Re: Exceptions in IPC::Open2

[perl5.git] / pod / perlfunc.pod

=head1 NAME

perlfunc - Perl builtin functions

=head1 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 L<perlop>.)  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 and list
contexts for its arguments.  If it does both, the scalar arguments will
be first, and the list argument will follow.  (Note that there can ever
be only one list argument.)  For instance, splice() has three scalar
arguments followed by a list.

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 I<LOOKS> like a function, therefore it I<IS> a
function, and precedence doesn't matter.  Otherwise it's a list
operator or unary operator, and precedence does matter.  And whitespace
between the function and left parenthesis doesn't count--so you need to
be careful sometimes:

    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 B<-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.

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 rule:

=over 8

=item  I<THERE IS NO GENERAL RULE FOR CONVERTING A LIST INTO A SCALAR!>

=back

Each operator and function decides which sort of value it would be most
appropriate to return in a scalar context.  Some operators return the
length of the list that would have been returned in a 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.

=head2 Perl Functions by Category

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

=over

=item Functions for SCALARs or strings

C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
C<length>, C<oct>, C<ord>, C<pack>, C<q>/STRING/, C<qq>/STRING/, C<reverse>,
C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y>///

=item Regular expressions and pattern matching

C<m>//, C<pos>, C<quotemeta>, C<s>///, C<split>, C<study>

=item Numeric functions

C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
C<sin>, C<sqrt>, C<srand>

=item Functions for real @ARRAYs

C<pop>, C<push>, C<shift>, C<splice>, C<unshift>

=item Functions for list data

C<grep>, C<join>, C<map>, C<qw>/STRING/, C<reverse>, C<sort>, C<unpack>

=item Functions for real %HASHes

C<delete>, C<each>, C<exists>, C<keys>, C<values>

=item Input and output functions

C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
C<warn>, C<write>

=item Functions for fixed length data or records

C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>

=item Functions for filehandles, files, or directories

C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>, C<readlink>,
C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>, C<unlink>, C<utime>

=item Keywords related to the control flow of your perl program

C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>

=item Keywords related to scoping

C<caller>, C<import>, C<local>, C<my>, C<package>, C<use>

=item Miscellaneous functions

C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>,
C<scalar>, C<undef>, C<wantarray>

=item Functions for processes and process groups

C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
C<pipe>, C<qx>/STRING/, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
C<times>, C<wait>, C<waitpid>

=item Keywords related to perl modules

C<do>, C<import>, C<no>, C<package>, C<require>, C<use>

=item Keywords related to classes and object-orientedness

C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
C<untie>, C<use>

=item Low-level socket functions

C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
C<socket>, C<socketpair>

=item System V interprocess communication functions

C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>

=item Fetching user and group info

C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
C<getpwuid>, C<setgrent>, C<setpwent>

=item Fetching network info

C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
C<setnetent>, C<setprotoent>, C<setservent>

=item Time-related functions

C<gmtime>, C<localtime>, C<time>, C<times>

=item Functions new in perl5

C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>,
C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>

* - C<sub> was a keyword in perl4, but in perl5 it is an
operator which can be used in expressions.

=item Functions obsoleted in perl5

C<dbmclose>, C<dbmopen>

=back

=head2 Alphabetical Listing of Perl Functions

=over 8

=item I<-X> FILEHANDLE

=item I<-X> EXPR

=item I<-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 C<-t>, which tests STDIN.
Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
the undefined value if the file doesn't exist.  Despite the funny
names, precedence is the same as any other named unary operator, and
the argument may be parenthesized like any other unary operator.  The
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.
    -s  File has nonzero size (returns size).

    -f  File is a plain file.
    -d  File is a directory.
    -l  File is a symbolic link.
    -p  File is a named pipe (FIFO).
    -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 a text file.
    -B  File is a binary file (opposite of -T).

    -M  Age of file in days when script started.
    -A  Same for access time.
    -C  Same for inode change time.

The interpretation of the file permission operators C<-r>, C<-R>, C<-w>,
C<-W>, C<-x>, and C<-X> is 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.  Also note that, for the superuser,
C<-r>, C<-R>, C<-w>, and C<-W> always return 1, and C<-x> and C<-X> return
1 if any execute bit is set in the mode.  Scripts run by the superuser may
thus need to do a stat() to determine the actual mode of the
file, or temporarily set the uid to something else.

Example:

    while (<>) {
        chop;
        next unless -f $_;      # ignore specials
        ...
    }

Note that C<-s/a/b/> does not do a negated substitution.  Saying
C<-exp($foo)> still works as expected, however--only single letters
following a minus are interpreted as file tests.

The C<-T> and C<-B> switches work as follows.  The first block or so of the
file is examined for odd characters such as strange control codes or
characters with the high bit set.  If too many odd characters (E<gt>30%)
are found, it's a C<-B> file, otherwise it's a C<-T> file.  Also, any file
containing null in the first block is considered a binary file.  If C<-T>
or C<-B> is used on a filehandle, the current stdio buffer is examined
rather than the first block.  Both C<-T> and C<-B> return TRUE on a null
file, or a file at EOF when testing a filehandle.  Because you have to
read a file to do the C<-T> test, on most occasions you want to use a C<-f>
against the file first, as in C<next unless -f $file && -T $file>.

If any of the file tests (or either the 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 C<-t>, and you need to remember
that lstat() and C<-l> will leave values in the stat structure for the
symbolic link, not the real file.)  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 _;

=item abs VALUE

=item abs

Returns the absolute value of its argument.
If VALUE is omitted, uses $_.

=item accept NEWSOCKET,GENERICSOCKET

Accepts an incoming socket connect, just as the accept(2) system call
does.  Returns the packed address if it succeeded, FALSE otherwise.
See example in L<perlipc/"Sockets: Client/Server Communication">.

=item alarm SECONDS

=item alarm

Arranges to have a SIGALRM delivered to this process after the
specified number of 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 than you
specified because of how seconds are counted.)  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
syscall() interface to access setitimer(2) if your system supports it,
or else see L</select()>.  It is usually a mistake to intermix alarm()
and sleep() calls.

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.

    eval {
        local $SIG{ALRM} = sub { die "alarm\n" };       # NB \n required
        alarm $timeout;
        $nread = sysread SOCKET, $buffer, $size;
        alarm 0;
    };
    die if $@ && $@ ne "alarm\n";       # propagate errors
    if ($@) {
        # timed out
    }
    else {
        # didn't
    }

=item atan2 Y,X

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

For the tangent operation, you may use the POSIX::tan()
function, or use the familiar relation:

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

=item bind SOCKET,NAME

Binds a network address to a socket, just as the bind system call
does.  Returns TRUE if it succeeded, FALSE otherwise.  NAME should be a
packed address of the appropriate type for the socket.  See the examples in
L<perlipc/"Sockets: Client/Server Communication">.

=item binmode FILEHANDLE

Arranges for the file to be read or written in "binary" mode in operating
systems that distinguish between binary and text files.  Files that are
not in binary mode have CR LF sequences translated to LF on input and LF
translated to CR LF on output.  Binmode has no effect under Unix; in MS-DOS
and similarly archaic systems, it may be imperative--otherwise your
MS-DOS-damaged C library may mangle your file.  The key distinction between
systems that need binmode and those that don't is their text file
formats.  Systems like Unix and Plan9 that delimit lines with a single
character, and that encode that character in C as '\n', do not need
C<binmode>.  The rest need it.  If FILEHANDLE is an expression, the value
is taken as the name of the filehandle.

=item bless REF,CLASSNAME

=item bless REF

This function tells the thingy referenced by REF that it is now
an object in the CLASSNAME package--or the current package if no CLASSNAME
is specified, which is often the case.  It returns the reference for
convenience, because a bless() is often the last thing in a constructor.
Always use the two-argument version if the function doing the blessing
might be inherited by a derived class.  See L<perlobj> for more about the
blessing (and blessings) of objects.

=item caller EXPR

=item caller

Returns the context of the current subroutine call.  In a 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 a 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, $is_require) = caller($i);

Here $subroutine may be C<"(eval)"> if the frame is not a subroutine
call, but an C<eval>.  In such a case additional elements $evaltext and
$is_require are set: $is_require is true if the frame is created by a
C<require> or C<use> statement, $evaltext contains the text of the
C<eval EXPR> statement.  In particular, for a C<eval BLOCK> statement,
$filename is C<"(eval)">, but $evaltext is undefined.  (Note also that
each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
frame.

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.

=item chdir EXPR

Changes the working directory to EXPR, if possible.  If EXPR is
omitted, changes to home directory.  Returns TRUE upon success, FALSE
otherwise.  See example under die().

=item 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 I<not> a string of octal digits:
C<0644> is okay, C<'0644'> is not.  Returns the number of files
successfully changed.  See also L</oct>, if all you have is a string.

    $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

=item chomp VARIABLE

=item chomp LIST

=item chomp

This is a slightly safer version of L</chop>.  It removes any
line ending that corresponds to the current value of C<$/> (also known as
$INPUT_RECORD_SEPARATOR in the C<English> module).  It returns the total
number of characters removed from all its arguments.  It's often used to
remove the newline from the end of an input record when you're worried
that the final record may be missing its newline.  When in paragraph mode
(C<$/ = "">), it removes all trailing newlines from the string.  If
VARIABLE is omitted, it chomps $_.  Example:

    while (<>) {
        chomp;  # avoid \n on last field
        @array = split(/:/);
        ...
    }

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.

=item chop VARIABLE

=item chop LIST

=item chop

Chops off the last character of a string and returns the character
chopped.  It's used primarily to remove the newline from the end of an
input record, but is much more efficient than C<s/\n//> because it neither
scans nor copies the string.  If VARIABLE is omitted, chops $_.
Example:

    while (<>) {
        chop;   # avoid \n on last field
        @array = split(/:/);
        ...
    }

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

    chop($cwd = `pwd`);
    chop($answer = <STDIN>);

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 C<substr($string, 0, -1)>.

=item chown LIST

Changes the owner (and group) of a list of files.  The first two
elements of the list must be the I<NUMERICAL> uid and gid, in that order.
Returns the number of files successfully changed.

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

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

    print "User: ";
    chop($user = <STDIN>);
    print "Files: "
    chop($pattern = <STDIN>);

    ($login,$pass,$uid,$gid) = getpwnam($user)
        or die "$user not in passwd file";

    @ary = <${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.

=item chr NUMBER

=item chr

Returns the character represented by that NUMBER in the character set.
For example, C<chr(65)> is "A" in ASCII.  For the reverse, use L</ord>.

If NUMBER is omitted, uses $_.

=item chroot FILENAME

=item chroot

This function works as 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 of 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 chroot to $_.

=item close FILEHANDLE

Closes the file or pipe associated with the file handle, returning TRUE
only if stdio successfully flushes buffers and closes the system file
descriptor.

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 C<close> will additionally
return FALSE if one of the other system calls involved fails or if the
program exits with non-zero status.  (If the only problem was that the
program exited non-zero $! will be set to 0.) Also, closing a pipe will
wait for the process executing on the pipe to complete, in case you
want to look at the output of the pipe afterwards.  Closing a pipe
explicitly also puts the exit status value of the command into C<$?>.
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 gives the real filehandle name.

=item closedir DIRHANDLE

Closes a directory opened by opendir().

=item connect SOCKET,NAME

Attempts to connect to a remote socket, just as the connect system call
does.  Returns TRUE if it succeeded, FALSE otherwise.  NAME should be a
packed address of the appropriate type for the socket.  See the examples in
L<perlipc/"Sockets: Client/Server Communication">.

=item continue BLOCK

Actually a flow control statement rather than a function.  If there is a
C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
C<foreach>), it is always executed just before the conditional is about to
be evaluated again, just like the third part of a C<for> loop in C.  Thus
it can be used to increment a loop variable, even when the loop has been
continued via the C<next> statement (which is similar to the C C<continue>
statement).

