerical mode, which should probably be an octal

number, and which definitely should not be a string of octal digits:

0644 is okay, '0644' is not. Returns the number of files

successfully changed. See also oct, if all you have is a string.

    $cnt = chmod 0755, 'foo', 'bar';

    chmod 0755, @executables;

    $mode = '0644'; chmod $mode, 'foo';      # !!! sets mode to

                                             # --w----r-T

    $mode = '0644'; chmod oct($mode), 'foo'; # this is better

    $mode = 0644;   chmod $mode, 'foo';      # this is best

On systems that support fchmod, you might pass file handles among the

files. On systems that don't support fchmod, passing file handles

produces a fatal error at run time.

    open(my $fh, "<", "foo");

    my $perm = (stat $fh)[2] & 07777;

    chmod($perm | 0600, $fh);

You can also import the symbolic S_I* constants from the Fcntl

module:

    use Fcntl ':mode';

    chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;

    # This is identical to the chmod 0755 of the above example.

chomp VARIABLE

chomp( LIST )

chomp

This safer version of chop removes any trailing string

that corresponds to the current value of $/ (also known as

$INPUT_RECORD_SEPARATOR in the 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 ($/ = ""), it removes all trailing newlines from the string.

When in slurp mode ($/ = undef) or fixed-length record mode ($/ is

a reference to an integer or the like, see the perlvar manpage) chomp() won't

remove anything.

If VARIABLE is omitted, it chomps $_. Example:

    while (<>) {

        chomp;  # avoid \n on last field

        @array = split(/:/);

        # ...

If VARIABLE is a hash, it chomps the hash's values, but not its keys.

You can actually chomp anything that's an lvalue, including an assignment:

    chomp($cwd = `pwd`);

    chomp($answer = <STDIN>);

If you chomp a list, each element is chomped, and the total number of

characters removed is returned.

If the encoding pragma is in scope then the lengths returned are

calculated from the length of $/ in Unicode characters, which is not

always the same as the length of $/ in the native encoding.

Note that parentheses are necessary when you're chomping anything

that is not a simple variable. This is because chomp $cwd = `pwd`;

is interpreted as (chomp $cwd) = `pwd`;, rather than as

chomp( $cwd = `pwd` ) which you might expect. Similarly,

chomp $a, $b is interpreted as chomp($a), $b rather than

as chomp($a, $b).

chop VARIABLE

chop( LIST )

chop

Chops off the last character of a string and returns the character

chopped. It is much more efficient than s/.$//s because it neither

scans nor copies the string. If VARIABLE is omitted, chops $_.

If VARIABLE is a hash, it chops the hash's values, but not its keys.

You can actually chop anything that's an lvalue, including an assignment.

If you chop a list, each element is chopped. Only the value of the

last chop is returned.

Note that chop returns the last character. To return all but the last

character, use substr($string, 0, -1).

See also the perlref manpage.

rename OLDNAME,NEWNAME

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 mv command sometimes compensates

for this. Other restrictions include whether it works on directories,

open files, or pre-existing files. Check the perlport manpage and either the

the rename(2) manpage manpage or equivalent system documentation for details.

require VERSION

require EXPR

require

Demands a version of Perl specified by VERSION, or demands some semantics

specified by EXPR or by $_ if EXPR is not supplied.

VERSION may be either a numeric argument such as 5.006, which will be

compared to $], or a literal of the form v5.6.1, which will be compared

to $^V (aka $PERL_VERSION). A fatal error is produced at run time if

VERSION is greater than the version of the current Perl interpreter.

Compare with use, which can do a similar check at compile time.

Specifying VERSION as a literal of the form v5.6.1 should generally be

avoided, because it leads to misleading error messages under earlier

versions of Perl that do not support this syntax. The equivalent numeric

version should be used instead.

    require v5.6.1;     # run time version check

    require 5.6.1;      # ditto

    require 5.006_001;  # ditto; preferred for backwards compatibility

Otherwise, require 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 eval. Has

semantics similar to the following subroutine:

    sub require {

       my ($filename) = @_;

       if (exists $INC{$filename}) {

           return 1 if $INC{$filename};

           die "Compilation failed in require";

       my ($realfilename,$result);

       ITER: {

           foreach $prefix (@INC) {

               $realfilename = "$prefix/$filename";

               if (-f $realfilename) {

                   $INC{$filename} = $realfilename;

                   $result = do $realfilename;

                   last ITER;

           die "Can't find $filename in \@INC";

       if ($@) {

           $INC{$filename} = undef;

           die $@;

       } elsif (!$result) {

           delete $INC{$filename};

           die "$filename did not return true value";

       } else {

           return $result;

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

successful execution of any initialization code, so it's customary to

end such a file with 1; unless you're sure it'll return true

otherwise. But it's better just to put the 1;, in case you add more

statements.

If EXPR is a bareword, the require assumes a ``.pm'' extension and

replaces ``::'' with ``/'' in the filename for you,

to make it easy to load standard modules. This form of loading of

modules does not risk altering your namespace.

In other words, if you try this:

        require Foo::Bar;    # a splendid bareword

The require function will actually look for the ``Foo/Bar.pm'' file in the

directories specified in the @INC array.

But if you try this:

        $class = 'Foo::Bar';

        require $class;      # $class is not a bareword

#or

        require "Foo::Bar";  # not a bareword because of the ""

The require function will look for the ``Foo::Bar'' file in the @INC array and

will complain about not finding ``Foo::Bar'' there. In this case you can do:

        eval "require $class";

Now that you understand how require looks for files in the case of

a bareword argument, there is a little extra functionality going on

behind the scenes. Before require looks for a ``.pm'' extension,

it will first look for a filename with a ``.pmc'' extension. A file

with this extension is assumed to be Perl bytecode generated by

B::Bytecode. If this file is found, and its modification

time is newer than a coinciding ``.pm'' non-compiled file, it will be

loaded in place of that non-compiled file ending in a ``.pm'' extension.

You can also insert hooks into the import facility, by putting directly

Perl code into the @INC array. There are three forms of hooks: subroutine

references, array references and blessed objects.

Subroutine references are the simplest case. When the inclusion system

walks through @INC and encounters a subroutine, this subroutine gets

called with two parameters, the first being a reference to itself, and the

second the name of the file to be included (e.g. ``Foo/Bar.pm''). The

subroutine should return undef or a filehandle, from which the file to

include will be read. If undef is returned, require will look at

the remaining elements of @INC.

If the hook is an array reference, its first element must be a subroutine

reference. This subroutine is called as above, but the first parameter is

the array reference. This enables to pass indirectly some arguments to

the subroutine.

In other words, you can write:

    push @INC, \&my_sub;

    sub my_sub {

        my ($coderef, $filename) = @_;  # $coderef is \&my_sub

...

or:

    push @INC, [ \&my_sub, $x, $y, ... ];

    sub my_sub {

        my ($arrayref, $filename) = @_;

        # Retrieve $x, $y, ...

        my @parameters = @$arrayref[1..$#$arrayref];

...

If the hook is an object, it must provide an INC method that will be

called as above, the first parameter being the object itself. (Note that

you must fully qualify the sub's name, as it is always forced into package

main.) Here is a typical code layout:

    # In Foo.pm

    package Foo;

    sub new { ... }

    sub Foo::INC {

        my ($self, $filename) = @_;

...

    # In the main program

    push @INC, new Foo(...);

Note that these hooks are also permitted to set the %INC entry

corresponding to the files they have loaded. See %INC in the perlvar manpage.

For a yet-more-powerful import facility, see use and the perlmod manpage.

reset EXPR

reset

Generally used in a continue block at the end of a loop to clear

variables and reset ?? searches so that they work again. The

expression is interpreted as a list of single characters (hyphens

allowed for ranges). All variables and arrays beginning with one of

those letters are reset to their pristine state. If the expression is

omitted, one-match searches (?pattern?) are reset to match again. Resets

only variables or searches in the current package. Always returns

1. Examples:

    reset 'X';          # reset all X variables

    reset 'a-z';        # reset lower case variables

    reset;              # just reset ?one-time? searches

Resetting "A-Z" is not recommended because you'll wipe out your

@ARGV and @INC arrays and your %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 my.

return EXPR

return

Returns from a subroutine, eval, or 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 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 an explicit return, a subroutine, eval,

or do FILE will automatically return the value of the last expression

evaluated.)

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 characters

in the opposite order.

    print reverse <>;           # line tac, last line first

    undef $/;                   # for efficiency of <>

    print scalar reverse <>;    # character tac, last line tsrif

Used without arguments in scalar context, reverse() reverses $_.

This operator is also handy for inverting a hash, although there are some

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, such as from a DBM file.

