r1 - 04 Mar 2006 - NelsonFerraz

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. Elements of the LIST should be separated by commas. 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, getprgp, 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

a 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 have 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])  }

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 the function doing the blessing might be inherited by a

derived class. 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, so 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

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.

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

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 chomp.

chown LIST

Changes the owner (and group) of a list of files. The first two

elements of the list must be the numeric uid and gid, in that

order. A value of -1 in either position is interpreted by most

systems to leave that value unchanged. Returns the number of files

successfully changed.

    $cnt = chown $uid, $gid, 'foo', 'bar';

    chown $uid, $gid, @filenames;

Here's an example that looks up nonnumeric uids in the passwd file:

    print "User: ";

    chomp($user = <STDIN>);

    print "Files: ";

    chomp($pattern = <STDIN>);

    ($login,$pass,$uid,$gid) = getpwnam($user)

        or die "$user not in passwd file";

    @ary = glob($pattern);      # expand filenames

    chown $uid, $gid, @ary;

On most systems, you are not allowed to change the ownership of the

file unless you're the superuser, although you should be able to change

the group to any of your secondary groups. On insecure systems, these

restrictions may be relaxed, but this is not a portable assumption.

On POSIX systems, you can detect this condition this way:

    use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);

    $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);

chr NUMBER

chr

Returns the character represented by that NUMBER in the character set.

For example, chr(65) is "A" in either ASCII or Unicode, and

chr(0x263a) is a Unicode smiley face. Note that characters from 128

to 255 (inclusive) are by default not encoded in UTF-8 Unicode for

backward compatibility reasons (but see the encoding manpage).

If NUMBER is omitted, uses $_.

For the reverse, use ord.

Note that under the bytes pragma the NUMBER is masked to

the low eight bits.

See the perlunicode manpage and the encoding manpage for more about Unicode.

chroot FILENAME

chroot

This function works like the system call by the same name: it makes the

named directory the new root directory for all further pathnames that

begin with a / by your process and all its children. (It doesn't

change your current working directory, which is unaffected.) For security

reasons, this call is restricted to the superuser. If FILENAME is

omitted, does a chroot to $_.

close FILEHANDLE

close

Closes the file or pipe associated with the file handle, returning

true only if IO buffers are successfully flushed and closes the system

file descriptor. Closes the currently selected filehandle if the

argument is omitted.

You don't have to close FILEHANDLE if you are immediately going to do

another open on it, because open will close it for you. (See

open.) However, an explicit close on an input file resets the line

counter ($.), while the implicit close done by open does not.

If the file handle came from a piped open, close will additionally

return false if one of the other system calls involved fails, or if the

program exits with non-zero status. (If the only problem was that the

program exited non-zero, $! will be set to 0.) Closing a pipe

also waits for the process executing on the pipe to complete, in case you

want to look at the output of the pipe afterwards, and

implicitly puts the exit status value of that command into $?.

Prematurely closing the read end of a pipe (i.e. before the process

writing to it at the other end has closed it) will result in a

SIGPIPE being delivered to the writer. If the other end can't

handle that, be sure to read all the data before closing the pipe.

Example:

    open(OUTPUT, '|sort >foo')  # pipe to sort

        or die "Can't start sort: $!";

    #...                        # print stuff to output

    close OUTPUT                # wait for sort to finish

        or warn $! ? "Error closing sort pipe: $!"

                   : "Exit status $? from sort";

    open(INPUT, 'foo')          # get sort's results

        or die "Can't open 'foo' for input: $!";

FILEHANDLE may be an expression whose value can be used as an indirect

filehandle, usually the real filehandle name.

closedir DIRHANDLE

Closes a directory opened by opendir and returns the success of that

system call.

connect SOCKET,NAME

Attempts to connect to a remote socket, just as the connect system call

does. Returns true if it succeeded, false otherwise. NAME should be a

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

Sockets: Client/Server Communication in the perlipc manpage.

continue BLOCK

Actually a flow control statement rather than a function. If there is a

continue BLOCK attached to a BLOCK (typically in a while or

foreach), it is always executed just before the conditional is about to

be evaluated again, just like the third part of a for loop in C. Thus

it can be used to increment a loop variable, even when the loop has been

continued via the next statement (which is similar to the C continue

statement).

last, next, or redo may appear within a continue

block. last and redo will behave as if they had been executed within

the main block. So will next, but since it will execute a continue

block, it may be more entertaining.

    while (EXPR) {

        ### redo always comes here

        do_something;

    } continue {

        ### next always comes here

        do_something_else;

        # then back the top to re-check EXPR

    ### last always comes here

Omitting the continue section is semantically equivalent to using an

empty one, logically enough. In that case, next goes directly back

to check the condition at the top of the loop.

cos EXPR

cos

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

takes cosine of $_.

For the inverse cosine operation, you may use the Math::Trig::acos()

function, or use this relation:

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

crypt PLAINTEXT,SALT

Encrypts a string exactly like the the crypt(3) manpage function in the C library

(assuming that you actually have a version there that has not been

extirpated as a potential munition). This can prove useful for checking

the password file for lousy passwords, amongst other things. Only the

guys wearing white hats should do this.

Note that crypt is intended to be a one-way function, much like

breaking eggs to make an omelette. There is no (known) corresponding

decrypt function (in other words, the crypt() is a one-way hash

function). As a result, this function isn't all that useful for

cryptography. (For that, see your nearby CPAN mirror.)