C<last>, C<next>, or C<redo> may appear within a C<continue>
block. C<last> and C<redo> will behave as if they had been executed within
the main block. So will C<next>, but since it will execute a C<continue>
block, 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 C<continue> section is semantically equivalent to using an
empty one, logically enough. In that case, C<next> goes directly back
to check the condition at the top of the loop.

=item cos EXPR

Returns the cosine of EXPR (expressed in radians).  If EXPR is omitted
takes cosine of $_.

For the inverse cosine operation, you may use the POSIX::acos()
function, or use this relation:

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

=item crypt PLAINTEXT,SALT

Encrypts a string exactly like the crypt(3) 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.  As a result, this function isn't all that useful for
cryptography.  (For that, see your nearby CPAN mirror.)

Here's an example that makes sure that whoever runs this program knows
their own password:

    $pwd = (getpwuid($<))[1];
    $salt = substr($pwd, 0, 2);

    system "stty -echo";
    print "Password: ";
    chop($word = <STDIN>);
    print "\n";
    system "stty echo";

    if (crypt($word, $salt) ne $pwd) {
        die "Sorry...\n";
    } else {
        print "ok\n";
    }

Of course, typing in your own password to whoever asks you
for it is unwise.

=item dbmclose HASH

[This function has been superseded by the untie() function.]

Breaks the binding between a DBM file and a hash.

=item dbmopen HASH,DBNAME,MODE

[This function has been 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 I<NOT> a filehandle, even though it looks like one).  DBNAME
is the name of the database (without the F<.dir> or F<.pag> extension if
any).  If the database does not exist, it is created with protection
specified by MODE (as modified by the 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 L<AnyDBM_File> for a more general description of the pros and
cons of the various dbm approaches, as well as L<DB_File> for a particularly
rich implementation.

=item defined EXPR

=item defined

Returns a Boolean value telling whether EXPR has a value other than
the undefined value C<undef>.  If EXPR is not present, C<$_> will be
checked.

Many operations return C<undef> to indicate failure, end of file,
system error, uninitialized variable, and other exceptional
conditions.  This function allows you to distinguish C<undef> from
other values.  (A simple Boolean test will not distinguish among
C<undef>, zero, the empty string, and "0", which are all equally
false.)  Note that since C<undef> is a valid scalar, its presence
doesn't I<necessarily> indicate an exceptional condition: pop()
returns C<undef> when its argument is an empty array, I<or> when the
element to return happens to be C<undef>.

You may also use defined() to check whether a subroutine exists.  On
the other hand, use of defined() upon aggregates (hashes and arrays)
is not guaranteed to produce intuitive results, and should probably be
avoided.

When used on a hash element, it tells you whether the value is defined,
not whether the key exists in the hash.  Use L</exists> for the latter
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 0 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.

Currently, using defined() on an entire array or hash reports whether
memory for that aggregate has ever been allocated.  So an array you set
to the empty list appears undefined initially, and one that once was full
and that you then set to the empty list still appears defined.  You
should instead use a simple test for size:

    if (@an_array) { print "has array elements\n" }
    if (%a_hash)   { print "has hash members\n"   }

Using undef() on these, however, does clear their memory and then report
them as not defined anymore, but you shoudln't do that unless you don't
plan to use them again, because it saves time when you load them up
again to have memory already ready to be filled.

This counterintuitive behaviour of defined() on aggregates may be
changed, fixed, or broken in a future release of Perl.

See also L</undef>, L</exists>, L</ref>.

=item delete EXPR

Deletes the specified key(s) and their associated values from a hash.
For each key, returns the deleted value associated with that key, or
the undefined value if there was no such key.  Deleting from C<$ENV{}>
modifies the environment.  Deleting from a hash tied to a DBM file
deletes the entry from the DBM file.  (But deleting from a tie()d hash
doesn't necessarily return anything.)

The following deletes all the values of a hash:

    foreach $key (keys %HASH) {
        delete $HASH{$key};
    }

And so does this:

    delete @HASH{keys %HASH}

(But both of these are slower than the undef() command.)  Note that the
EXPR can be arbitrarily complicated as long as the final operation is a
hash element lookup or hash slice:

    delete $ref->[$x][$y]{$key};
    delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};

=item die LIST

Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
the current value of C<$!> (errno).  If C<$!> is 0, exits with the value of
C<($? E<gt>E<gt> 8)> (backtick `command` status).  If C<($? E<gt>E<gt> 8)>
is 0, exits with 255.  Inside an eval(), the error message is stuffed into
C<$@>, and the eval() is terminated with the undefined value; this makes
die() the way to raise an exception.

Equivalent examples:

    die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
    chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"

If the value of EXPR does not end in a newline, the current script line
number and input line number (if any) are also printed, and a newline
is supplied.  Hint: sometimes appending ", stopped" to your message
will cause it to make better sense when the string "at foo line 123" is
appended.  Suppose you are running script "canasta".

    die "/etc/games is no good";
    die "/etc/games is no good, stopped";

produce, respectively

    /etc/games is no good at canasta line 123.
    /etc/games is no good, stopped at canasta line 123.

See also exit() and warn().

If LIST is empty and $@ already contains a value (typically from a
previous eval) that value is reused after appending "\t...propagated".
This is useful for propagating exceptions:

    eval { ... };
    die unless $@ =~ /Expected exception/;

If $@ is empty then the string "Died" is used.

You can arrange for a callback to be called just before the die() does
its deed, by setting the C<$SIG{__DIE__}> hook.  The associated handler
will be called with the error text and can change the error message, if
it sees fit, by calling die() again.  See L<perlvar/$SIG{expr}> for details on
setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.

Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
blocks/strings.  If one wants the hook to do nothing in such
situations, put

        die @_ if $^S;

as the first line of the handler (see L<perlvar/$^S>).

=item do BLOCK

Not really a function.  Returns the value of the last command in the
sequence of commands indicated by BLOCK.  When modified by a loop
modifier, executes the BLOCK once before testing the loop condition.
(On other statements the loop modifiers test the conditional first.)

=item do SUBROUTINE(LIST)

A deprecated form of subroutine call.  See L<perlsub>.

=item do EXPR

Uses the value of EXPR as a filename and executes the contents of the
file as a Perl script.  Its primary use is to include subroutines
from a Perl subroutine library.

    do 'stat.pl';

is just like

    scalar eval `cat stat.pl`;

except that it's more efficient, more concise, keeps track of the
current filename for error messages, and searches all the B<-I>
libraries if the file isn't in the current directory (see also the @INC
array in L<perlvar/Predefined Names>).  It is also different in how
code evaluated with C<do FILENAME> doesn't see lexicals in the enclosing
scope like C<eval STRING> does.  It's the same, however, in that it does
reparse the file every time you call it, so you probably don't want to
do this inside a loop.

Note that inclusion of library modules is better done with the
use() and require() operators, which also do error checking
and raise an exception if there's a problem.

=item dump LABEL

This causes an immediate core dump.  Primarily this is so that you can
use the B<undump> program to turn your core dump into an executable binary
after having initialized all your variables at the beginning of the
program.  When the new binary is executed it will begin by executing a
C<goto LABEL> (with all the restrictions that C<goto> suffers).  Think of
it as a goto with an intervening core dump and reincarnation.  If LABEL
is omitted, restarts the program from the top.  WARNING: any files
opened at the time of the dump will NOT be open any more when the
program is reincarnated, with possible resulting confusion on the part
of Perl.  See also B<-u> option in L<perlrun>.

Example:

    #!/usr/bin/perl
    require 'getopt.pl';
    require 'stat.pl';
    %days = (
        'Sun' => 1,
        'Mon' => 2,
        'Tue' => 3,
        'Wed' => 4,
        'Thu' => 5,
        'Fri' => 6,
        'Sat' => 7,
    );

    dump QUICKSTART if $ARGV[0] eq '-d';

    QUICKSTART:
    Getopt('f');

=item each HASH

When called in a list context, returns a 2-element list consisting of the
key and value for the next element of a hash, so that you can iterate over
it.  When called in a scalar context, returns the key for only the next
element in the hash.  (Note: Keys may be "0" or "", which are logically
false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
for this reason.)

Entries are returned in an apparently random order.  When the hash is
entirely read, a null array is returned in list context (which when
assigned produces a FALSE (0) value), and C<undef> is returned in a
scalar context.  The next call to each() after that will start iterating
again.  There is a single iterator for each hash, shared by all each(),
keys(), and values() function calls in the program; it can be reset by
reading all the elements from the hash, or by evaluating C<keys HASH> or
C<values HASH>.  If you add or delete elements of a hash while you're
iterating over it, you may get entries skipped or duplicated, so don't.

The following prints out your environment like the printenv(1) program,
only in a different order:

    while (($key,$value) = each %ENV) {
        print "$key=$value\n";
    }

See also keys() and values().

=item eof FILEHANDLE

=item eof ()

=item eof

Returns 1 if the next read on FILEHANDLE will return end of file, or if
FILEHANDLE is not open.  FILEHANDLE may be an expression whose value
gives the real filehandle name.  (Note that this function actually
reads a character and then ungetc()s it, so it is not very useful in an
interactive context.)  Do not read from a terminal file (or call
C<eof(FILEHANDLE)> on it) after end-of-file is reached.  Filetypes such
as terminals may lose the end-of-file condition if you do.

An C<eof> without an argument uses the last file read as argument.
Empty parentheses () may be used to indicate the pseudo file formed of
the files listed on the command line, i.e., C<eof()> is reasonable to
use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
last file.  Use C<eof(ARGV)> or eof without the parentheses to test
I<EACH> file in a while (E<lt>E<gt>) loop.  Examples:

    # reset line numbering on each input file
    while (<>) {
        print "$.\t$_";
        close(ARGV) if (eof);   # Not eof().
    }

    # insert dashes just before last line of last file
    while (<>) {
        if (eof()) {
            print "--------------\n";
            close(ARGV);        # close or break; is needed if we
                                # are reading from the terminal
        }
        print;
    }

Practical hint: you almost never need to use C<eof> in Perl, because the
input operators return undef when they run out of data.

=item eval EXPR

=item eval BLOCK

In the first form, the return value of EXPR is parsed and executed as if it
were a little Perl program.  The value of the expression (which is itself
determined within a scalar context) is first parsed, and if there are no
errors, executed in the context of the current Perl program, so that any
variable settings or subroutine and format definitions remain afterwards.
Note that the value is parsed every time the eval executes.  If EXPR is
omitted, evaluates C<$_>.  This form is typically used to delay parsing
and subsequent execution of the text of EXPR until run time.

In the second form, the code within the BLOCK is parsed only once--at the
same time the code surrounding the eval itself was parsed--and executed
within the context of the current Perl program.  This form is typically
used to trap exceptions more efficiently than the first (see below), while
also providing the benefit of checking the code within BLOCK at compile
time.

The final semicolon, if any, may be omitted from the value of EXPR or within
the BLOCK.

In both forms, the value returned is the value of the last expression
evaluated inside the mini-program, or a return statement may be used, just
as with subroutines.  The expression providing the return value is evaluated
in void, scalar or array context, depending on the context of the eval itself.
See L</wantarray> for more on how the evaluation context can be determined.

If there is a syntax error or runtime error, or a die() statement is
executed, an undefined value is returned by eval(), and C<$@> is set to the
error message.  If there was no error, C<$@> is guaranteed to be a null
string.  Beware that using eval() neither silences perl from printing
warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
To do either of those, you have to use the C<$SIG{__WARN__}> facility.  See
L</warn> and L<perlvar>.

Note that, because eval() traps otherwise-fatal errors, it is useful for
determining whether a particular feature (such as socket() or symlink())
is implemented.  It is also Perl's exception trapping mechanism, where
the die operator is used to raise exceptions.