    %by_name = reverse %by_address;     # Invert the hash

rewinddir DIRHANDLE

Sets the current position to the beginning of the directory for the

readdir routine on DIRHANDLE.

rindex STR,SUBSTR,POSITION

rindex STR,SUBSTR

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 beginning at or before that position.

rmdir FILENAME

rmdir

Deletes the directory specified by FILENAME if that directory is

empty. If it succeeds it returns true, otherwise it returns false and

sets $! (errno). If FILENAME is omitted, uses $_.

s///

The substitution operator. See the perlop manpage.

scalar EXPR

Forces EXPR to be interpreted in scalar context and returns the value

of EXPR.

    @counts = ( scalar @a, scalar @b, scalar @c );

There is no equivalent operator to force an expression to

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 @{[ (some expression) ]}, but usually a simple

(some expression) suffices.

Because 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 the perlop manpage for more details on unary operators and the comma operator.

seek FILEHANDLE,POSITION,WHENCE

Sets FILEHANDLE's position, just like the fseek call of stdio.

FILEHANDLE may be an expression whose value gives the name of the

filehandle. The values for WHENCE are 0 to set the new position

in bytes to POSITION, 1 to set it to the current position plus

POSITION, and 2 to set it to EOF plus POSITION (typically

negative). For WHENCE you may use the constants SEEK_SET,

SEEK_CUR, and SEEK_END (start of the file, current position, end

of the file) from the Fcntl module. Returns 1 upon success, 0

otherwise.

Note the in bytes: even if the filehandle has been set to

operate on characters (for example by using the :utf8 open

layer), tell() will return byte offsets, not character offsets

(because implementing that would render seek() and tell() rather slow).

If you want to position file for sysread or syswrite, don't use

seek--buffering makes its effect on the file's system position

unpredictable and non-portable. Use 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 1 (SEEK_CUR) is useful for not moving the file position:

    seek(TEST,0,1);

This is also useful for applications emulating 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 seek doesn't change the current position,

but it does clear the end-of-file condition on the handle, so that the

next <FILE> makes Perl try again to read something. We hope.

If that doesn't work (some IO implementations are particularly

cantankerous), then you may need something more like this:

    for (;;) {

        for ($curpos = tell(FILE); $_ = <FILE>;

             $curpos = tell(FILE)) {

            # search for some stuff and put it into files

        sleep($for_a_while);

        seek(FILE, $curpos, 0);

seekdir DIRHANDLE,POS

Sets the current position for the readdir routine on DIRHANDLE. POS

must be a value returned by telldir. seekdir also has the same caveats

about possible directory compaction as the corresponding system library

routine.

select FILEHANDLE

select

Returns the currently selected filehandle. Sets the current default

filehandle for output, if FILEHANDLE is supplied. This has two

effects: first, a write or a 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

do the following:

    select(REPORT1);

    $^ = 'report1_top';

    select(REPORT2);

    $^ = 'report2_top';

FILEHANDLE may be an expression whose value gives the name of the

actual filehandle. Thus:

    $oldfh = select(STDERR); $| = 1; select($oldfh);

Some programmers may prefer to think of filehandles as objects with

methods, preferring to write the last example as:

    use IO::Handle;

    STDERR->autoflush(1);

select RBITS,WBITS,EBITS,TIMEOUT

This calls the the select(2) manpage system call with the bit masks specified, which

can be constructed using fileno and vec, along these lines:

    $rin = $win = $ein = '';

    vec($rin,fileno(STDIN),1) = 1;

    vec($win,fileno(STDOUT),1) = 1;

    $ein = $rin | $win;

If you want to select on many filehandles you might wish to write a

subroutine:

    sub fhbits {

        my(@fhlist) = split(' ',$_[0]);

        my($bits);

        for (@fhlist) {

            vec($bits,fileno($_),1) = 1;

        $bits;

    $rin = fhbits('STDIN TTY SOCK');

The usual idiom is:

    ($nfound,$timeleft) =

      select($rout=$rin, $wout=$win, $eout=$ein, $timeout);

or to block until something becomes ready just do this

    $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);

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 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);

Note that whether select gets restarted after signals (say, SIGALRM)

is implementation-dependent. See also the perlport manpage for notes on the

portability of select.

On error, select behaves like the the select(2) manpage system call : it returns

-1 and sets $!.

Note: on some Unixes, the the select(2) manpage system call may report a socket file

descriptor as ``ready for reading'', when actually no data is available,

thus a subsequent read blocks. It can be avoided using always the

O_NONBLOCK flag on the socket. See the select(2) manpage and the fcntl(2) manpage for further

details.

WARNING: One should not attempt to mix buffered I/O (like read

or <FH>) with select, except as permitted by POSIX, and even

then only on POSIX systems. You have to use sysread instead.

semctl ID,SEMNUM,CMD,ARG

Calls the System V IPC function 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 that will hold the returned

semid_ds structure or semaphore value array. Returns like ioctl:

the undefined value for error, ``0 but true'' for zero, or the actual

return value otherwise. The ARG must consist of a vector of native

short integers, which may be created with pack("s!",(0)x$nsem).

See also SysV IPC in the perlipc manpage, IPC::SysV, IPC::Semaphore

documentation.

semget KEY,NSEMS,FLAGS

Calls the System V IPC function semget. Returns the semaphore id, or

the undefined value if there is an error. See also

SysV IPC in the perlipc manpage, IPC::SysV, IPC::SysV::Semaphore

documentation.

semop KEY,OPSTRING

Calls the System V IPC function semop to perform semaphore operations

such as signalling and waiting. OPSTRING must be a packed array of

semop structures. Each semop structure can be generated with

pack("s!3", $semnum, $semop, $semflag). The length of OPSTRING

implies the number of semaphore operations. Returns true if

successful, or false if there is an error. As an example, the

following code waits on semaphore $semnum of semaphore id $semid:

    $semop = pack("s!3", $semnum, -1, 0);

    die "Semaphore trouble: $!\n" unless semop($semid, $semop);

To signal the semaphore, replace -1 with 1. See also

SysV IPC in the perlipc manpage, IPC::SysV, and IPC::SysV::Semaphore

documentation.

send SOCKET,MSG,FLAGS,TO

send SOCKET,MSG,FLAGS

Sends a message on a socket. Attempts to send the scalar MSG to the

SOCKET filehandle. Takes the same flags as the system call of the

same name. On unconnected sockets you must specify a destination to

send TO, in which case it does a C sendto. Returns the number of

characters sent, or the undefined value if there is an error. The C

system call the sendmsg(2) manpage is currently unimplemented. See

UDP: Message Passing in the perlipc manpage for examples.

Note the characters: depending on the status of the socket, either

(8-bit) bytes or characters are sent. By default all sockets operate

on bytes, but for example if the socket has been changed using

binmode() to operate with the :utf8 I/O layer (see open, or the

open pragma, the open manpage), the I/O will operate on UTF-8 encoded

Unicode characters, not bytes. Similarly for the :encoding pragma:

in that case pretty much any characters can be sent.

setpgrp PID,PGRP

Sets the current process group for the specified PID, 0 for the current

process. Will produce a fatal error if used on a machine that doesn't

implement POSIX the setpgid(2) manpage or BSD setpgrp(2). If the arguments are omitted,

it defaults to 0,0. Note that the BSD 4.2 version of setpgrp does not

accept any arguments, so only setpgrp(0,0) is portable. See also

POSIX::setsid().

setpriority WHICH,WHO,PRIORITY

Sets the current priority for a process, a process group, or a user.

(See setpriority(2).) Will produce a fatal error if used on a machine

that doesn't implement setpriority(2).

setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL

Sets the socket option requested. Returns undefined if there is an

error. Use integer constants provided by the Socket module for

LEVEL and OPNAME. Values for LEVEL can also be obtained from

getprotobyname. OPTVAL might either be a packed string or an integer.

An integer OPTVAL is shorthand for pack(``i'', OPTVAL).

An example disabling the Nagle's algorithm for a socket:

    use Socket qw(IPPROTO_TCP TCP_NODELAY);

    setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);

shift ARRAY

shift

Shifts the first value of the array off and returns it, shortening the

array by 1 and moving everything down. If there are no elements in the

array, returns the undefined value. If ARRAY is omitted, shifts the

@_ array within the lexical scope of subroutines and formats, and the

@ARGV array at file scopes or within the lexical scopes established by

the eval '', BEGIN {}, INIT {}, CHECK {}, and END {}

constructs.

See also unshift, push, and pop. shift and unshift do the

same thing to the left end of an array that pop and push do to the

right end.

shmctl ID,CMD,ARG

Calls the System V IPC function shmctl. You'll probably have to say

    use IPC::SysV;

first to get the correct constant definitions. If CMD is IPC_STAT,

then ARG must be a variable that will hold the returned shmid_ds

structure. Returns like ioctl: the undefined value for error, ``0 but

true`` for zero, or the actual return value otherwise.