When verifying an existing encrypted string you should use the

encrypted text as the salt (like crypt($plain, $crypted) eq

$crypted>). This allows your code to work with the standard crypt

and with more exotic implementations. In other words, do not assume

anything about the returned string itself, or how many bytes in

the encrypted string matter.

Traditionally the result is a string of 13 bytes: two first bytes of

the salt, followed by 11 bytes from the set [./0-9A-Za-z], and only

the first eight bytes of the encrypted string mattered, but

alternative hashing schemes (like MD5), higher level security schemes

(like C2), and implementations on non-UNIX platforms may produce

different strings.

When choosing a new salt create a random two character string whose

characters come from the set [./0-9A-Za-z] (like join '', ('.',

'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of

characters is just a recommendation; the characters allowed in

the salt depend solely on your system's crypt library, and Perl can't

restrict what salts crypt() accepts.

Here's an example that makes sure that whoever runs this program knows

their own password:

    $pwd = (getpwuid($<))[1];

    system "stty -echo";

    print "Password: ";

    chomp($word = <STDIN>);

    print "\n";

    system "stty echo";

    if (crypt($word, $pwd) ne $pwd) {

        die "Sorry...\n";

    } else {

        print "ok\n";

Of course, typing in your own password to whoever asks you

for it is unwise.

The crypt function is unsuitable for encrypting large quantities

of data, not least of all because you can't get the information

back. Look at the by-module/Crypt and by-module/PGP directories

on your favorite CPAN mirror for a slew of potentially useful

modules.

If using crypt() on a Unicode string (which potentially has

characters with codepoints above 255), Perl tries to make sense

of the situation by trying to downgrade (a copy of the string)

the string back to an eight-bit byte string before calling crypt()

(on that copy). If that works, good. If not, crypt() dies with

Wide character in crypt.

dbmclose HASH

[This function has been largely superseded by the untie function.]

Breaks the binding between a DBM file and a hash.

dbmopen HASH,DBNAME,MASK

[This function has been largely superseded by the tie function.]

This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a

hash. HASH is the name of the hash. (Unlike normal open, the first

argument is not a filehandle, even though it looks like one). DBNAME

is the name of the database (without the .dir or .pag extension if

any). If the database does not exist, it is created with protection

specified by MASK (as modified by the umask). If your system supports

only the older DBM functions, you may perform only one dbmopen in your

program. In older versions of Perl, if your system had neither DBM nor

ndbm, calling dbmopen produced a fatal error; it now falls back to

sdbm(3).

If you don't have write access to the DBM file, you can only read hash

variables, not set them. If you want to test whether you can write,

either use file tests or try setting a dummy hash entry inside an eval,

which will trap the error.

Note that functions such as keys and values may return huge lists

when used on large DBM files. You may prefer to use the each

function to iterate over large DBM files. Example:

    # print out history file offsets

    dbmopen(%HIST,'/usr/lib/news/history',0666);

    while (($key,$val) = each %HIST) {

        print $key, ' = ', unpack('L',$val), "\n";

    dbmclose(%HIST);

See also the AnyDBM ? _File manpage for a more general description of the pros and

cons of the various dbm approaches, as well as the DB_File manpage for a particularly

rich implementation.

You can control which DBM library you use by loading that library

before you call dbmopen():

    use DB_File;

    dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")

        or die "Can't open netscape history file: $!";

defined EXPR

defined

Returns a Boolean value telling whether EXPR has a value other than

the undefined value undef. If EXPR is not present, $_ will be

checked.

Many operations return undef to indicate failure, end of file,

system error, uninitialized variable, and other exceptional

conditions. This function allows you to distinguish undef from

other values. (A simple Boolean test will not distinguish among

undef, zero, the empty string, and "0", which are all equally

false.) Note that since undef is a valid scalar, its presence

doesn't necessarily indicate an exceptional condition: pop

returns undef when its argument is an empty array, or when the

element to return happens to be undef.

You may also use defined(&func) to check whether subroutine &func

has ever been defined. The return value is unaffected by any forward

declarations of &func. Note that a subroutine which is not defined

may still be callable: its package may have an AUTOLOAD method that

makes it spring into existence the first time that it is called -- see

the perlsub manpage.

Use of defined on aggregates (hashes and arrays) is deprecated. It

used to report whether memory for that aggregate has ever been

allocated. This behavior may disappear in future versions of Perl.

You should instead use a simple test for size:

    if (@an_array) { print "has array elements\n" }

    if (%a_hash)   { print "has hash members\n"   }

When used on a hash element, it tells you whether the value is defined,

not whether the key exists in the hash. Use exists for the latter

purpose.

Examples:

    print if defined $switch{'D'};

    print "$val\n" while defined($val = pop(@ary));

    die "Can't readlink $sym: $!"

        unless defined($value = readlink $sym);

    sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }

    $debugging = 0 unless defined $debugging;

Note: Many folks tend to overuse defined, and then are surprised to

discover that the number 0 and "" (the zero-length string) are, in fact,

defined values. For example, if you say

    "ab" =~ /a(.*)b/;

The pattern match succeeds, and $1 is defined, despite the fact that it

matched ``nothing''. But it didn't really match nothing--rather, it

matched something that happened to be zero characters long. This is all

very above-board and honest. When a function returns an undefined value,

it's an admission that it couldn't give you an honest answer. So you

should use defined only when you're questioning the integrity of what

you're trying to do. At other times, a simple comparison to 0 or "" is

what you want.

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NAME

DESCRIPTION

Perl Functions by Category

Portability

Alphabetical Listing of Perl Functions