If the code to be executed doesn't vary, you may use the eval-BLOCK
form to trap run-time errors without incurring the penalty of
recompiling each time.  The error, if any, is still returned in C<$@>.
Examples:

    # make divide-by-zero nonfatal
    eval { $answer = $a / $b; }; warn $@ if $@;

    # same thing, but less efficient
    eval '$answer = $a / $b'; warn $@ if $@;

    # a compile-time error
    eval { $answer = };

    # a run-time error
    eval '$answer =';   # sets $@

When using the eval{} form as an exception trap in libraries, you may
wish not to trigger any C<__DIE__> hooks that user code may have
installed.  You can use the C<local $SIG{__DIE__}> construct for this
purpose, as shown in this example:

    # a very private exception trap for divide-by-zero
    eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; warn $@ if $@;

This is especially significant, given that C<__DIE__> hooks can call
die() again, which has the effect of changing their error messages:

    # __DIE__ hooks may modify error messages
    {
       local $SIG{'__DIE__'} = sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
       eval { die "foo lives here" };
       print $@ if $@;                # prints "bar lives here"
    }

With an eval(), you should be especially careful to remember what's
being looked at when:

    eval $x;            # CASE 1
    eval "$x";          # CASE 2

    eval '$x';          # CASE 3
    eval { $x };        # CASE 4

    eval "\$$x++"       # CASE 5
    $$x++;              # CASE 6

Cases 1 and 2 above behave identically: they run the code contained in
the variable $x.  (Although case 2 has misleading double quotes making
the reader wonder what else might be happening (nothing is).)  Cases 3
and 4 likewise behave in the same way: they run the code '$x', which
does nothing but return the value of C<$x>.  (Case 4 is preferred for
purely visual reasons, but it also has the advantage of compiling at
compile-time instead of at run-time.)  Case 5 is a place where
normally you I<WOULD> like to use double quotes, except that in this
particular situation, you can just use symbolic references instead, as
in case 6.

=item exec LIST

=item exec PROGRAM LIST

The exec() function executes a system command I<AND NEVER RETURNS> -
use system() instead of exec() if you want it to return. It fails and
returns FALSE only if the command does not exist I<and> it is executed
directly instead of via your system's command shell (see below).

Since it's a common mistake to use system() instead of exec(), Perl
warns you if there is a following statement which isn't die(), warn()
or exit() (if C<-w> is set  -  but you always do that).   If you
I<really> want to follow an exec() with some other statement, you
can use one of these styles to avoid the warning:

    exec ('foo') or print STDERR "couldn't exec foo";
    { exec ('foo') }; print STDERR "couldn't exec foo";

If there is more than one argument in LIST, or if LIST is an array with
more than one value, calls execvp(3) with the arguments in LIST.  If
there is only one scalar argument, the argument is checked for shell
metacharacters, and if there are any, the entire argument is passed to
the system's command shell for parsing (this is C</bin/sh -c> on Unix
platforms, but varies on other platforms).  If there are no shell
metacharacters in the argument, it is split into words and passed
directly to execvp(), which is more efficient.  Note: exec() and
system() do not flush your output buffer, so you may need to set C<$|>
to avoid lost output.  Examples:

    exec '/bin/echo', 'Your arguments are: ', @ARGV;
    exec "sort $outfile | uniq";

If you don't really want to execute the first argument, but want to lie
to the program you are executing about its own name, you can specify
the program you actually want to run as an "indirect object" (without a
comma) in front of the LIST.  (This always forces interpretation of the
LIST as a multivalued list, even if there is only a single scalar in
the list.)  Example:

    $shell = '/bin/csh';
    exec $shell '-sh';          # pretend it's a login shell

or, more directly,

    exec {'/bin/csh'} '-sh';    # pretend it's a login shell

When the arguments get executed via the system shell, results will
be subject to its quirks and capabilities.  See L<perlop/"`STRING`">
for details.

=item exists EXPR

Returns TRUE if the specified hash key exists in its hash array, even
if the corresponding value is undefined.

    print "Exists\n" if exists $array{$key};
    print "Defined\n" if defined $array{$key};
    print "True\n" if $array{$key};

A hash element can be TRUE only if it's defined, and defined if
it exists, but the reverse doesn't necessarily hold true.

Note that the EXPR can be arbitrarily complicated as long as the final
operation is a hash key lookup:

    if (exists $ref->[$x][$y]{$key}) { ... }

=item exit EXPR

Evaluates EXPR and exits immediately with that value.  (Actually, it
calls any defined C<END> routines first, but the C<END> routines may not
abort the exit.  Likewise any object destructors that need to be called
are called before exit.)  Example:

    $ans = <STDIN>;
    exit 0 if $ans =~ /^[Xx]/;

See also die().  If EXPR is omitted, exits with 0 status.  The only
universally portable values for EXPR are 0 for success and 1 for error;
all other values are subject to unpredictable interpretation depending
on the environment in which the Perl program is running.

You shouldn't use exit() to abort a subroutine if there's any chance that
someone might want to trap whatever error happened.  Use die() instead,
which can be trapped by an eval().

=item exp EXPR

=item exp

Returns I<e> (the natural logarithm base) to the power of EXPR.
If EXPR is omitted, gives C<exp($_)>.

=item fcntl FILEHANDLE,FUNCTION,SCALAR

Implements the fcntl(2) function.  You'll probably have to say

    use Fcntl;

first to get the correct function definitions.  Argument processing and
value return works just like ioctl() below.  Note that fcntl() will produce
a fatal error if used on a machine that doesn't implement fcntl(2).
For example:

    use Fcntl;
    fcntl($filehandle, F_GETLK, $packed_return_buffer);

=item fileno FILEHANDLE

Returns the file descriptor for a filehandle.  This is useful for
constructing bitmaps for select().  If FILEHANDLE is an expression, the
value is taken as the name of the filehandle.

=item flock FILEHANDLE,OPERATION

Calls flock(2), or an emulation of it, on FILEHANDLE.  Returns TRUE for
success, FALSE on failure.  Produces a fatal error if used on a machine
that doesn't implement flock(2), fcntl(2) locking, or lockf(3).  flock()
is Perl's portable file locking interface, although it locks only entire
files, not records.

On many platforms (including most versions or clones of Unix), locks
established by flock() are B<merely advisory>.  This means that files
locked with flock() may be modified by programs which do not also use
flock().  Windows NT and OS/2, however, are among the platforms which
supply mandatory locking.  See your local documentation for details.

OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
LOCK_NB.  These constants are traditionally valued 1, 2, 8 and 4, but
you can use the symbolic names if import them from the Fcntl module,
either individually, or as a group using the ':flock' tag.  LOCK_SH
requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
releases a previously requested lock.  If LOCK_NB is added to LOCK_SH or
LOCK_EX then flock() will return immediately rather than blocking
waiting for the lock (check the return status to see if you got it).

To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
before (un)locking it.

Note that the emulation built with lockf(3) doesn't provide shared
locks, and it requires that FILEHANDLE be open with write intent.  These
are the semantics that lockf(3) implements.  Most (all?) systems
implement lockf(3) in terms of fcntl(2) locking, though, so the
differing semantics shouldn't bite too many people.

Note also that some versions of flock() cannot lock things over the
network; you would need to use the more system-specific fcntl() for
that.  If you like you can force Perl to ignore your system's flock(2)
function, and so provide its own fcntl(2)-based emulation, by passing
the switch C<-Ud_flock> to the F<Configure> program when you configure
perl.

Here's a mailbox appender for BSD systems.

    use Fcntl ':flock'; # import LOCK_* constants

    sub lock {
        flock(MBOX,LOCK_EX);
        # and, in case someone appended
        # while we were waiting...
        seek(MBOX, 0, 2);
    }

    sub unlock {
        flock(MBOX,LOCK_UN);
    }

    open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
            or die "Can't open mailbox: $!";

    lock();
    print MBOX $msg,"\n\n";
    unlock();

See also L<DB_File> for other flock() examples.

=item fork

Does a fork(2) system call.  Returns the child pid to the parent process
and 0 to the child process, or C<undef> if the fork is unsuccessful.
Note: unflushed buffers remain unflushed in both processes, which means
you may need to set C<$|> ($AUTOFLUSH in English) or call the autoflush()
method of IO::Handle to avoid duplicate output.

If you fork() without ever waiting on your children, you will accumulate
zombies:

    $SIG{CHLD} = sub { wait };

There's also the double-fork trick (error checking on
fork() returns omitted);

    unless ($pid = fork) {
        unless (fork) {
            exec "what you really wanna do";
            die "no exec";
            # ... or ...
            ## (some_perl_code_here)
            exit 0;
        }
        exit 0;
    }
    waitpid($pid,0);

See also L<perlipc> for more examples of forking and reaping
moribund children.

Note that if your forked child inherits system file descriptors like
STDIN and STDOUT that are actually connected by a pipe or socket, even
if you exit, the remote server (such as, say, httpd or rsh) won't think
you're done.  You should reopen those to /dev/null if it's any issue.

=item format

Declare a picture format for use by the write() function.  For
example:

    format Something =
        Test: @<<<<<<<< @||||| @>>>>>
              $str,     $%,    '$' . int($num)
    .

    $str = "widget";
    $num = $cost/$quantity;
    $~ = 'Something';
    write;

See L<perlform> for many details and examples.


=item formline PICTURE,LIST

This is an internal function used by C<format>s, though you may call it
too.  It formats (see L<perlform>) a list of values according to the
contents of PICTURE, placing the output into the format output
accumulator, C<$^A> (or $ACCUMULATOR in English).
Eventually, when a write() is done, the contents of
C<$^A> are written to some filehandle, but you could also read C<$^A>
yourself and then set C<$^A> back to "".  Note that a format typically
does one formline() per line of form, but the formline() function itself
doesn't care how many newlines are embedded in the PICTURE.  This means
that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
You may therefore need to use multiple formlines to implement a single
record format, just like the format compiler.

Be careful if you put double quotes around the picture, because an "C<@>"
character may be taken to mean the beginning of an array name.
formline() always returns TRUE.  See L<perlform> for other examples.

=item getc FILEHANDLE

=item getc

Returns the next character from the input file attached to FILEHANDLE,
or a null string at end of file.  If FILEHANDLE is omitted, reads from STDIN.
This is not particularly efficient.  It cannot be used to get unbuffered
single-characters, however.  For that, try something more like:

    if ($BSD_STYLE) {
        system "stty cbreak </dev/tty >/dev/tty 2>&1";
    }
    else {
        system "stty", '-icanon', 'eol', "\001";
    }

    $key = getc(STDIN);

    if ($BSD_STYLE) {
        system "stty -cbreak </dev/tty >/dev/tty 2>&1";
    }
    else {
        system "stty", 'icanon', 'eol', '^@'; # ASCII null
    }
    print "\n";

Determination of whether $BSD_STYLE should be set
is left as an exercise to the reader.

The POSIX::getattr() function can do this more portably on systems
alleging POSIX compliance.
See also the C<Term::ReadKey> module from your nearest CPAN site;
details on CPAN can be found on L<perlmod/CPAN>.

=item getlogin

Returns the current login from F</etc/utmp>, if any.  If null, use
getpwuid().

    $login = getlogin || getpwuid($<) || "Kilroy";

Do not consider getlogin() for authentication: it is not as
secure as getpwuid().

=item getpeername SOCKET

Returns the packed sockaddr address of other end of the SOCKET connection.

    use Socket;
    $hersockaddr    = getpeername(SOCK);
    ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
    $herhostname    = gethostbyaddr($iaddr, AF_INET);
    $herstraddr     = inet_ntoa($iaddr);

=item getpgrp PID

Returns the current process group for the specified PID.  Use
a PID of 0 to get the current process group for the
current process.  Will raise an exception if used on a machine that
doesn't implement getpgrp(2).  If PID is omitted, returns process
group of current process.  Note that the POSIX version of getpgrp()
does not accept a PID argument, so only PID==0 is truly portable.

=item getppid

Returns the process id of the parent process.

=item getpriority WHICH,WHO

Returns the current priority for a process, a process group, or a user.
(See L<getpriority(2)>.)  Will raise a fatal exception if used on a
machine that doesn't implement getpriority(2).