See also SysV IPC in the perlipc manpage and IPC::SysV documentation.

shmget KEY,SIZE,FLAGS

Calls the System V IPC function shmget. Returns the shared memory

segment id, or the undefined value if there is an error.

See also SysV IPC in the perlipc manpage and IPC::SysV documentation.

shmread ID,VAR,POS,SIZE

shmwrite ID,STRING,POS,SIZE

Reads or writes the System V shared memory segment ID starting at

position POS for size SIZE by attaching to it, copying in/out, and

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.

shmread() taints the variable. See also SysV IPC in the perlipc manpage,

IPC::SysV documentation, and the IPC::Shareable module from CPAN.

shutdown SOCKET,HOW

Shuts down a socket connection in the manner indicated by HOW, which

has the same interpretation as in the system call of the same name.

    shutdown(SOCKET, 0);    # I/we have stopped reading data

    shutdown(SOCKET, 1);    # I/we have stopped writing data

    shutdown(SOCKET, 2);    # I/we have stopped using this socket

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 file descriptor in any forked copies in other

processes.

sin EXPR

sin

Returns the sine of EXPR (expressed in radians). If EXPR is omitted,

returns sine of $_.

For the inverse sine operation, you may use the Math::Trig::asin

function, or use this relation:

    sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }

sleep EXPR

sleep

Causes the script to sleep for EXPR seconds, or forever if no EXPR.

May be interrupted if the process receives a signal such as SIGALRM.

Returns the number of seconds actually slept. You probably cannot

mix alarm and sleep calls, because sleep is often implemented

using 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

always sleep the full amount. They may appear to sleep longer than that,

however, because your process might not be scheduled right away in a

busy multitasking system.

For delays of finer granularity than one second, you may use Perl's

syscall interface to access the setitimer(2) manpage if your system supports

it, or else see select above. The Time::HiRes module (from CPAN,

and starting from Perl 5.8 part of the standard distribution) may also

help.

See also the POSIX module's pause function.

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 use Socket first

to get the proper definitions imported. See the examples in

Sockets: Client/Server Communication in the perlipc manpage.

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 $^F in the perlvar manpage.

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.

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 $^F in the perlvar manpage.

Some systems defined pipe in terms of socketpair, in which a call

to pipe(Rdr, Wtr) is essentially:

    use Socket;

    socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);

    shutdown(Rdr, 1);        # no more writing for reader

    shutdown(Wtr, 0);        # no more reading for writer

See the perlipc manpage for an example of socketpair use. Perl 5.8 and later will

emulate socketpair using IP sockets to localhost if your system implements

sockets but not socketpair.

sort SUBNAME LIST

sort BLOCK LIST

sort LIST

In list context, this sorts the LIST and returns the sorted list value.

In scalar context, the behaviour of sort() is undefined.

If SUBNAME or BLOCK is omitted, sorts 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 0,

depending on how the elements of the list are to be ordered. (The <

<=> >> and cmp operators are extremely useful in such routines.)

SUBNAME may be a scalar variable name (unsubscripted), in which case

the value provides the name of (or a reference to) the actual

subroutine to use. In place of a SUBNAME, you can provide a BLOCK as

an anonymous, in-line sort subroutine.

If the subroutine's prototype is ($$), the elements to be compared

are passed by reference in @_, as for a normal subroutine. This is

slower than unprototyped subroutines, where the elements to be

compared are passed into the subroutine

as the package global variables $a and $b (see example below). Note that

in the latter case, it is usually counter-productive to declare $a and

$b as lexicals.

In either case, the subroutine may not be recursive. The values to be

compared are always passed by reference and should not be modified.

You also cannot exit out of the sort block or subroutine using any of the

loop control operators described in the perlsyn manpage or with goto.

When use locale is in effect, sort LIST sorts LIST according to the

current collation locale. See the perllocale manpage.

sort() 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 sort() (for example, in a foreach, map or grep)

actually modifies the element in the original list. This is usually

something to be avoided when writing clear code.

Perl 5.6 and earlier used a quicksort algorithm to implement sort.

That algorithm was not stable, and could go quadratic. (A stable sort

preserves the input order of elements that compare equal. Although

quicksort's run time is O(NlogN? ) when averaged over all arrays of

length N, the time can be O(N**2), quadratic behavior, for some

inputs.) In 5.7, the quicksort implementation was replaced with

a stable mergesort algorithm whose worst-case behavior is O(NlogN? ).

But benchmarks indicated that for some inputs, on some platforms,

the original quicksort was faster. 5.8 has a sort pragma for

limited control of the sort. Its rather blunt control of the

underlying algorithm may not persist into future Perls, but the

ability to characterize the input or output in implementation

independent ways quite probably will. See the sort manpage.

Examples:

    # sort lexically

    @articles = sort @files;

    # same thing, but with explicit sort routine

    @articles = sort {$a cmp $b} @files;

    # now case-insensitively

    @articles = sort {uc($a) cmp uc($b)} @files;

    # same thing in reversed order

    @articles = sort {$b cmp $a} @files;

    # sort numerically ascending

    @articles = sort {$a <=> $b} @files;

    # 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;

    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;

            # prints xdogcatCainAbel

    print sort @george, 'to', @harry;

            # prints AbelAxedCainPunishedcatchaseddoggonetoxyz

    # inefficiently sort by descending numeric compare using

    # the first integer after the first = sign, or the

    # whole record case-insensitively otherwise

    @new = sort {

        ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]

||

                    uc($a)  cmp  uc($b)

    } @old;

    # same thing, but much more efficiently;

    # we'll build auxiliary indices instead

    # for speed

    @nums = @caps = ();

    for (@old) {

        push @nums, /=(\d+)/;

        push @caps, uc($_);

    @new = @old[ sort {

                        $nums[$b] <=> $nums[$a]

||

                        $caps[$a] cmp $caps[$b]

                       } 0..$#old

];

    # same thing, but without any temps

    @new = map { $_->[0] }

           sort { $b->[1] <=> $a->[1]

||

                  $a->[2] cmp $b->[2]

           } map { [$_, /=(\d+)/, uc($_)] } @old;

    # using a prototype allows you to use any comparison subroutine

    # as a sort subroutine (including other package's subroutines)

    package other;

    sub backwards ($$) { $_[1] cmp $_[0]; }     # $a and $b are not set here

    package main;

    @new = sort other::backwards @old;

    # guarantee stability, regardless of algorithm

    use sort 'stable';

    @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;

    # force use of mergesort (not portable outside Perl 5.8)

    use sort '_mergesort';  # note discouraging _

    @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;

If you're using strict, you must not declare $a

and $b as lexicals. They are package globals. That means

if you're in the main package and type

    @articles = sort {$b <=> $a} @files;

then $a and $b are $main::a and $main::b (or $::a and $::b),

but if you're in the FooPack package, it's the same as typing

    @articles = sort {$FooPack::b <=> $FooPack::a} @files;

The comparison function is required to behave. If it returns

inconsistent results (sometimes saying $x[1] is less than $x[2] and

sometimes saying the opposite, for example) the results are not

well-defined.

Because <=> returns undef when either operand is NaN

(not-a-number), and because sort will trigger a fatal error unless the

result of a comparison is defined, when sorting with a comparison function

like $a <=> $b, be careful about lists that might contain a NaN.

The following example takes advantage of the fact that NaN = NaN? to

eliminate any NaNs from the input.

    @result = sort { $a <=> $b } grep { $_ == $_ } @input;

splice ARRAY,OFFSET,LENGTH,LIST

splice ARRAY,OFFSET,LENGTH

splice ARRAY,OFFSET

splice ARRAY

Removes the elements designated by OFFSET and LENGTH from an array, and

replaces them with the elements of LIST, if any. In list context,

returns the elements removed from the array. In scalar context,

returns the last element removed, or undef if no elements are

removed. The array grows or shrinks as necessary.

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, removes the elements from OFFSET onward

except for -LENGTH elements at the end of the array.

If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is

past the end of the array, perl issues a warning, and splices at the

end of the array.

The following equivalences hold (assuming $[ = 0 and $#a > $i )

    push(@a,$x,$y)      splice(@a,@a,0,$x,$y)

    pop(@a)             splice(@a,-1)

    shift(@a)           splice(@a,0,1)

    unshift(@a,$x,$y)   splice(@a,0,0,$x,$y)

    $a[$i] = $y         splice(@a,$i,1,$y)

Example, assuming array lengths are passed before arrays:

    sub aeq {   # compare two list values

        my(@a) = splice(@_,0,shift);

        my(@b) = splice(@_,0,shift);

        return 0 unless @a == @b;       # same len?

        while (@a) {

            return 0 if pop(@a) ne pop(@b);

        return 1;

    if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }

split /PATTERN/,EXPR,LIMIT

split /PATTERN/,EXPR

split /PATTERN/

split

Splits the string EXPR into a list of strings and returns that list. By

default, empty leading fields are preserved, and empty trailing ones are

deleted. (If all fields are empty, they are considered to be trailing.)

In scalar context, returns the number of fields found and splits into

the @_ array. Use of split in scalar context is deprecated, however,

because it clobbers your subroutine arguments.

If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,

splits on whitespace (after skipping any leading whitespace). Anything

matching PATTERN is taken to be a delimiter separating the fields. (Note

that the delimiter may be longer than one character.)

If LIMIT is specified and positive, it represents the maximum number

of fields the EXPR will be split into, though the actual number of

fields returned depends on the number of times PATTERN matches within

EXPR. If LIMIT is unspecified or zero, trailing null fields are

stripped (which potential users of pop would do well to remember).

If LIMIT is negative, it is treated as if an arbitrarily large LIMIT

had been specified. Note that splitting an EXPR that evaluates to the

empty string always returns the empty list, regardless of the LIMIT

specified.

A pattern matching the null string (not to be confused with

a null pattern //, which is just one member of the set of patterns

matching a null string) will split the value of EXPR into separate

characters at each point it matches that way. For example:

    print join(':', split(/ */, 'hi there'));

produces the output 'h:i:t:h:e:r:e'.

As a special case for split, using the empty pattern // specifically

matches only the null string, and is not be confused with the regular use

of // to mean ``the last successful pattern match''. So, for split,

the following:

    print join(':', split(//, 'hi there'));

produces the output 'h:i: :t:h:e:r:e'.

Empty leading (or trailing) fields are produced when there are positive

width matches at the beginning (or end) of the string; a zero-width match

at the beginning (or end) of the string does not produce an empty field.

For example:

   print join(':', split(/(?=\w)/, 'hi there!'));

produces the output 'h:i :t:h:e:r:e!'.

The LIMIT parameter can be used to split a line partially

    ($login, $passwd, $remainder) = split(/:/, $_, 3);

When assigning to a list, if LIMIT is omitted, or zero, Perl supplies

a LIMIT one larger than the number of variables in the list, to avoid

unnecessary work. For the list above LIMIT would have been 4 by

default. In time critical applications it behooves you not to split

into more fields than you really need.

If the PATTERN contains parentheses, additional list elements are

created from each matching substring in the delimiter.

    split(/([,-])/, "1-10,20", 3);

produces the list value

    (1, '-', 10, ',', 20)

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

    %hdrs   =  (UNIX_FROM => split /^(\S*?):\s*/m, $header);

The pattern /PATTERN/ may be replaced with an expression to specify

patterns that vary at runtime. (To do runtime compilation only once,

use /$variable/o.)

As a special case, specifying a PATTERN of space (' ') will split on

white space just as split with no arguments does. Thus, <a href="#item_split">split(' ') can

be used to emulate awk's default behavior, whereas <a href="#item_split">split(/ /)

will give you as many null initial fields as there are leading spaces.

A split on /\s+/ is like a <a href="#item_split">split(' ') except that any leading

whitespace produces a null first field. A split with no arguments

really does a <a href="#item_split">split(' ', $_) internally.

A PATTERN of /^/ is treated as if it were /^/m, since it isn't

much use otherwise.

Example:

    open(PASSWD, '/etc/passwd');

    while (<PASSWD>) {

        chomp;

        ($login, $passwd, $uid, $gid,

         $gcos, $home, $shell) = split(/:/);

        #...

As with regular pattern matching, any capturing parentheses that are not

matched in a split() will be set to undef when returned:

    @fields = split /(A)|B/, "1A2B3";

    # @fields is (1, 'A', 2, undef, 3)

sprintf FORMAT, LIST

Returns a string formatted by the usual printf conventions of the C

library function sprintf. See below for more details

and see the sprintf(3) manpage or the printf(3) manpage on your system for an explanation of

the general principles.

For example:

        # Format number with up to 8 leading zeroes

        $result = sprintf("%08d", $number);

        # Round number to 3 digits after decimal point

        $rounded = sprintf("%.3f", $number);

Perl does its own sprintf formatting--it emulates the C

function 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 sprintf are not

available from Perl.

Unlike printf, sprintf does not do what you probably mean when you

pass it an array as your first argument. The array is given scalar context,

and instead of using the 0th element of the array as the format, Perl will

use the count of elements in the array as the format, which is almost never

useful.

Perl's sprintf permits the following universally-known conversions:

   %%   a percent sign

   %c   a character with the given number

   %s   a string

   %d   a signed integer, in decimal

   %u   an unsigned integer, in decimal

   %o   an unsigned integer, in octal

   %x   an unsigned integer, in hexadecimal

   %e   a floating-point number, in scientific notation

   %f   a floating-point number, in fixed decimal notation

   %g   a floating-point number, in %e or %f notation

In addition, Perl permits the following widely-supported conversions:

   %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

Finally, for backward (and we do mean ``backward'') compatibility, Perl

permits these unnecessary but widely-supported conversions:

   %i   a synonym for %d

   %D   a synonym for %ld

   %U   a synonym for %lu

   %O   a synonym for %lo

   %F   a synonym for %f

Note that the number of exponent digits in the scientific notation produced

by %e, %E, %g and %G for numbers with the modulus of the

exponent less than 100 is system-dependent: it may be three or less

(zero-padded as necessary). In other words, 1.23 times ten to the

99th may be either ``1.23e99'' or ``1.23e099''.

Between the % and the format letter, you may specify a number of

additional attributes controlling the interpretation of the format.

In order, these are:

format parameter index

An explicit format parameter index, such as 2$. By default sprintf

will format the next unused argument in the list, but this allows you

to take the arguments out of order, e.g.:

  printf '%2$d %1$d', 12, 34;      # prints "34 12"

  printf '%3$d %d %1$d', 1, 2, 3;  # prints "3 1 1"

flags

one or more of:

   space   prefix positive number with a space

   +       prefix positive number with a plus sign

   -       left-justify within the field

   0       use zeros, not spaces, to right-justify

   #       prefix non-zero octal with "0", non-zero hex with "0x",

           non-zero binary with "0b"

For example:

  printf '<% d>', 12;   # prints "< 12>"

  printf '<%+d>', 12;   # prints "<+12>"

  printf '<%6s>', 12;   # prints "<    12>"

  printf '<%-6s>', 12;  # prints "<12    >"

  printf '<%06s>', 12;  # prints "<000012>"

  printf '<%#x>', 12;   # prints "<0xc>"

vector flag

This flag tells perl to interpret the supplied string as a vector of

integers, one for each character in the string. Perl applies the format to

each integer in turn, then joins the resulting strings with a separator (a

dot . by default). This can be useful for displaying ordinal values of

characters in arbitrary strings:

  printf "%vd", "AB\x{100}";           # prints "65.66.256"

  printf "version is v%vd\n", $^V;     # Perl's version

Put an asterisk * before the v to override the string to

use to separate the numbers:

  printf "address is %*vX\n", ":", $addr;   # IPv6 address

  printf "bits are %0*v8b\n", " ", $bits;   # random bitstring

You can also explicitly specify the argument number to use for

the join string using e.g. *2$v:

  printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":";   # 3 IPv6 addresses

(minimum) width

Arguments are usually formatted to be only as wide as required to

display the given value. You can override the width by putting

a number here, or get the width from the next argument (with *)

or from a specified argument (with e.g. *2$):

  printf '<%s>', "a";       # prints "<a>"

  printf '<%6s>', "a";      # prints "<     a>"

  printf '<%*s>', 6, "a";   # prints "<     a>"

  printf '<%*2$s>', "a", 6; # prints "<     a>"

  printf '<%2s>', "long";   # prints "<long>" (does not truncate)

If a field width obtained through * is negative, it has the same

effect as the - flag: left-justification.

precision, or maximum width

You can specify a precision (for numeric conversions) or a maximum

width (for string conversions) by specifying a . followed by a number.