=item getpwnam NAME

=item getgrnam NAME

=item gethostbyname NAME

=item getnetbyname NAME

=item getprotobyname NAME

=item getpwuid UID

=item getgrgid GID

=item getservbyname NAME,PROTO

=item gethostbyaddr ADDR,ADDRTYPE

=item getnetbyaddr ADDR,ADDRTYPE

=item getprotobynumber NUMBER

=item getservbyport PORT,PROTO

=item getpwent

=item getgrent

=item gethostent

=item getnetent

=item getprotoent

=item getservent

=item setpwent

=item setgrent

=item sethostent STAYOPEN

=item setnetent STAYOPEN

=item setprotoent STAYOPEN

=item setservent STAYOPEN

=item endpwent

=item endgrent

=item endhostent

=item endnetent

=item endprotoent

=item endservent

These routines perform the same functions as their counterparts in the
system library.  Within a list context, the return values from the
various get routines are as follows:

    ($name,$passwd,$uid,$gid,
       $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
    ($name,$passwd,$gid,$members) = getgr*
    ($name,$aliases,$addrtype,$length,@addrs) = gethost*
    ($name,$aliases,$addrtype,$net) = getnet*
    ($name,$aliases,$proto) = getproto*
    ($name,$aliases,$port,$proto) = getserv*

(If the entry doesn't exist you get a null list.)

Within a scalar context, you get the name, unless the function was a
lookup by name, in which case you get the other thing, whatever it is.
(If the entry doesn't exist you get the undefined value.)  For example:

    $uid = getpwnam
    $name = getpwuid
    $name = getpwent
    $gid = getgrnam
    $name = getgrgid
    $name = getgrent
    etc.

In I<getpw*()> the fields $quota, $comment, and $expire are special
cases in the sense that in many systems they are unsupported.  If the
$quota is unsupported, it is an empty scalar.  If it is supported, it
usually encodes the disk quota.  If the $comment field is unsupported,
it is an empty scalar.  If it is supported it usually encodes some
administrative comment about the user.  In some systems the $quota
field may be $change or $age, fields that have to do with password
aging.  In some systems the $comment field may be $class.  The $expire
field, if present, encodes the expiration period of the account or the
password.  For the availability and the exact meaning of these fields
in your system, please consult your getpwnam(3) documentation and your
<pwd.h> file.  You can also find out from within Perl which meaning
your $quota and $comment fields have and whether you have the $expire
field by using the Config module and the values d_pwquota, d_pwage,
d_pwchange, d_pwcomment, and d_pwexpire.

The $members value returned by I<getgr*()> is a space separated list of
the login names of the members of the group.

For the I<gethost*()> functions, if the C<h_errno> variable is supported in
C, it will be returned to you via C<$?> if the function call fails.  The
@addrs value returned by a successful call is a list of the raw
addresses returned by the corresponding system library call.  In the
Internet domain, each address is four bytes long and you can unpack it
by saying something like:

    ($a,$b,$c,$d) = unpack('C4',$addr[0]);

=item getsockname SOCKET

Returns the packed sockaddr address of this end of the SOCKET connection.

    use Socket;
    $mysockaddr = getsockname(SOCK);
    ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);

=item getsockopt SOCKET,LEVEL,OPTNAME

Returns the socket option requested, or undefined if there is an error.

=item glob EXPR

=item glob

Returns the value of EXPR with filename expansions such as a shell would
do.  This is the internal function implementing the C<E<lt>*.cE<gt>>
operator, but you can use it directly.  If EXPR is omitted, $_ is used.
The C<E<lt>*.cE<gt>> operator is discussed in more detail in
L<perlop/"I/O Operators">.

=item gmtime EXPR

Converts a time as returned by the time function to a 9-element array
with the time localized for the standard Greenwich time zone.
Typically used as follows:

    #  0    1    2     3     4    5     6     7     8
    ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
                                            gmtime(time);

All array elements are numeric, and come straight out of a struct tm.
In particular this means that $mon has the range 0..11 and $wday has
the range 0..6 with sunday as day 0.  Also, $year is the number of
years since 1900, I<not> simply the last two digits of the year.

If EXPR is omitted, does C<gmtime(time())>.

In a scalar context, returns the ctime(3) value:

    $now_string = gmtime;  # e.g., "Thu Oct 13 04:54:34 1994"

Also see the timegm() function provided by the Time::Local module,
and the strftime(3) function available via the POSIX module.

=item goto LABEL

=item goto EXPR

=item goto &NAME

The goto-LABEL form finds the statement labeled with LABEL and resumes
execution there.  It may not be used to go into any construct that
requires initialization, such as a subroutine or a foreach loop.  It
also can't be used to go into a construct that is optimized away,
or to get out of a block or subroutine given to sort().
It can be used to go almost anywhere else within the dynamic scope,
including out of subroutines, but it's usually better to use some other
construct such as last or die.  The author of Perl has never felt the
need to use this form of goto (in Perl, that is--C is another matter).

The goto-EXPR form expects a label name, whose scope will be resolved
dynamically.  This allows for computed gotos per FORTRAN, but isn't
necessarily recommended if you're optimizing for maintainability:

    goto ("FOO", "BAR", "GLARCH")[$i];

The goto-&NAME form is highly magical, and substitutes a call to the
named subroutine for the currently running subroutine.  This is used by
AUTOLOAD subroutines that wish to load another subroutine and then
pretend that the other subroutine had been called in the first place
(except that any modifications to @_ in the current subroutine are
propagated to the other subroutine.)  After the goto, not even caller()
will be able to tell that this routine was called first.

=item grep BLOCK LIST

=item grep EXPR,LIST

This is similar in spirit to, but not the same as, grep(1)
and its relatives.  In particular, it is not limited to using
regular expressions.

Evaluates the BLOCK or EXPR for each element of LIST (locally setting
$_ to each element) and returns the list value consisting of those
elements for which the expression evaluated to TRUE.  In a scalar
context, returns the number of times the expression was TRUE.

    @foo = grep(!/^#/, @bar);    # weed out comments

or equivalently,

    @foo = grep {!/^#/} @bar;    # weed out comments

Note that, because $_ is a reference into the list value, it can be used
to modify the elements of the array.  While this is useful and
supported, it can cause bizarre results if the LIST is not a named
array.  Similarly, grep returns aliases into the original list,
much like the way that a for loops's index variable aliases the list
elements.  That is, modifying an element of a list returned by grep
(for example, in a C<foreach>, C<map> or another C<grep>)
actually modifies the element in the original list.

See also L</map> for an array composed of the results of the BLOCK or EXPR.

=item hex EXPR

=item hex

Interprets EXPR as a hex string and returns the corresponding
value.  (To convert strings that might start with either 0 or 0x
see L</oct>.)  If EXPR is omitted, uses $_.

    print hex '0xAf'; # prints '175'
    print hex 'aF';   # same

=item import

There is no builtin import() function.  It is merely an ordinary
method (subroutine) defined (or inherited) by modules that wish to export
names to another module.  The use() function calls the import() method
for the package used.  See also L</use()>, L<perlmod>, and L<Exporter>.

=item index STR,SUBSTR,POSITION

=item index STR,SUBSTR

Returns the position of the first occurrence of SUBSTR in STR at or after
POSITION.  If POSITION is omitted, starts searching from the beginning of
the string.  The return value is based at 0 (or whatever you've set the C<$[>
variable to--but don't do that).  If the substring is not found, returns
one less than the base, ordinarily -1.

=item int EXPR

=item int

Returns the integer portion of EXPR.  If EXPR is omitted, uses $_.

=item ioctl FILEHANDLE,FUNCTION,SCALAR

Implements the ioctl(2) function.  You'll probably have to say

    require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph

first to get the correct function definitions.  If F<ioctl.ph> doesn't
exist or doesn't have the correct definitions you'll have to roll your
own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
(There is a Perl script called B<h2ph> that comes with the Perl kit which
may help you in this, but it's nontrivial.)  SCALAR will be read and/or
written depending on the FUNCTION--a pointer to the string value of SCALAR
will be passed as the third argument of the actual ioctl call.  (If SCALAR
has no string value but does have a numeric value, that value will be
passed rather than a pointer to the string value.  To guarantee this to be
TRUE, add a 0 to the scalar before using it.)  The pack() and unpack()
functions are useful for manipulating the values of structures used by
ioctl().  The following example sets the erase character to DEL.

    require 'ioctl.ph';
    $getp = &TIOCGETP;
    die "NO TIOCGETP" if $@ || !$getp;
    $sgttyb_t = "ccccs";                # 4 chars and a short
    if (ioctl(STDIN,$getp,$sgttyb)) {
        @ary = unpack($sgttyb_t,$sgttyb);
        $ary[2] = 127;
        $sgttyb = pack($sgttyb_t,@ary);
        ioctl(STDIN,&TIOCSETP,$sgttyb)
            || die "Can't ioctl: $!";
    }

The return value of ioctl (and fcntl) is as follows:

        if OS returns:          then Perl returns:
            -1                    undefined value
             0                  string "0 but true"
        anything else               that number

Thus Perl returns TRUE on success and FALSE on failure, yet you can
still easily determine the actual value returned by the operating
system:

    ($retval = ioctl(...)) || ($retval = -1);
    printf "System returned %d\n", $retval;

=item join EXPR,LIST

Joins the separate strings of LIST into a single string with
fields separated by the value of EXPR, and returns the string.
Example:

    $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);

See L<perlfunc/split>.

=item keys HASH

Returns a list consisting of all the keys of the named hash.  (In a
scalar context, returns the number of keys.)  The keys are returned in
an apparently random order, but it is the same order as either the
values() or each() function produces (given that the hash has not been
modified).  As a side effect, it resets HASH's iterator.

Here is yet another way to print your environment:

    @keys = keys %ENV;
    @values = values %ENV;
    while ($#keys >= 0) {
        print pop(@keys), '=', pop(@values), "\n";
    }

or how about sorted by key:

    foreach $key (sort(keys %ENV)) {
        print $key, '=', $ENV{$key}, "\n";
    }

To sort an array by value, you'll need to use a C<sort> function.
Here's a descending numeric sort of a hash by its values:

    foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
        printf "%4d %s\n", $hash{$key}, $key;
    }

As an lvalue C<keys> allows you to increase the number of hash buckets
allocated for the given hash.  This can gain you a measure of efficiency if
you know the hash is going to get big.  (This is similar to pre-extending
an array by assigning a larger number to $#array.)  If you say

    keys %hash = 200;

then C<%hash> will have at least 200 buckets allocated for it.  These
buckets will be retained even if you do C<%hash = ()>, use C<undef
%hash> if you want to free the storage while C<%hash> is still in scope.
You can't shrink the number of buckets allocated for the hash using
C<keys> in this way (but you needn't worry about doing this by accident,
as trying has no effect).

=item kill LIST

Sends a signal to a list of processes.  The first element of
the list must be the signal to send.  Returns the number of
processes successfully signaled.

    $cnt = kill 1, $child1, $child2;
    kill 9, @goners;

Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
process groups instead of processes.  (On System V, a negative I<PROCESS>
number will also kill process groups, but that's not portable.)  That
means you usually want to use positive not negative signals.  You may also
use a signal name in quotes.  See L<perlipc/"Signals"> for details.

=item last LABEL

=item last

The C<last> command is like the C<break> statement in C (as used in
loops); it immediately exits the loop in question.  If the LABEL is
omitted, the command refers to the innermost enclosing loop.  The
C<continue> block, if any, is not executed:

    LINE: while (<STDIN>) {
        last LINE if /^$/;      # exit when done with header
        ...
    }

See also L</continue> for an illustration of how C<last>, C<next>, and
C<redo> work.

=item lc EXPR

=item lc

Returns an lowercased version of EXPR.  This is the internal function
implementing the \L escape in double-quoted strings.
Respects current LC_CTYPE locale if C<use locale> in force.  See L<perllocale>.

If EXPR is omitted, uses $_.

=item lcfirst EXPR

=item lcfirst

Returns the value of EXPR with the first character lowercased.  This is
the internal function implementing the \l escape in double-quoted strings.
Respects current LC_CTYPE locale if C<use locale> in force.  See L<perllocale>.