For floating point formats, with the exception of 'g' and 'G', this specifies

the number of decimal places to show (the default being 6), e.g.:

  # these examples are subject to system-specific variation

  printf '<%f>', 1;    # prints "<1.000000>"

  printf '<%.1f>', 1;  # prints "<1.0>"

  printf '<%.0f>', 1;  # prints "<1>"

  printf '<%e>', 10;   # prints "<1.000000e+01>"

  printf '<%.1e>', 10; # prints "<1.0e+01>"

For 'g' and 'G', this specifies the maximum number of digits to show,

including prior to the decimal point as well as after it, e.g.:

  # these examples are subject to system-specific variation

  printf '<%g>', 1;        # prints "<1>"

  printf '<%.10g>', 1;     # prints "<1>"

  printf '<%g>', 100;      # prints "<100>"

  printf '<%.1g>', 100;    # prints "<1e+02>"

  printf '<%.2g>', 100.01; # prints "<1e+02>"

  printf '<%.5g>', 100.01; # prints "<100.01>"

  printf '<%.4g>', 100.01; # prints "<100>"

For integer conversions, specifying a precision implies that the

output of the number itself should be zero-padded to this width:

  printf '<%.6x>', 1;      # prints "<000001>"

  printf '<%#.6x>', 1;     # prints "<0x000001>"

  printf '<%-10.6x>', 1;   # prints "<000001    >"

For string conversions, specifying a precision truncates the string

to fit in the specified width:

  printf '<%.5s>', "truncated";   # prints "<trunc>"

  printf '<%10.5s>', "truncated"; # prints "<     trunc>"

You can also get the precision from the next argument using .*:

  printf '<%.6x>', 1;       # prints "<000001>"

  printf '<%.*x>', 6, 1;    # prints "<000001>"

You cannot currently get the precision from a specified number,

but it is intended that this will be possible in the future using

e.g. .*2$:

  printf '<%.*2$x>', 1, 6;   # INVALID, but in future will print "<000001>"

size

For numeric conversions, you can specify the size to interpret the

number as using l, h, V, q, L, or ll. For integer

conversions (d u o x X b i D U O), numbers are usually assumed to be

whatever the default integer size is on your platform (usually 32 or 64

bits), but you can override this to use instead one of the standard C types,

as supported by the compiler used to build Perl:

   l           interpret integer as C type "long" or "unsigned long"

   h           interpret integer as C type "short" or "unsigned short"

   q, L or ll  interpret integer as C type "long long", "unsigned long long".

               or "quads" (typically 64-bit integers)

The last will produce errors if Perl does not understand ``quads'' in your

installation. (This requires that either the platform natively supports quads

or Perl was specifically compiled to support quads.) You can find out

whether your Perl supports quads via the Config manpage:

        use Config;

        ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&

                print "quads\n";

For floating point conversions (e f g E F G), numbers are usually assumed

to be the default floating point size on your platform (double or long double),

but you can force 'long double' with q, L, or ll if your

platform supports them. You can find out whether your Perl supports long

doubles via the Config manpage:

        use Config;

        $Config{d_longdbl} eq 'define' && print "long doubles\n";

You can find out whether Perl considers 'long double' to be the default

floating point size to use on your platform via the Config manpage:

        use Config;

        ($Config{uselongdouble} eq 'define') &&

                print "long doubles by default\n";

It can also be the case that long doubles and doubles are the same thing:

        use Config;

        ($Config{doublesize} == $Config{longdblsize}) &&

                print "doubles are long doubles\n";

The size specifier V has no effect for Perl code, but it is supported

for compatibility with XS code; it means 'use the standard size for

a Perl integer (or floating-point number)', which is already the

default for Perl code.

order of arguments

Normally, sprintf takes the next unused argument as the value to

format for each format specification. If the format specification

uses * to require additional arguments, these are consumed from

the argument list in the order in which they appear in the format

specification before the value to format. Where an argument is

specified using an explicit index, this does not affect the normal

order for the arguments (even when the explicitly specified index

would have been the next argument in any case).

So:

  printf '<%*.*s>', $a, $b, $c;

would use $a for the width, $b for the precision and $c

as the value to format, while:

  print '<%*1$.*s>', $a, $b;

would use $a for the width and the precision, and $b as the

value to format.

Here are some more examples - beware that when using an explicit

index, the $ may need to be escaped:

  printf "%2\$d %d\n",    12, 34;               # will print "34 12\n"

  printf "%2\$d %d %d\n", 12, 34;               # will print "34 12 34\n"

  printf "%3\$d %d %d\n", 12, 34, 56;           # will print "56 12 34\n"

  printf "%2\$*3\$d %d\n", 12, 34, 3;           # will print " 34 12\n"

use localethe perllocale manpage

sqrt EXPR

sqrt

Return the square root of EXPR. If EXPR is omitted, returns square

root of $_. Only works on non-negative operands, unless you've

loaded the standard Math::Complex module.

    use Math::Complex;

    print sqrt(-2);    # prints 1.4142135623731i

srand EXPR

srand

Sets the random number seed for the rand operator.

The point of the function is to ``seed'' the rand function so that

rand can produce a different sequence each time you run your

program.

If srand() is not called explicitly, it is called implicitly at the

first use of the rand operator. However, this was not the case in

versions of Perl before 5.004, so if your script will run under older

Perl versions, it should call srand.

Most programs won't even call srand() at all, except those that

need a cryptographically-strong starting point rather than the

generally acceptable default, which is based on time of day,

process ID, and memory allocation, or the /dev/urandom device,

if available.

You can call srand($seed) with the same $seed to reproduce the

same sequence from rand(), but this is usually reserved for

generating predictable results for testing or debugging.

Otherwise, don't call srand() more than once in your program.

Do not call srand() (i.e. without an argument) more than once in

a script. The internal state of the random number generator should

contain more entropy than can be provided by any seed, so calling

srand() again actually loses randomness.

Most implementations of srand take an integer and will silently

truncate decimal numbers. This means srand(42) will usually

produce the same results as srand(42.1). To be safe, always pass

srand an integer.

In versions of Perl prior to 5.004 the default seed was just the

current time. This isn't a particularly good seed, so many old

programs supply their own seed value (often time ^ $$ or time ^

($$ + ($$ << 15))>), but that isn't necessary any more.

For cryptographic purposes, however, you need something much more random

than the default seed. Checksumming the compressed output of one or more

rapidly changing operating system status programs is the usual method. For

example:

    srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);

If you're particularly concerned with this, see the Math::TrulyRandom

module in CPAN.

Frequently called programs (like CGI scripts) that simply use

    time ^ $$

for a seed can fall prey to the mathematical property that

    a^b == (a+1)^(b+1)

one-third of the time. So don't do that.

stat FILEHANDLE

stat EXPR

stat

Returns a 13-element list giving the status info for a file, either

the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,

it stats $_. Returns a null list if the stat fails. Typically used

as follows:

    ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,

       $atime,$mtime,$ctime,$blksize,$blocks)

           = stat($filename);

Not all fields are supported on all filesystem types. Here are the

meanings of the fields:

  0 dev      device number of filesystem

  1 ino      inode number

  2 mode     file mode  (type and permissions)

  3 nlink    number of (hard) links to the file

  4 uid      numeric user ID of file's owner

  5 gid      numeric group ID of file's owner

  6 rdev     the device identifier (special files only)

  7 size     total size of file, in bytes

  8 atime    last access time in seconds since the epoch

  9 mtime    last modify time in seconds since the epoch

 10 ctime    inode change time in seconds since the epoch (*)

 11 blksize  preferred block size for file system I/O

 12 blocks   actual number of blocks allocated

(The epoch was at 00:00 January 1, 1970 GMT.)

(*) Not all fields are supported on all filesystem types. Notably, the

ctime field is non-portable. In particular, you cannot expect it to be a

``creation time'', see Files and Filesystems in the perlport manpage for details.

If stat is passed the special filehandle consisting of an underline, no

stat is done, but the current contents of the stat structure from the

last stat, lstat, or filetest are returned. Example:

    if (-x $file && (($d) = stat(_)) && $d < 0) {

        print "$file is executable NFS file\n";

(This works on machines only for which the device number is negative

under NFS.)

Because the mode contains both the file type and its permissions, you

should mask off the file type portion and (s)printf using a "%o"

if you want to see the real permissions.

    $mode = (stat($filename))[2];

    printf "Permissions are %04o\n", $mode & 07777;

In scalar context, stat returns a boolean value indicating success

or failure, and, if successful, sets the information associated with

the special filehandle _.

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;

You can import symbolic mode constants (S_IF*) and functions

(S_IS*) from the Fcntl module:

    use Fcntl ':mode';

    $mode = (stat($filename))[2];

    $user_rwx      = ($mode & S_IRWXU) >> 6;

    $group_read    = ($mode & S_IRGRP) >> 3;

    $other_execute =  $mode & S_IXOTH;

    printf "Permissions are %04o\n", S_IMODE($mode), "\n";

    $is_setuid     =  $mode & S_ISUID;

    $is_setgid     =  S_ISDIR($mode);

You could write the last two using the -u and -d operators.

The commonly available S_IF* constants are

    # Permissions: read, write, execute, for user, group, others.

    S_IRWXU S_IRUSR S_IWUSR S_IXUSR

    S_IRWXG S_IRGRP S_IWGRP S_IXGRP

    S_IRWXO S_IROTH S_IWOTH S_IXOTH

    # Setuid/Setgid/Stickiness/SaveText.

    # Note that the exact meaning of these is system dependent.

    S_ISUID S_ISGID S_ISVTX S_ISTXT

    # File types.  Not necessarily all are available on your system.

    S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT

    # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.

    S_IREAD S_IWRITE S_IEXEC

and the S_IF* functions are

    S_IMODE($mode)      the part of $mode containing the permission bits

                        and the setuid/setgid/sticky bits

    S_IFMT($mode)       the part of $mode containing the file type

                        which can be bit-anded with e.g. S_IFREG

                        or with the following functions

    # The operators -f, -d, -l, -b, -c, -p, and -S.

    S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)

    S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)

    # No direct -X operator counterpart, but for the first one

    # the -g operator is often equivalent.  The ENFMT stands for

    # record flocking enforcement, a platform-dependent feature.

    S_ISENFMT($mode) S_ISWHT($mode)

See your native the chmod(2) manpage and the stat(2) manpage documentation for more details

about the S_* constants. To get status info for a symbolic link

instead of the target file behind the link, use the lstat function.

study SCALAR

study

Takes extra time to study SCALAR ($_ if unspecified) in anticipation of

doing many pattern matches on the string before it is next modified.

This may or may not save time, depending on the nature and number of

patterns you are searching on, and on the distribution of character

frequencies in the string to be searched--you probably want to compare

run times with and without it to see which runs faster. Those loops

that scan for many short constant strings (including the constant

parts of more complex patterns) will benefit most. You may have only

one study active at a time--if you study a different scalar the first

is ``unstudied''. (The way 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 'k' characters are. From each search string,

the rarest character is selected, based on some static frequency tables

constructed from some C programs and English text. Only those places

that contain this ``rarest'' character are examined.)

For example, here is a loop that inserts index producing entries

before any line containing a certain pattern:

    while (<>) {

        study;

        print ".IX foo\n"       if /\bfoo\b/;

        print ".IX bar\n"       if /\bbar\b/;

        print ".IX blurfl\n"    if /\bblurfl\b/;

        # ...

        print;

In searching for /\bfoo\b/, only those locations in $_ that contain f

will be looked at, because f is rarer than o. In general, this is

a big win except in pathological cases. The only question is whether

it saves you more time than it took to build the linked list in the

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 eval that to

avoid recompiling all your patterns all the time. Together with

undefining $/ to input entire files as one record, this can be very

fast, often faster than specialized programs like fgrep(1). The following

scans a list of files (@files) for a list of words (@words), and prints

out the names of those files that contain a match:

    $search = 'while (<>) { study;';

    foreach $word (@words) {

        $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";

    $search .= "}";

    @ARGV = @files;

    undef $/;

    eval $search;               # this screams

    $/ = "\n";          # put back to normal input delimiter

    foreach $file (sort keys(%seen)) {

        print $file, "\n";

sub NAME BLOCK

sub NAME (PROTO) BLOCK

sub NAME : ATTRS BLOCK

sub NAME (PROTO) : ATTRS BLOCK

This is subroutine definition, not a real function per se.

Without a BLOCK it's just a forward declaration. Without a NAME,

it's an anonymous function declaration, and does actually return

a value: the CODE ref of the closure you just created.

See the perlsub manpage and the perlref manpage for details about subroutines and

references, and the attributes manpage and the Attribute::Handlers manpage for more

information about attributes.

substr EXPR,OFFSET,LENGTH,REPLACEMENT

substr EXPR,OFFSET,LENGTH

substr EXPR,OFFSET

Extracts a substring out of EXPR and returns it. First character is at

offset 0, or whatever you've set $[ to (but don't do that).

If OFFSET is negative (or more precisely, less than $[), starts

that far from the end of the string. If LENGTH is omitted, returns

everything to the end of the string. If LENGTH is negative, leaves that

many characters off the end of the string.

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 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,

just as you can with splice().

symlink OLDFILE,NEWFILE

Creates a new filename symbolically linked to the old filename.

Returns 1 for success, 0 otherwise. On systems that don't support

symbolic links, produces a fatal error at run time. To check for that,

use eval:

    $symlink_exists = eval { symlink("",""); 1 };

syscall NUMBER, LIST

Calls the system call specified as the first element of the list,

passing the remaining elements as arguments to the system call. If

unimplemented, produces a fatal error. The arguments are interpreted

as follows: if a given argument is numeric, the argument is passed as

an int. If not, the pointer to the string value is passed. You are

responsible to make sure a string is pre-extended long enough to

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 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 0 to them to force them to look

like numbers. This emulates the syswrite function (or vice versa):

    require 'syscall.ph';               # may need to run h2ph

    $s = "hi there\n";

    syscall(&SYS_write, fileno(STDOUT), $s, length $s);

Note that Perl supports passing of up to only 14 arguments to your system call,

which in practice should usually suffice.

Syscall returns whatever value returned by the system call it calls.

If the system call fails, syscall returns -1 and sets $! (errno).

Note that some system calls can legitimately return -1. The proper

way to handle such calls is to assign $!=0; before the call and

check the value of $! if syscall returns -1.

There's a problem with 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 pipe instead.

sysopen FILEHANDLE,FILENAME,MODE

sysopen FILEHANDLE,FILENAME,MODE,PERMS

Opens the file whose filename is given by FILENAME, and associates it

with FILEHANDLE. If FILEHANDLE is an expression, its value is used as

the name of the real filehandle wanted. This function calls the

underlying operating system's 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 Fcntl.

See the documentation of your operating system's open to see which

values and flag bits are available. You may combine several flags

using the |-operator.

Some of the most common values are O_RDONLY for opening the file in

read-only mode, O_WRONLY for opening the file in write-only mode,

and O_RDWR for opening the file in read-write mode.

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 not work under

OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to

use them in new code.

If the file named by FILENAME does not exist and the open call creates

it (typically because MODE includes the O_CREAT flag), then the value of

PERMS specifies the permissions of the newly created file. If you omit

the PERMS argument to sysopen, Perl uses the octal value 0666.

These permission values need to be in octal, and are modified by your

process's current umask.

In many systems the O_EXCL flag is available for opening files in

exclusive mode. This is not locking: exclusiveness means here that

if the file already exists, sysopen() fails. O_EXCL may not work

on network filesystems, and has no effect unless the O_CREAT flag

is set as well. Setting O_CREAT|O_EXCL prevents the file from

being opened if it is a symbolic link. It does not protect against

symbolic links in the file's path.

Sometimes you may want to truncate an already-existing file. This

can be done using the O_TRUNC flag. The behavior of

O_TRUNC with O_RDONLY is undefined.

You should seldom if ever use 0644 as argument to sysopen, because

that takes away the user's option to have a more permissive umask.

Better to omit it. See the the perlfunc(1) manpage entry on umask for more

on this.

Note that sysopen depends on the fdopen() C library function.

On many UNIX systems, fdopen() is known to fail when file descriptors

exceed a certain value, typically 255. If you need more file

descriptors than that, consider rebuilding Perl to use the sfio

library, or perhaps using the POSIX::open() function.

See the perlopentut manpage for a kinder, gentler explanation of opening files.

sysread FILEHANDLE,SCALAR,LENGTH,OFFSET

sysread FILEHANDLE,SCALAR,LENGTH

Attempts to read LENGTH bytes of data into variable SCALAR from the

specified FILEHANDLE, using the system call read(2). It bypasses

buffered IO, so mixing this with other kinds of reads, print,

write, seek, tell, or eof can cause confusion because the

perlio or stdio layers usually buffers data. Returns the number of

bytes actually read, 0 at end of file, or undef if there was an

error (in the latter case $! is also set). SCALAR will be grown or

shrunk so that the last byte actually read is the last byte of the

scalar after the read.

An OFFSET may be specified to place the read data at some place in the

string other than the beginning. A negative OFFSET specifies

placement at that many characters counting backwards from the end of

the string. A positive OFFSET greater than the length of SCALAR

results in the string being padded to the required size with "\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.

Note that if the filehandle has been marked as :utf8 Unicode

characters are read instead of bytes (the LENGTH, OFFSET, and the

return value of sysread() are in Unicode characters).

The :encoding(...) layer implicitly introduces the :utf8 layer.

See binmode, open, and the open pragma, the open manpage.

sysseek FILEHANDLE,POSITION,WHENCE

Sets FILEHANDLE's system position in bytes using the system call

lseek(2). FILEHANDLE may be an expression whose value gives the name

of the filehandle. The values for WHENCE are 0 to set the new

position to POSITION, 1 to set the it to the current position plus

POSITION, and 2 to set it to EOF plus POSITION (typically

negative).