If EXPR is omitted, uses $_.

=item length EXPR

=item length

Returns the length in characters of the value of EXPR.  If EXPR is
omitted, returns length of $_.

=item link OLDFILE,NEWFILE

Creates a new filename linked to the old filename.  Returns 1 for
success, 0 otherwise.

=item listen SOCKET,QUEUESIZE

Does the same thing that the listen system call does.  Returns TRUE if
it succeeded, FALSE otherwise.  See example in L<perlipc/"Sockets: Client/Server Communication">.

=item local EXPR

A local modifies the listed variables to be local to the enclosing block,
subroutine, C<eval{}>, or C<do>.  If more than one value is listed, the
list must be placed in parentheses.  See L<perlsub/"Temporary Values via
local()"> for details, including issues with tied arrays and hashes.

You really probably want to be using my() instead, because local() isn't
what most people think of as "local".  See L<perlsub/"Private Variables
via my()"> for details.

=item localtime EXPR

Converts a time as returned by the time function to a 9-element array
with the time analyzed for the local time zone.  Typically used as
follows:

    #  0    1    2     3     4    5     6     7     8
    ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
                                                localtime(time);

All array elements are numeric, and come straight out of a struct tm.
In particular this means that $mon has the range 0..11 and $wday has
the range 0..6 with sunday as day 0.  Also, $year is the number of
years since 1900, that is, $year is 123 in year 2023.

If EXPR is omitted, uses the current time (C<localtime(time)>).

In a scalar context, returns the ctime(3) value:

    $now_string = localtime;  # e.g., "Thu Oct 13 04:54:34 1994"

This scalar value is B<not> locale dependent, see L<perllocale>, but
instead a Perl builtin.  Also see the Time::Local module, and the
strftime(3) and mktime(3) function available via the POSIX module.  To
get somewhat similar but locale dependent date strings, set up your
locale environment variables appropriately (please see L<perllocale>)
and try for example

        use POSIX qw(strftime)
        $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;

Note that the C<%a> and C<%b>, the short forms of the day of the week
and the month of the year, may not necessarily be three characters wide.

=item log EXPR

=item log

Returns logarithm (base I<e>) of EXPR.  If EXPR is omitted, returns log
of $_.

=item lstat FILEHANDLE

=item lstat EXPR

=item lstat

Does the same thing as the stat() function, but stats a symbolic link
instead of the file the symbolic link points to.  If symbolic links are
unimplemented on your system, a normal stat() is done.

If EXPR is omitted, stats $_.

=item m//

The match operator.  See L<perlop>.

=item map BLOCK LIST

=item map EXPR,LIST

Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
element) and returns the list value composed of the results of each such
evaluation.  Evaluates BLOCK or EXPR in a list context, so each element of LIST
may produce zero, one, or more elements in the returned value.

    @chars = map(chr, @nums);

translates a list of numbers to the corresponding characters.  And

    %hash = map { getkey($_) => $_ } @array;

is just a funny way to write

    %hash = ();
    foreach $_ (@array) {
        $hash{getkey($_)} = $_;
    }

Note that, because $_ is a reference into the list value, it can be used
to modify the elements of the array.  While this is useful and
supported, it can cause bizarre results if the LIST is not a named
array.  See also L</grep> for an array composed of those items of the 
original list for which the BLOCK or EXPR evaluates to true.

=item mkdir FILENAME,MODE

Creates the directory specified by FILENAME, with permissions specified
by MODE (as modified by umask).  If it succeeds it returns 1, otherwise
it returns 0 and sets C<$!> (errno).

=item msgctl ID,CMD,ARG

Calls the System V IPC function msgctl(2).  If CMD is &IPC_STAT, then ARG
must be a variable which will hold the returned msqid_ds structure.
Returns like ioctl: the undefined value for error, "0 but true" for
zero, or the actual return value otherwise.

=item msgget KEY,FLAGS

Calls the System V IPC function msgget(2).  Returns the message queue id,
or the undefined value if there is an error.

=item msgsnd ID,MSG,FLAGS

Calls the System V IPC function msgsnd to send the message MSG to the
message queue ID.  MSG must begin with the long integer message type,
which may be created with C<pack("l", $type)>.  Returns TRUE if
successful, or FALSE if there is an error.

=item msgrcv ID,VAR,SIZE,TYPE,FLAGS

Calls the System V IPC function msgrcv to receive a message from
message queue ID into variable VAR with a maximum message size of
SIZE.  Note that if a message is received, the message type will be the
first thing in VAR, and the maximum length of VAR is SIZE plus the size
of the message type.  Returns TRUE if successful, or FALSE if there is
an error.

=item my EXPR

A "my" declares the listed variables to be local (lexically) to the
enclosing block, subroutine, C<eval>, or C<do/require/use>'d file.  If
more than one value is listed, the list must be placed in parentheses.  See
L<perlsub/"Private Variables via my()"> for details.

=item next LABEL

=item next

The C<next> command is like the C<continue> statement in C; it starts
the next iteration of the loop:

    LINE: while (<STDIN>) {
        next LINE if /^#/;      # discard comments
        ...
    }

Note that if there were a C<continue> block on the above, it would get
executed even on discarded lines.  If the LABEL is omitted, the command
refers to the innermost enclosing loop.

See also L</continue> for an illustration of how C<last>, C<next>, and
C<redo> work.

=item no Module LIST

See the "use" function, which "no" is the opposite of.

=item oct EXPR

=item oct

Interprets EXPR as an octal string and returns the corresponding
value.  (If EXPR happens to start off with 0x, interprets it as
a hex string instead.)  The following will handle decimal, octal, and
hex in the standard Perl or C notation:

    $val = oct($val) if $val =~ /^0/;

If EXPR is omitted, uses $_.  This function is commonly used when
a string such as "644" needs to be converted into a file mode, for
example. (Although perl will automatically convert strings into
numbers as needed, this automatic conversion assumes base 10.)

=item open FILEHANDLE,EXPR

=item open FILEHANDLE

Opens the file whose filename is given by EXPR, and associates it with
FILEHANDLE.  If FILEHANDLE is an expression, its value is used as the
name of the real filehandle wanted.  If EXPR is omitted, the scalar
variable of the same name as the FILEHANDLE contains the filename.
(Note that lexical variables--those declared with C<my>--will not work
for this purpose; so if you're using C<my>, specify EXPR in your call
to open.)

If the filename begins with '<' or nothing, the file is opened for input.
If the filename begins with '>', the file is truncated and opened for
output.  If the filename begins with '>>', the file is opened for
appending.  You can put a '+' in front of the '>' or '<' to indicate that
you want both read and write access to the file; thus '+<' is almost
always preferred for read/write updates--the '+>' mode would clobber the
file first.  The prefix and the filename may be separated with spaces.
These various prefixes correspond to the fopen(3) modes of 'r', 'r+', 'w',
'w+', 'a', and 'a+'.

If the filename begins with "|", the filename is interpreted as a command
to which output is to be piped, and if the filename ends with a "|", the
filename is interpreted See L<perlipc/"Using open() for IPC"> for more
examples of this.  as command which pipes input to us.  (You may not have
a raw open() to a command that pipes both in I<and> out, but see
L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
for alternatives.)

Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT.  Open returns
nonzero upon success, the undefined value otherwise.  If the open
involved a pipe, the return value happens to be the pid of the
subprocess.

If you're unfortunate enough to be running Perl on a system that
distinguishes between text files and binary files (modern operating
systems don't care), then you should check out L</binmode> for tips for
dealing with this.  The key distinction between systems that need binmode
and those that don't is their text file formats.  Systems like Unix and
Plan9 that delimit lines with a single character, and that encode that
character in C as '\n', do not need C<binmode>.  The rest need it.

When opening a file, it's usually a bad idea to continue normal execution
if the request failed, so C<open> is frequently used in connection with
C<die>. Even if C<die> won't do what you want (say, in a CGI script,
where you want to make a nicely formatted error message (but there are
modules which can help with that problem)) you should always check
the return value from opening a file. The infrequent exception is when
working with an unopened filehandle is actually what you want to do.

Examples:

    $ARTICLE = 100;
    open ARTICLE or die "Can't find article $ARTICLE: $!\n";
    while (<ARTICLE>) {...

    open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
    # if the open fails, output is discarded

    open(DBASE, '+<dbase.mine')             # open for update
        or die "Can't open 'dbase.mine' for update: $!";

    open(ARTICLE, "caesar <$article |")     # decrypt article
        or die "Can't start caesar: $!";

    open(EXTRACT, "|sort >/tmp/Tmp$$")      # $$ is our process id
        or die "Can't start sort: $!";

    # process argument list of files along with any includes

    foreach $file (@ARGV) {
        process($file, 'fh00');
    }

    sub process {
        local($filename, $input) = @_;
        $input++;               # this is a string increment
        unless (open($input, $filename)) {
            print STDERR "Can't open $filename: $!\n";
            return;
        }

        while (<$input>) {              # note use of indirection
            if (/^#include "(.*)"/) {
                process($1, $input);
                next;
            }
            ...         # whatever
        }
    }

You may also, in the Bourne shell tradition, specify an EXPR beginning
with "E<gt>&", in which case the rest of the string is interpreted as the
name of a filehandle (or file descriptor, if numeric) which is to be
duped and opened.  You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
+E<gt>E<gt>, and +E<lt>.  The
mode you specify should match the mode of the original filehandle.
(Duping a filehandle does not take into account any existing contents of
stdio buffers.)
Here is a script that saves, redirects, and restores STDOUT and
STDERR:

    #!/usr/bin/perl
    open(SAVEOUT, ">&STDOUT");
    open(SAVEERR, ">&STDERR");

    open(STDOUT, ">foo.out") || die "Can't redirect stdout";
    open(STDERR, ">&STDOUT") || die "Can't dup stdout";

    select(STDERR); $| = 1;     # make unbuffered
    select(STDOUT); $| = 1;     # make unbuffered

    print STDOUT "stdout 1\n";  # this works for
    print STDERR "stderr 1\n";  # subprocesses too

    close(STDOUT);
    close(STDERR);

    open(STDOUT, ">&SAVEOUT");
    open(STDERR, ">&SAVEERR");

    print STDOUT "stdout 2\n";
    print STDERR "stderr 2\n";


If you specify "E<lt>&=N", where N is a number, then Perl will do an
equivalent of C's fdopen() of that file descriptor; this is more
parsimonious of file descriptors.  For example:

    open(FILEHANDLE, "<&=$fd")

If you open a pipe on the command "-", i.e., either "|-" or "-|", then
there is an implicit fork done, and the return value of open is the pid
of the child within the parent process, and 0 within the child
process.  (Use C<defined($pid)> to determine whether the open was successful.)
The filehandle behaves normally for the parent, but i/o to that
filehandle is piped from/to the STDOUT/STDIN of the child process.
In the child process the filehandle isn't opened--i/o happens from/to
the new STDOUT or STDIN.  Typically this is used like the normal
piped open when you want to exercise more control over just how the
pipe command gets executed, such as when you are running setuid, and
don't want to have to scan shell commands for metacharacters.
The following pairs are more or less equivalent:

    open(FOO, "|tr '[a-z]' '[A-Z]'");
    open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';

    open(FOO, "cat -n '$file'|");
    open(FOO, "-|") || exec 'cat', '-n', $file;

See L<perlipc/"Safe Pipe Opens"> for more examples of this.

NOTE: On any operation which may do a fork, unflushed buffers remain
unflushed in both processes, which means you may need to set C<$|> to
avoid duplicate output.

Closing any piped filehandle causes the parent process to wait for the
child to finish, and returns the status value in C<$?>.