Note the in bytes: even if the filehandle has been set to operate

on characters (for example by using the :utf8 I/O layer), tell()

will return byte offsets, not character offsets (because implementing

that would render sysseek() very slow).

sysseek() bypasses normal buffered IO, so mixing this with reads (other

than sysread, for example <> or read()) print, write,

seek, tell, or eof may cause confusion.

For WHENCE, you may also use the constants SEEK_SET, SEEK_CUR,

and SEEK_END (start of the file, current position, end of the file)

from the Fcntl module. Use of the constants is also more portable

than relying on 0, 1, and 2. For example to define a ``systell'' function:

        use Fcntl 'SEEK_CUR';

        sub systell { sysseek($_[0], 0, SEEK_CUR) }

Returns the new position, or the undefined value on failure. A position

of zero is returned as the string "0 but true"; thus sysseek returns

true on success and false on failure, yet you can still easily determine

the new position.

system LIST

system PROGRAM LIST

Does exactly the same thing as 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 /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 execvp, which is

more efficient.

Beginning with v5.6.0, Perl will attempt to flush all files opened for

output before any operation that may do a fork, but this may not be

supported on some platforms (see the perlport manpage). To be safe, you may need

to set $| ($AUTOFLUSH in English) or call the autoflush() method

of IO::Handle on any open handles.

The return value is the exit status of the program as returned by the

wait call. To get the actual exit value, shift right by eight (see

below). See also exec. This is not what you want to use to capture

the output from a command, for that you should use merely backticks or

qx//, as described in `STRING` in the perlop manpage. Return value of -1

indicates a failure to start the program or an error of the the wait(2) manpage system

call (inspect $! for the reason).

Like exec, system allows you to lie to a program about its name if

you use the system PROGRAM LIST syntax. Again, see exec.

Since SIGINT and SIGQUIT are ignored during the execution of

system, if you expect your program to terminate on receipt of these

signals you will need to arrange to do so yourself based on the return

value.

    @args = ("command", "arg1", "arg2");

    system(@args) == 0

         or die "system @args failed: $?"

You can check all the failure possibilities by inspecting

$? like this:

    if ($? == -1) {

        print "failed to execute: $!\n";

    elsif ($? & 127) {

        printf "child died with signal %d, %s coredump\n",

            ($? & 127),  ($? & 128) ? 'with' : 'without';

    else {

        printf "child exited with value %d\n", $? >> 8;

or more portably by using the W*() calls of the POSIX extension;

see the perlport manpage for more information.

When the arguments get executed via the system shell, results

and return codes will be subject to its quirks and capabilities.

See `STRING` in the perlop manpage and exec for details.

syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET

syswrite FILEHANDLE,SCALAR,LENGTH

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 buffered IO, so

mixing this with reads (other than sysread()), print, write,

seek, tell, or eof may cause confusion because the perlio and

stdio layers usually buffers data. Returns the number of bytes

actually written, or undef if there was an error (in this case the

errno variable $! is also set). 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

that many characters counting backwards from the end of the string.

In the case the SCALAR is empty you can use OFFSET but only zero offset.

Note that if the filehandle has been marked as :utf8, Unicode

characters are written instead of bytes (the LENGTH, OFFSET, and the

return value of syswrite() are in UTF-8 encoded Unicode characters).

The :encoding(...) layer implicitly introduces the :utf8 layer.

See binmode, open, and the open pragma, the open manpage.

tell FILEHANDLE

tell

Returns the current position in bytes for FILEHANDLE, or -1 on

error. FILEHANDLE may be an expression whose value gives the name of

the actual filehandle. If FILEHANDLE is omitted, assumes the file

last read.

Note the in bytes: even if the filehandle has been set to

operate on characters (for example by using the :utf8 open

layer), tell() will return byte offsets, not character offsets

(because that would render seek() and tell() rather slow).

The return value of tell() for the standard streams like the STDIN

depends on the operating system: it may return -1 or something else.

tell() on pipes, fifos, and sockets usually returns -1.

There is no systell function. Use sysseek(FH, 0, 1) for that.

Do not use tell() (or other buffered I/O operations) on a file handle

that has been manipulated by sysread(), syswrite() or sysseek().

Those functions ignore the buffering, while tell() does not.

telldir DIRHANDLE

Returns the current position of the readdir routines on DIRHANDLE.

Value may be given to seekdir to access a particular location in a

directory. telldir has the same caveats about possible directory

compaction as the corresponding system library routine.

tie VARIABLE,CLASSNAME,LIST

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 new

method of the class (meaning TIESCALAR, TIEHANDLE, TIEARRAY,

or TIEHASH). Typically these are arguments such as might be passed

to the dbm_open() function of C. The object returned by the new

method is also returned by the tie function, which would be useful

if you want to access other methods in CLASSNAME.

Note that functions such as keys and values may return huge lists

when used on large objects, like DBM files. You may prefer to use the

each function to iterate over such. Example:

    # print out history file offsets

    use NDBM_File;

    tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);

    while (($key,$val) = each %HIST) {

        print $key, ' = ', unpack('L',$val), "\n";

    untie(%HIST);

A class implementing a hash should have the following methods:

    TIEHASH classname, LIST

    FETCH this, key

    STORE this, key, value

    DELETE this, key

    CLEAR this

    EXISTS this, key

    FIRSTKEY this

    NEXTKEY this, lastkey

    SCALAR this

    DESTROY this

    UNTIE this

A class implementing an ordinary array should have the following methods:

    TIEARRAY classname, LIST

    FETCH this, key

    STORE this, key, value

    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

    UNTIE 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

    BINMODE this

    EOF this

    FILENO this

    SEEK this, position, whence

    TELL this

    OPEN this, mode, LIST

    CLOSE this

    DESTROY this

    UNTIE this

A class implementing a scalar should have the following methods:

    TIESCALAR classname, LIST

    FETCH this,

    STORE this, value

    DESTROY this

    UNTIE this

Not all methods indicated above need be implemented. See the perltie manpage,

the Tie::Hash manpage, the Tie::Array manpage, the Tie::Scalar manpage, and the Tie::Handle manpage.

Unlike dbmopen, the tie function will not use or require a module

for you--you need to do that explicitly yourself. See the DB_File manpage

or the Config module for interesting tie implementations.

For further details see the perltie manpage, tied VARIABLE.

tied VARIABLE

Returns a reference to the object underlying VARIABLE (the same value

that was originally returned by the tie call that bound the variable

to a package.) Returns the undefined value if VARIABLE isn't tied to a

package.

time

Returns the number of non-leap seconds since whatever time the system

considers to be the epoch, suitable for feeding to gmtime and

localtime. On most systems the epoch is 00:00:00 UTC, January 1, 1970;

a prominent exception being Mac OS Classic which uses 00:00:00, January 1,

1904 in the current local time zone for its epoch.

For measuring time in better granularity than one second,

you may use either the Time::HiRes module (from CPAN, and starting from

Perl 5.8 part of the standard distribution), or if you have

gettimeofday(2), you may be able to use the syscall interface of Perl.

See the perlfaq8 manpage for details.

times

Returns a four-element list giving the user and system times, in

seconds, for this process and the children of this process.

    ($user,$system,$cuser,$csystem) = times;

In scalar context, times returns $user.

tr///

The transliteration operator. Same as y///. See the perlop manpage.

truncate FILEHANDLE,LENGTH

truncate EXPR,LENGTH

Truncates the file opened on FILEHANDLE, or named by EXPR, to the

specified length. Produces a fatal error if truncate isn't implemented

on your system. Returns true if successful, the undefined value

otherwise.

The behavior is undefined if LENGTH is greater than the length of the

file.

uc EXPR

uc

Returns an uppercased version of EXPR. This is the internal function

implementing the \U escape in double-quoted strings. Respects

current LC_CTYPE locale if use locale in force. See the perllocale manpage

and the perlunicode manpage for more details about locale and Unicode support.

It does not attempt to do titlecase mapping on initial letters. See

ucfirst for that.

If EXPR is omitted, uses $_.

ucfirst EXPR

ucfirst

Returns the value of EXPR with the first character in uppercase

(titlecase in Unicode). This is the internal function implementing

the \u escape in double-quoted strings. Respects current LC_CTYPE

locale if use locale in force. See the perllocale manpage and the perlunicode manpage

for more details about locale and Unicode support.

If EXPR is omitted, uses $_.

umask EXPR

umask

Sets the umask for the process to EXPR and returns the previous value.

If EXPR is omitted, merely returns the current umask.

The Unix permission rwxr-x--- is represented as three sets of three

bits, or three octal digits: 0750 (the leading 0 indicates octal

and isn't one of the digits). The umask value is such a number

representing disabled permissions bits. The permission (or ``mode'')

values you pass mkdir or sysopen are modified by your umask, so

even if you tell sysopen to create a file with permissions 0777,

if your umask is 0022 then the file will actually be created with

permissions 0755. If your umask were 0027 (group can't

write; others can't read, write, or execute), then passing

sysopen 0666 would create a file with mode 0640 (0666 &~

027> is 0640).