Using the constructor from the IO::Handle package (or one of its
subclasses, such as IO::File or IO::Socket),
you can generate anonymous filehandles which have the scope of whatever
variables hold references to them, and automatically close whenever
and however you leave that scope:

    use IO::File;
    ...
    sub read_myfile_munged {
        my $ALL = shift;
        my $handle = new IO::File;
        open($handle, "myfile") or die "myfile: $!";
        $first = <$handle>
            or return ();     # Automatically closed here.
        mung $first or die "mung failed";       # Or here.
        return $first, <$handle> if $ALL;       # Or here.
        $first;                                 # Or here.
    }

The filename that is passed to open will have leading and trailing
whitespace deleted.  To open a file with arbitrary weird
characters in it, it's necessary to protect any leading and trailing
whitespace thusly:

    $file =~ s#^(\s)#./$1#;
    open(FOO, "< $file\0");

If you want a "real" C open() (see L<open(2)> on your system), then
you should use the sysopen() function.  This is another way to
protect your filenames from interpretation.  For example:

    use IO::Handle;
    sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
        or die "sysopen $path: $!";
    HANDLE->autoflush(1);
    HANDLE->print("stuff $$\n");
    seek(HANDLE, 0, 0);
    print "File contains: ", <HANDLE>;

See L</seek()> for some details about mixing reading and writing.

=item opendir DIRHANDLE,EXPR

Opens a directory named EXPR for processing by readdir(), telldir(),
seekdir(), rewinddir(), and closedir().  Returns TRUE if successful.
DIRHANDLEs have their own namespace separate from FILEHANDLEs.

=item ord EXPR

=item ord

Returns the numeric ascii value of the first character of EXPR.  If
EXPR is omitted, uses $_.  For the reverse, see L</chr>.

=item pack TEMPLATE,LIST

Takes an array or list of values and packs it into a binary structure,
returning the string containing the structure.  The TEMPLATE is a
sequence of characters that give the order and type of values, as
follows:

    A   An ascii string, will be space padded.
    a   An ascii string, will be null padded.
    b   A bit string (ascending bit order, like vec()).
    B   A bit string (descending bit order).
    h   A hex string (low nybble first).
    H   A hex string (high nybble first).

    c   A signed char value.
    C   An unsigned char value.

    s   A signed short value.
    S   An unsigned short value.
          (This 'short' is _exactly_ 16 bits, which may differ from
           what a local C compiler calls 'short'.)

    i   A signed integer value.
    I   An unsigned integer value.
          (This 'integer' is _at_least_ 32 bits wide.  Its exact size
           depends on what a local C compiler calls 'int', and may
           even be larger than the 'long' described in the next item.)

    l   A signed long value.
    L   An unsigned long value.
          (This 'long' is _exactly_ 32 bits, which may differ from
           what a local C compiler calls 'long'.)

    n   A short in "network" (big-endian) order.
    N   A long in "network" (big-endian) order.
    v   A short in "VAX" (little-endian) order.
    V   A long in "VAX" (little-endian) order.
          (These 'shorts' and 'longs' are _exactly_ 16 bits and
           _exactly_ 32 bits, respectively.)

    f   A single-precision float in the native format.
    d   A double-precision float in the native format.

    p   A pointer to a null-terminated string.
    P   A pointer to a structure (fixed-length string).

    u   A uuencoded string.

    w   A BER compressed integer.  Its bytes represent an unsigned
        integer in base 128, most significant digit first, with as few
        digits as possible.  Bit eight (the high bit) is set on each
        byte except the last.

    x   A null byte.
    X   Back up a byte.
    @   Null fill to absolute position.

Each letter may optionally be followed by a number which gives a repeat
count.  With all types except "a", "A", "b", "B", "h", "H", and "P" the
pack function will gobble up that many values from the LIST.  A * for the
repeat count means to use however many items are left.  The "a" and "A"
types gobble just one value, but pack it as a string of length count,
padding with nulls or spaces as necessary.  (When unpacking, "A" strips
trailing spaces and nulls, but "a" does not.)  Likewise, the "b" and "B"
fields pack a string that many bits long.  The "h" and "H" fields pack a
string that many nybbles long.  The "p" type packs a pointer to a null-
terminated string.  You are responsible for ensuring the string is not a
temporary value (which can potentially get deallocated before you get
around to using the packed result).  The "P" packs a pointer to a structure
of the size indicated by the length. A NULL pointer is created if the 
corresponding value for "p" or "P" is C<undef>.
Real numbers (floats and doubles) are
in the native machine format only; due to the multiplicity of floating
formats around, and the lack of a standard "network" representation, no
facility for interchange has been made.  This means that packed floating
point data written on one machine may not be readable on another - even if
both use IEEE floating point arithmetic (as the endian-ness of the memory
representation is not part of the IEEE spec).  Note that Perl uses doubles
internally for all numeric calculation, and converting from double into
float and thence back to double again will lose precision (i.e.,
C<unpack("f", pack("f", $foo)>) will not in general equal $foo).

Examples:

    $foo = pack("cccc",65,66,67,68);
    # foo eq "ABCD"
    $foo = pack("c4",65,66,67,68);
    # same thing

    $foo = pack("ccxxcc",65,66,67,68);
    # foo eq "AB\0\0CD"

    $foo = pack("s2",1,2);
    # "\1\0\2\0" on little-endian
    # "\0\1\0\2" on big-endian

    $foo = pack("a4","abcd","x","y","z");
    # "abcd"

    $foo = pack("aaaa","abcd","x","y","z");
    # "axyz"

    $foo = pack("a14","abcdefg");
    # "abcdefg\0\0\0\0\0\0\0"

    $foo = pack("i9pl", gmtime);
    # a real struct tm (on my system anyway)

    sub bintodec {
        unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
    }

The same template may generally also be used in the unpack function.

=item package NAMESPACE

Declares the compilation unit as being in the given namespace.  The scope
of the package declaration is from the declaration itself through the end of
the enclosing block (the same scope as the local() operator).  All further
unqualified dynamic identifiers will be in this namespace.  A package
statement affects only dynamic variables--including those you've used
local() on--but I<not> lexical variables created with my().  Typically it
would be the first declaration in a file to be included by the C<require>
or C<use> operator.  You can switch into a package in more than one place;
it influences merely which symbol table is used by the compiler for the
rest of that block.  You can refer to variables and filehandles in other
packages by prefixing the identifier with the package name and a double
colon:  C<$Package::Variable>.  If the package name is null, the C<main>
package as assumed.  That is, C<$::sail> is equivalent to C<$main::sail>.

See L<perlmod/"Packages"> for more information about packages, modules,
and classes.  See L<perlsub> for other scoping issues.

=item pipe READHANDLE,WRITEHANDLE

Opens a pair of connected pipes like the corresponding system call.
Note that if you set up a loop of piped processes, deadlock can occur
unless you are very careful.  In addition, note that Perl's pipes use
stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
after each command, depending on the application.

See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
for examples of such things.

=item pop ARRAY

=item pop

Pops and returns the last value of the array, shortening the array by
1.  Has a similar effect to

    $tmp = $ARRAY[$#ARRAY--];

If there are no elements in the array, returns the undefined value.
If ARRAY is omitted, pops the
@ARGV array in the main program, and the @_ array in subroutines, just
like shift().

=item pos SCALAR

=item pos

Returns the offset of where the last C<m//g> search left off for the variable
is in question ($_ is used when the variable is not specified).  May be
modified to change that offset.  Such modification will also influence
the C<\G> zero-width assertion in regular expressions.  See L<perlre> and
L<perlop>.

=item print FILEHANDLE LIST

=item print LIST

=item print

Prints a string or a comma-separated list of strings.  Returns TRUE
if successful.  FILEHANDLE may be a scalar variable name, in which case
the variable contains the name of or a reference to the filehandle, thus introducing one
level of indirection.  (NOTE: If FILEHANDLE is a variable and the next
token is a term, it may be misinterpreted as an operator unless you
interpose a + or put parentheses around the arguments.)  If FILEHANDLE is
omitted, prints by default to standard output (or to the last selected
output channel--see L</select>).  If LIST is also omitted, prints $_ to
STDOUT.  To set the default output channel to something other than
STDOUT use the select operation.  Note that, because print takes a
LIST, anything in the LIST is evaluated in a list context, and any
subroutine that you call will have one or more of its expressions
evaluated in a list context.  Also be careful not to follow the print
keyword with a left parenthesis unless you want the corresponding right
parenthesis to terminate the arguments to the print--interpose a + or
put parentheses around all the arguments.

Note that if you're storing FILEHANDLES in an array or other expression,
you will have to use a block returning its value instead:

    print { $files[$i] } "stuff\n";
    print { $OK ? STDOUT : STDERR } "stuff\n";

=item printf FILEHANDLE FORMAT, LIST

=item printf FORMAT, LIST

Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that $\
(the output record separator) is not appended.  The first argument
of the list will be interpreted as the printf format.  If C<use locale> is
in effect, the character used for the decimal point in formatted real numbers
is affected by the LC_NUMERIC locale.  See L<perllocale>.

Don't fall into the trap of using a printf() when a simple
print() would do.  The print() is more efficient, and less
error prone.

=item prototype FUNCTION

Returns the prototype of a function as a string (or C<undef> if the
function has no prototype).  FUNCTION is a reference to, or the name of,
the function whose prototype you want to retrieve.

If FUNCTION is a string starting with C<CORE::>, the rest is taken as
a name for Perl builtin.  If builtin is not I<overridable> (such as
C<qw>) or its arguments cannot be expressed by a prototype (such as
C<system>) - in other words, the builtin does not behave like a Perl
function - returns C<undef>.  Otherwise, the string describing the
equivalent prototype is returned.

=item push ARRAY,LIST

Treats ARRAY as a stack, and pushes the values of LIST
onto the end of ARRAY.  The length of ARRAY increases by the length of
LIST.  Has the same effect as

    for $value (LIST) {
        $ARRAY[++$#ARRAY] = $value;
    }

but is more efficient.  Returns the new number of elements in the array.

=item q/STRING/

=item qq/STRING/

=item qx/STRING/

=item qw/STRING/

Generalized quotes.  See L<perlop>.

=item quotemeta EXPR

=item quotemeta

Returns the value of EXPR with all non-alphanumeric
characters backslashed.  (That is, all characters not matching
C</[A-Za-z_0-9]/> will be preceded by a backslash in the
returned string, regardless of any locale settings.)
This is the internal function implementing
the \Q escape in double-quoted strings.

If EXPR is omitted, uses $_.

=item rand EXPR

=item rand

Returns a random fractional number greater than or equal to 0 and less
than the value of EXPR.  (EXPR should be positive.)  If EXPR is
omitted, the value 1 is used.  Automatically calls srand() unless
srand() has already been called.  See also srand().

(Note: If your rand function consistently returns numbers that are too
large or too small, then your version of Perl was probably compiled
with the wrong number of RANDBITS.)

=item read FILEHANDLE,SCALAR,LENGTH,OFFSET

=item read FILEHANDLE,SCALAR,LENGTH

Attempts to read LENGTH bytes of data into variable SCALAR from the
specified FILEHANDLE.  Returns the number of bytes actually read, or
undef if there was an error.  SCALAR will be grown or shrunk to the
length actually read.  An OFFSET may be specified to place the read
data at some other place than the beginning of the string.  This call
is actually implemented in terms of stdio's fread call.  To get a true
read system call, see sysread().

=item readdir DIRHANDLE

Returns the next directory entry for a directory opened by opendir().
If used in a list context, returns all the rest of the entries in the
directory.  If there are no more entries, returns an undefined value in
a scalar context or a null list in a list context.

If you're planning to filetest the return values out of a readdir(), you'd
better prepend the directory in question.  Otherwise, because we didn't
chdir() there, it would have been testing the wrong file.

    opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
    @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
    closedir DIR;

=item readline EXPR

Reads from the file handle EXPR.  In scalar context, a single line
is read and returned.  In list context, reads until end-of-file is
reached and returns a list of lines (however you've defined lines
with $/ or $INPUT_RECORD_SEPARATOR).
This is the internal function implementing the C<E<lt>EXPRE<gt>>
operator, but you can use it directly.  The C<E<lt>EXPRE<gt>>
operator is discussed in more detail in L<perlop/"I/O Operators">.