Here's some advice: supply a creation mode of 0666 for regular

files (in sysopen) and one of 0777 for directories (in

mkdir) and executable files. This gives users the freedom of

choice: if they want protected files, they might choose process umasks

of 022, 027, or even the particularly antisocial mask of 077.

Programs should rarely if ever make policy decisions better left to

the user. The exception to this is when writing files that should be

kept private: mail files, web browser cookies, .rhosts files, and

so on.

If the umask(2) manpage is not implemented on your system and you are trying to

restrict access for yourself (i.e., (EXPR & 0700) > 0), produces a

fatal error at run time. If the umask(2) manpage is not implemented and you are

not trying to restrict access for yourself, returns undef.

Remember that a umask is a number, usually given in octal; it is not a

string of octal digits. See also oct, if all you have is a string.

undef EXPR

undef

Undefines the value of EXPR, which must be an lvalue. Use only on a

scalar value, an array (using @), a hash (using %), a subroutine

(using &), or a typeglob (using *). (Saying undef $hash{$key}

will probably not do what you expect on most predefined variables or

DBM list values, so don't do that; see the delete manpage.) Always returns the

undefined value. You can omit the EXPR, in which case nothing is

undefined, but you still get an undefined value that you could, for

instance, return from a subroutine, assign to a variable or pass as a

parameter. Examples:

    undef $foo;

    undef $bar{'blurfl'};      # Compare to: delete $bar{'blurfl'};

    undef @ary;

    undef %hash;

    undef &mysub;

    undef *xyz;       # destroys $xyz, @xyz, %xyz, &xyz, etc.

    return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;

    select undef, undef, undef, 0.25;

    ($a, $b, undef, $c) = &foo;       # Ignore third value returned

Note that this is a unary operator, not a list operator.

unlink LIST

unlink

Deletes a list of files. Returns the number of files successfully

deleted.

    $cnt = unlink 'a', 'b', 'c';

    unlink @goners;

    unlink <*.bak>;

Note: unlink will not attempt to delete directories unless you are superuser

and the -U flag is supplied to Perl. Even if these conditions are

met, be warned that unlinking a directory can inflict damage on your

filesystem. Finally, using unlink on directories is not supported on

many operating systems. Use rmdir instead.

If LIST is omitted, uses $_.

unpack TEMPLATE,EXPR

unpack does the reverse of pack: it takes a string

and expands it out into a list of values.

(In scalar context, it returns merely the first value produced.)

The string is broken into chunks described by the TEMPLATE. Each chunk

is converted separately to a value. Typically, either the string is a result

of pack, or the bytes of the string represent a C structure of some

kind.

The TEMPLATE has the same format as in the pack function.

Here's a subroutine that does substring:

    sub substr {

        my($what,$where,$howmuch) = @_;

        unpack("x$where a$howmuch", $what);

and then there's

    sub ordinal { unpack("c",$_[0]); } # same as ord()

In addition to fields allowed in pack(), you may prefix a field with

a %<number> to indicate that

you want a <number>-bit checksum of the items instead of the items

themselves. Default is a 16-bit checksum. Checksum is calculated by

summing numeric values of expanded values (for string fields the sum of

ord($char) is taken, for bit fields the sum of zeroes and ones).

For example, the following

computes the same number as the System V sum program:

    $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 p and P formats should be used with care. Since Perl

has no way of checking whether the value passed to unpack()

corresponds to a valid memory location, passing a pointer value that's

not known to be valid is likely to have disastrous consequences.

If there are more pack codes or if the repeat count of a field or a group

is larger than what the remainder of the input string allows, the result

is not well defined: in some cases, the repeat count is decreased, or

unpack() will produce null strings or zeroes, or terminate with an

error. If the input string is longer than one described by the TEMPLATE,

the rest is ignored.

See pack for more examples and notes.

untie VARIABLE

Breaks the binding between a variable and a package. (See tie.)

Has no effect if the variable is not tied.

unshift ARRAY,LIST

Does the opposite of a shift. Or the opposite of a 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 reverse to do the

reverse.

use Module VERSION LIST

use Module VERSION

use Module LIST

use Module

use VERSION

Imports some semantics into the current package from the named module,

generally by aliasing certain subroutine or variable names into your

package. It is exactly equivalent to

    BEGIN { require Module; import Module LIST; }

except that Module must be a bareword.

VERSION may be either a numeric argument such as 5.006, which will be

compared to $], or a literal of the form v5.6.1, which will be compared

to $^V (aka $PERL_VERSION. A fatal error is produced if VERSION is

greater than the version of the current Perl interpreter; Perl will not

attempt to parse the rest of the file. Compare with require, which can

do a similar check at run time.

Specifying VERSION as a literal of the form v5.6.1 should generally be

avoided, because it leads to misleading error messages under earlier

versions of Perl that do not support this syntax. The equivalent numeric

version should be used instead.

    use v5.6.1;         # compile time version check

    use 5.6.1;          # ditto

    use 5.006_001;      # ditto; preferred for backwards compatibility

This is often useful if you need to check the current Perl version before

useing library modules that have changed in incompatible ways from

older versions of Perl. (We try not to do this more than we have to.)

The BEGIN forces the require and import to happen at compile time. The

require makes sure the module is loaded into memory if it hasn't been

yet. The import is not a builtin--it's just an ordinary static method

call into the Module package to tell the module to import the list of

features back into the current package. The module can implement its

import method any way it likes, though most modules just choose to

derive their import method via inheritance from the Exporter class that

is defined in the Exporter module. See the Exporter manpage. If no import

method can be found then the call is skipped.

If you do not want to call the package's import method (for instance,

to stop your namespace from being altered), explicitly supply the empty list:

    use Module ();

That is exactly equivalent to

    BEGIN { require Module }

If the VERSION argument is present between Module and LIST, then the

use will call the VERSION method in class Module with the given

version as an argument. The default VERSION method, inherited from

the UNIVERSAL class, croaks if the given version is larger than the

value of the variable $Module::VERSION.

Again, there is a distinction between omitting LIST (import called

with no arguments) and an explicit empty LIST () (import not

called). Note that there is no comma after VERSION!

Because this is a wide-open interface, pragmas (compiler directives)

are also implemented this way. Currently implemented pragmas are:

    use constant;

    use diagnostics;

    use integer;

    use sigtrap  qw(SEGV BUS);

    use strict   qw(subs vars refs);

    use subs     qw(afunc blurfl);

    use warnings qw(all);

    use sort     qw(stable _quicksort _mergesort);

Some of these pseudo-modules import semantics into the current

block scope (like strict or integer, unlike ordinary modules,

which import symbols into the current package (which are effective

through the end of the file).

There's a corresponding no command that unimports meanings imported

by use, i.e., it calls unimport Module LIST instead of import.

    no integer;

    no strict 'refs';

    no warnings;

See the perlmodlib manpage for a list of standard modules and pragmas. See the perlrun manpage

for the -M and -m command-line options to perl that give use

functionality from the command-line.

utime LIST

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 change time of each file is set

to the current time. For example, this code has the same effect as the

Unix the touch(1) manpage command when the files already exist and belong to

the user running the program:

    #!/usr/bin/perl

    $atime = $mtime = time;

    utime $atime, $mtime, @ARGV;

Since perl 5.7.2, if the first two elements of the list are undef, then

the the utime(2) manpage function in the C library will be called with a null second

argument. On most systems, this will set the file's access and

modification times to the current time (i.e. equivalent to the example

above) and will even work on other users' files where you have write

permission:

    utime undef, undef, @ARGV;

Under NFS this will use the time of the NFS server, not the time of

the local machine. If there is a time synchronization problem, the

NFS server and local machine will have different times. The Unix

the touch(1) manpage command will in fact normally use this form instead of the

one shown in the first example.

Note that only passing one of the first two elements as undef will

be equivalent of passing it as 0 and will not have the same effect as

described when they are both undef. This case will also trigger an

uninitialized warning.

values HASH

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. 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 keys or each

function would produce on the same (unmodified) hash. Since Perl

5.8.1 the ordering is different even between different runs of Perl

for security reasons (see Algorithmic Complexity Attacks in the perlsec manpage).

As a side effect, calling values() resets the HASH's internal iterator,

see each. (In particular, calling values() in void context resets

the iterator with no other overhead.)

Note that the values are not copied, which means modifying them will

modify the contents of the hash:

    for (values %hash)      { s/foo/bar/g }   # modifies %hash values

    for (@hash{keys %hash}) { s/foo/bar/g }   # same

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Perl Functions by Category

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Alphabetical Listing of Perl Functions