=item readlink EXPR

=item readlink

Returns the value of a symbolic link, if symbolic links are
implemented.  If not, gives a fatal error.  If there is some system
error, returns the undefined value and sets C<$!> (errno).  If EXPR is
omitted, uses $_.

=item readpipe EXPR

EXPR is interpolated and then executed as a system command.
The collected standard output of the command is returned.
In scalar context, it comes back as a single (potentially
multi-line) string.  In list context, returns a list of lines
(however you've defined lines with $/ or $INPUT_RECORD_SEPARATOR).
This is the internal function implementing the C<qx/EXPR/>
operator, but you can use it directly.  The C<qx/EXPR/>
operator is discussed in more detail in L<perlop/"I/O Operators">.

=item recv SOCKET,SCALAR,LEN,FLAGS

Receives a message on a socket.  Attempts to receive LENGTH bytes of
data into variable SCALAR from the specified SOCKET filehandle.
Actually does a C recvfrom(), so that it can return the address of the
sender.  Returns the undefined value if there's an error.  SCALAR will
be grown or shrunk to the length actually read.  Takes the same flags
as the system call of the same name.
See L<perlipc/"UDP: Message Passing"> for examples.

=item redo LABEL

=item redo

The C<redo> command restarts the loop block without evaluating the
conditional again.  The C<continue> block, if any, is not executed.  If
the LABEL is omitted, the command refers to the innermost enclosing
loop.  This command is normally used by programs that want to lie to
themselves about what was just input:

    # a simpleminded Pascal comment stripper
    # (warning: assumes no { or } in strings)
    LINE: while (<STDIN>) {
        while (s|({.*}.*){.*}|$1 |) {}
        s|{.*}| |;
        if (s|{.*| |) {
            $front = $_;
            while (<STDIN>) {
                if (/}/) {      # end of comment?
                    s|^|$front{|;
                    redo LINE;
                }
            }
        }
        print;
    }

See also L</continue> for an illustration of how C<last>, C<next>, and
C<redo> work.

=item ref EXPR

=item ref

Returns a TRUE value if EXPR is a reference, FALSE otherwise.  If EXPR
is not specified, $_ will be used.  The value returned depends on the
type of thing the reference is a reference to.
Builtin types include:

    REF
    SCALAR
    ARRAY
    HASH
    CODE
    GLOB

If the referenced object has been blessed into a package, then that package
name is returned instead.  You can think of ref() as a typeof() operator.

    if (ref($r) eq "HASH") {
        print "r is a reference to a hash.\n";
    }
    if (!ref ($r) {
        print "r is not a reference at all.\n";
    }

See also L<perlref>.

=item rename OLDNAME,NEWNAME

Changes the name of a file.  Returns 1 for success, 0 otherwise.  Will
not work across file system boundaries.

=item require EXPR

=item require

Demands some semantics specified by EXPR, or by $_ if EXPR is not
supplied.  If EXPR is numeric, demands that the current version of Perl
(C<$]> or $PERL_VERSION) be equal or greater than EXPR.

Otherwise, demands that a library file be included if it hasn't already
been included.  The file is included via the do-FILE mechanism, which is
essentially just a variety of eval().  Has semantics similar to the following
subroutine:

    sub require {
        local($filename) = @_;
        return 1 if $INC{$filename};
        local($realfilename,$result);
        ITER: {
            foreach $prefix (@INC) {
                $realfilename = "$prefix/$filename";
                if (-f $realfilename) {
                    $result = do $realfilename;
                    last ITER;
                }
            }
            die "Can't find $filename in \@INC";
        }
        die $@ if $@;
        die "$filename did not return true value" unless $result;
        $INC{$filename} = $realfilename;
        $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 "C<1;>", in case you add more
statements.

If EXPR is a bareword, the require assumes a "F<.pm>" extension and
replaces "F<::>" with "F</>" 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";

For a yet-more-powerful import facility, see L</use> and L<perlmod>.

=item reset EXPR

=item reset

Generally used in a C<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 ?? searches

Resetting "A-Z" is not recommended because you'll wipe out your
ARGV and ENV arrays.  Resets only package variables--lexical variables
are unaffected, but they clean themselves up on scope exit anyway,
so you'll probably want to use them instead.  See L</my>.

=item return EXPR

=item return

Returns from a subroutine, eval(), or do FILE with the value of the
given EXPR.  Evaluation of EXPR may be in a 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 a list context, an undefined value in
a scalar context, or nothing in a void context.

(Note that in the absence of a return, a subroutine, eval, or do FILE
will automatically return the value of the last expression evaluated.)

=item reverse LIST

In a list context, returns a list value consisting of the elements
of LIST in the opposite order.  In a scalar context, concatenates the
elements of LIST, and returns a string value consisting of those bytes,
but in the opposite order.

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

    undef $/;                   # for efficiency of <>
    print scalar reverse <>;    # byte tac, last line tsrif

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.

    %by_name = reverse %by_address;     # Invert the hash

=item rewinddir DIRHANDLE

Sets the current position to the beginning of the directory for the
readdir() routine on DIRHANDLE.

=item rindex STR,SUBSTR,POSITION

=item rindex STR,SUBSTR

Works just like index except that it returns the position of the LAST
occurrence of SUBSTR in STR.  If POSITION is specified, returns the
last occurrence at or before that position.

=item rmdir FILENAME

=item rmdir

Deletes the directory specified by FILENAME if it is empty.  If it
succeeds it returns 1, otherwise it returns 0 and sets C<$!> (errno).  If
FILENAME is omitted, uses $_.

=item s///

The substitution operator.  See L<perlop>.

=item scalar EXPR

Forces EXPR to be interpreted in a 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 a list context because it's in practice never
needed.  If you really wanted to do so, however, you could use
the construction C<@{[ (some expression) ]}>, but usually a simple
C<(some expression)> suffices.

=item 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 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 from either the
IO::Seekable or the POSIX module.  Returns 1 upon success, 0 otherwise.

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.

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 C<tail -f>.  Once you hit
EOF on your read, and then sleep for a while, you might have to stick in a
seek() to reset things.  The seek() doesn't change the current position,
but it I<does> clear the end-of-file condition on the handle, so that the
next C<E<lt>FILEE<gt>> makes Perl try again to read something.  We hope.

If that doesn't work (some stdios 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);
    }

=item seekdir DIRHANDLE,POS

Sets the current position for the readdir() routine on DIRHANDLE.  POS
must be a value returned by telldir().  Has the same caveats about
possible directory compaction as the corresponding system library
routine.

=item select FILEHANDLE

=item select

Returns the currently selected filehandle.  Sets the current default
filehandle for output, if FILEHANDLE is supplied.  This has two
effects: first, a C<write> or a C<print> without a filehandle will
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);

=item select RBITS,WBITS,EBITS,TIMEOUT

This calls the select(2) 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 {
        local(@fhlist) = split(' ',$_[0]);
        local($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 a 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);

B<WARNING>: Do not attempt to mix buffered I/O (like read() or E<lt>FHE<gt>)
with select().  You have to use sysread() instead.

=item semctl ID,SEMNUM,CMD,ARG

Calls the System V IPC function semctl.  If CMD is &IPC_STAT or
&GETALL, then ARG must be a variable which will hold the returned
semid_ds structure or semaphore value array.  Returns like ioctl: the
undefined value for error, "0 but true" for zero, or the actual return
value otherwise.

=item semget KEY,NSEMS,FLAGS

Calls the System V IPC function semget.  Returns the semaphore id, or
the undefined value if there is an error.

=item semop KEY,OPSTRING

Calls the System V IPC function semop to perform semaphore operations
such as signaling and waiting.  OPSTRING must be a packed array of
semop structures.  Each semop structure can be generated with
C<pack("sss", $semnum, $semop, $semflag)>.  The number of semaphore
operations is implied by the length of OPSTRING.  Returns TRUE if
successful, or FALSE if there is an error.  As an example, the
following code waits on semaphore $semnum of semaphore id $semid:

    $semop = pack("sss", $semnum, -1, 0);
    die "Semaphore trouble: $!\n" unless semop($semid, $semop);

To signal the semaphore, replace "-1" with "1".

=item send SOCKET,MSG,FLAGS,TO

=item send SOCKET,MSG,FLAGS

Sends a message on a socket.  Takes the same flags as the system call
of the same name.  On unconnected sockets you must specify a
destination to send TO, in which case it does a C sendto().  Returns
the number of characters sent, or the undefined value if there is an
error.
See L<perlipc/"UDP: Message Passing"> for examples.

=item 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 setpgrp(2).  If the arguments are omitted, it defaults to
0,0.  Note that the POSIX version of setpgrp() does not accept any
arguments, so only setpgrp 0,0 is portable.

=item 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).

=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL

Sets the socket option requested.  Returns undefined if there is an
error.  OPTVAL may be specified as undef if you don't want to pass an
argument.

=item shift ARRAY

=item 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 C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> 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.

=item shmctl ID,CMD,ARG

Calls the System V IPC function shmctl.  If CMD is &IPC_STAT, then ARG
must be a variable which 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.

=item 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.

=item shmread ID,VAR,POS,SIZE

=item 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 which 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.

=item 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.

=item sin EXPR

=item 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 POSIX::asin()
function, or use this relation:

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

=item sleep EXPR

=item 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.

For delays of finer granularity than one second, you may use Perl's
syscall() interface to access setitimer(2) if your system supports it,
or else see L</select()> above.

See also the POSIX module's sigpause() function.

=item socket SOCKET,DOMAIN,TYPE,PROTOCOL

Opens a socket of the specified kind and attaches it to filehandle
SOCKET.  DOMAIN, TYPE, and PROTOCOL are specified the same as for the
system call of the same name.  You should "use Socket;" first to get
the proper definitions imported.  See the example in L<perlipc/"Sockets: Client/Server Communication">.

=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL

Creates an unnamed pair of sockets in the specified domain, of the
specified type.  DOMAIN, TYPE, and PROTOCOL are specified the same as
for the system call of the same name.  If unimplemented, yields a fatal
error.  Returns TRUE if successful.

=item sort SUBNAME LIST

=item sort BLOCK LIST

=item sort LIST

Sorts the LIST and returns the sorted list value.  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 array are to be ordered.  (The C<E<lt>=E<gt>> and C<cmp>
operators are extremely useful in such routines.)  SUBNAME may be a
scalar variable name (unsubscripted), in which case the value provides
the name of (or a reference to) the actual subroutine to use.  In place
of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
subroutine.

In the interests of efficiency the normal calling code for subroutines is
bypassed, with the following effects: the subroutine may not be a
recursive subroutine, and the two elements to be compared are passed into
the subroutine not via @_ but as the package global variables $a and
$b (see example below).  They are passed by reference, so don't
modify $a and $b.  And don't try to declare them as lexicals either.

You also cannot exit out of the sort block or subroutine using any of the
loop control operators described in L<perlsyn> or with goto().

When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
current collation locale.  See L<perllocale>.

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;

    # sort using explicit subroutine name
    sub byage {
        $age{$a} <=> $age{$b};  # presuming numeric
    }
    @sortedclass = sort byage @class;

    # this sorts the %age hash by value instead of key
    # using an in-line function
    @eldest = sort { $age{$b} <=> $age{$a} } keys %age;

    sub backwards { $b cmp $a; }
    @harry = ('dog','cat','x','Cain','Abel');
    @george = ('gone','chased','yz','Punished','Axed');
    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 using a Schwartzian Transform (no temps)
    @new = map { $_->[0] }
        sort { $b->[1] <=> $a->[1]
                        ||
               $a->[2] cmp $b->[2]
        } map { [$_, /=(\d+)/, uc($_)] } @old;

If you're using strict, you I<MUST NOT> declare $a
and $b as lexicals.  They are package globals.  That means
if you're in the C<main> package, it's

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

or just

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

but if you're in the C<FooPack> package, it's

    @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 Perl interpreter will
probably crash and dump core.  This is entirely due to and dependent
upon your system's qsort(3) library routine; this routine often avoids
sanity checks in the interest of speed.

=item splice ARRAY,OFFSET,LENGTH,LIST

=item splice ARRAY,OFFSET,LENGTH

=item splice ARRAY,OFFSET

Removes the elements designated by OFFSET and LENGTH from an array, and
replaces them with the elements of LIST, if any.  In a list context,
returns the elements removed from the array.  In a scalar context,
returns the last element removed, or C<undef> if no elements are
removed.  The array grows or shrinks as necessary.  If LENGTH is
omitted, removes everything from OFFSET onward.  The following
equivalences hold (assuming C<$[ == 0>):

    push(@a,$x,$y)      splice(@a,$#a+1,0,$x,$y)
    pop(@a)             splice(@a,-1)
    shift(@a)           splice(@a,0,1)
    unshift(@a,$x,$y)   splice(@a,0,0,$x,$y)
    $a[$x] = $y         splice(@a,$x,1,$y);

Example, assuming array lengths are passed before arrays:

    sub aeq {   # compare two list values
        local(@a) = splice(@_,0,shift);
        local(@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)) { ... }

=item split /PATTERN/,EXPR,LIMIT

=item split /PATTERN/,EXPR

=item split /PATTERN/

=item split

Splits a string into an array of strings, and returns it.

If not in a list context, returns the number of fields found and splits into
the @_ array.  (In a list context, you can force the split into @_ by
using C<??> as the pattern delimiters, but it still returns the list
value.)  The use of implicit split to @_ is deprecated, however.

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 is positive, splits into no more than that
many fields (though it may split into fewer).  If LIMIT is unspecified
or zero, trailing null fields are stripped (which potential users
of pop() would do well to remember).  If LIMIT is negative, it is
treated as if an arbitrarily large LIMIT had been specified.

A pattern matching the null string (not to be confused with
a null pattern C<//>, which is just one member of the set of patterns
matching a null string) will split the value of EXPR into separate
characters at each point it matches that way.  For example:

    print join(':', split(/ */, '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, 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 array 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 C</PATTERN/> may be replaced with an expression to specify
patterns that vary at runtime.  (To do runtime compilation only once,
use C</$variable/o>.)

As a special case, specifying a PATTERN of space (C<' '>) will split on
white space just as split with no arguments does.  Thus, split(' ') can
be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
will give you as many null initial fields as there are leading spaces.
A split on /\s+/ is like a split(' ') except that any leading
whitespace produces a null first field.  A split with no arguments
really does a C<split(' ', $_)> internally.

Example:

    open(passwd, '/etc/passwd');
    while (<passwd>) {
        ($login, $passwd, $uid, $gid, $gcos,
            $home, $shell) = split(/:/);
        ...
    }

(Note that $shell above will still have a newline on it.  See L</chop>,
L</chomp>, and L</join>.)

=item sprintf FORMAT, LIST

Returns a string formatted by the usual printf conventions of the
C library function sprintf().  See L<sprintf(3)> or L<printf(3)>
on your system for an explanation of the general principles.

Perl does all of 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.

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)
   %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

Perl permits the following universally-known flags between the C<%>
and the conversion letter:

   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"
   number  minimum field width
   .number "precision": digits after decimal point for floating-point,
           max length for string, minimum length for integer
   l       interpret integer as C type "long" or "unsigned long"
   h       interpret integer as C type "short" or "unsigned short"

There is also one Perl-specific flag:

   V       interpret integer as Perl's standard integer type

Where a number would appear in the flags, an asterisk ("*") may be
used instead, in which case Perl uses the next item in the parameter
list as the given number (that is, as the field width or precision).
If a field width obtained through "*" is negative, it has the same
effect as the '-' flag: left-justification.

If C<use locale> is in effect, the character used for the decimal
point in formatted real numbers is affected by the LC_NUMERIC locale.
See L<perllocale>.

=item sqrt EXPR

=item sqrt

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

=item srand EXPR

=item srand

Sets the random number seed for the C<rand> operator.  If EXPR is
omitted, uses a semi-random value based on the current time and process
ID, among other things.  In versions of Perl prior to 5.004 the default
seed was just the current time().  This isn't a particularly good seed,
so many old programs supply their own seed value (often C<time ^ $$> or
C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.

In fact, it's usually not necessary to call srand() at all, because if
it is not called explicitly, it is called implicitly at the first use of
the C<rand> operator.  However, this was not the case in version of Perl
before 5.004, so if your script will run under older Perl versions, it
should call srand().

Note that you need something much more random than the default seed for
cryptographic purposes.  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.

Do I<not> call srand() multiple times in your program unless you know
exactly what you're doing and why you're doing it.  The point of the
function is to "seed" the rand() function so that rand() can produce
a different sequence each time you run your program.  Just do it once at the
top of your program, or you I<won't> get random numbers out of rand()!

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.

=item stat FILEHANDLE

=item stat EXPR

=item 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
meaning 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 since the epoch
  9 mtime    last modify time since the epoch
 10 ctime    inode change time (NOT creation time!) 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.)

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 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.)

In scalar context, C<stat> returns a boolean value indicating success
or failure, and, if successful, sets the information associated with
the special filehandle C<_>.

=item study SCALAR

=item study

Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
doing many pattern matches on the string before it is next modified.
This may or may not save time, depending on the nature and number of
patterns you are searching on, and on the distribution of character
frequencies in the string to be searched -- you probably want to compare
run times with and without it to see which runs faster.  Those loops
which 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 which 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 (C<@files>) for a list of words (C<@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";
    }

=item sub BLOCK

=item sub NAME

=item sub NAME BLOCK

This is subroutine definition, not a real function I<per se>.  With just a
NAME (and possibly prototypes), it's just a forward declaration.  Without
a NAME, it's an anonymous function declaration, and does actually return a
value: the CODE ref of the closure you just created.  See L<perlsub> and
L<perlref> for details.

=item substr EXPR,OFFSET,LEN,REPLACEMENT

=item substr EXPR,OFFSET,LEN

=item substr EXPR,OFFSET

Extracts a substring out of EXPR and returns it.  First character is at
offset 0, or whatever you've set C<$[> to (but don't do that).
If OFFSET is negative (or more precisely, less than C<$[>), starts
that far from the end of the string.  If LEN is omitted, returns
everything to the end of the string.  If LEN is negative, leaves that
many characters off the end of the string.

If you specify a substring which is partly outside the string, the part
within the string is returned.    If the substring is totally outside
the string a warning is produced.

You can use the substr() function
as an lvalue, in which case EXPR must be an lvalue.  If you assign
something shorter than LEN, the string will shrink, and if you assign
something longer than LEN, the string will grow to accommodate it.  To
keep the string the same length you may need to pad or chop your value
using sprintf().

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.
In this case LEN can be C<undef> if you want to affect everything to
the end of the string.

=item 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("","")};, $@ eq '');

=item syscall 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.

    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 C<$!> (errno).
Note that some system calls can legitimately return -1.  The proper
way to handle such calls is to assign C<$!=0;> before the call and
check the value of C<$!> if syscall returns -1.

There's a problem with C<syscall(&SYS_pipe)>: it returns the file
number of the read end of the pipe it creates.  There is no way
to retrieve the file number of the other end.  You can avoid this 
problem by using C<pipe> instead.

=item sysopen FILEHANDLE,FILENAME,MODE

=item sysopen FILEHANDLE,FILENAME,MODE,PERMS

Opens the file whose filename is given by FILENAME, and associates it
with FILEHANDLE.  If FILEHANDLE is an expression, its value is used as
the name of the real filehandle wanted.  This function calls the
underlying operating system's C<open> function with the parameters
FILENAME, MODE, PERMS.

The possible values and flag bits of the MODE parameter are
system-dependent; they are available via the standard module C<Fcntl>.
However, for historical reasons, some values are universal: zero means
read-only, one means write-only, and two means read/write.

If the file named by FILENAME does not exist and the C<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 PERMS is omitted, the default value is 0666, which allows
read and write for all.  This default is reasonable: see C<umask>.

The IO::File module provides a more object-oriented approach, if you're
into that kind of thing.

=item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET

=item sysread FILEHANDLE,SCALAR,LENGTH

Attempts to read LENGTH bytes of data into variable SCALAR from the
specified FILEHANDLE, using the system call read(2).  It bypasses
stdio, so mixing this with other kinds of reads, print(), write(),
seek(), or tell() can cause confusion because stdio usually buffers
data.  Returns the number of bytes actually read, or undef if there
was an error.  SCALAR will be grown or shrunk so that the last byte
actually read is the last byte of the 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 bytes 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.

=item sysseek FILEHANDLE,POSITION,WHENCE

Sets FILEHANDLE's system position using the system call lseek(2).  It
bypasses stdio, so mixing this with reads (other than sysread()),
print(), write(), seek(), or tell() may cause confusion.  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).  For WHENCE, you may use the
constants SEEK_SET, SEEK_CUR, and SEEK_END from either the IO::Seekable
or the POSIX module.

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.

=item system LIST

=item 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.  The return value is the exit status of the program as
returned by the wait() call.  To get the actual exit value divide by
256.  See also L</exec>.  This is I<NOT> what you want to use to capture
the output from a command, for that you should use merely backticks or
qx//, as described in L<perlop/"`STRING`">.

Like exec(), system() allows you to lie to a program about its name if
you use the "system PROGRAM LIST" syntax.  Again, see L</exec>.

Because system() and backticks block SIGINT and SIGQUIT, killing the
program they're running doesn't actually interrupt your program.

    @args = ("command", "arg1", "arg2");
    system(@args) == 0
         or die "system @args failed: $?"

Here's a more elaborate example of analysing the return value from
system() on a Unix system to check for all possibilities, including for
signals and core dumps.

    $! = 0;
    $rc = system @args;
    printf "system(%s) returned %#04x: ", "@args", $rc;
    if ($rc == 0) {
        print "ran with normal exit\n";
    }
    elsif ($rc == 0xff00) {
        # Note that $! can be an empty string if the command that
        # system() tried to execute was not found, not executable, etc.
        # These errors occur in the child process after system() has
        # forked, so the errno value is not visible in the parent.
        printf "command failed: %s\n", ($! || "Unknown system() error");
    }
    elsif (($rc & 0xff) == 0) {
        $rc >>= 8;
        print "ran with non-zero exit status $rc\n";
    }
    else {
        print "ran with ";
        if ($rc &   0x80) {
            $rc &= ~0x80;
            print "core dump from ";
        }
        print "signal $rc\n"
    }
    $ok = ($rc != 0);

When the arguments get executed via the system shell, results will
be subject to its quirks and capabilities.  See L<perlop/"`STRING`">
for details.

=item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET

=item syswrite FILEHANDLE,SCALAR,LENGTH

Attempts to write LENGTH bytes of data from variable SCALAR to the
specified FILEHANDLE, using the system call write(2).  It bypasses
stdio, so mixing this with reads (other than sysread()), print(),
write(), seek(), or tell() may cause confusion because stdio usually
buffers data.  Returns the number of bytes actually written, or undef
if there was an error.  If the LENGTH is greater than the available
data in the SCALAR after the OFFSET, only as much data as is available
will be written.

An OFFSET may be specified to write the data from some part of the
string other than the beginning.  A negative OFFSET specifies writing
that many bytes counting backwards from the end of the string.  In the
case the SCALAR is empty you can use OFFSET but only zero offset.

=item tell FILEHANDLE

=item tell

Returns the current position for FILEHANDLE.  FILEHANDLE may be an
expression whose value gives the name of the actual filehandle.  If
FILEHANDLE is omitted, assumes the file last read.

=item 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.  Has the same caveats about possible directory compaction as
the corresponding system library routine.

=item 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, 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
    DESTROY this
    FETCH this, key
    STORE this, key, value
    DELETE this, key
    EXISTS this, key
    FIRSTKEY this
    NEXTKEY this, lastkey

A class implementing an ordinary array should have the following methods:

    TIEARRAY classname, LIST
    DESTROY this
    FETCH this, key
    STORE this, key, value
    [others TBD]

A class implementing a scalar should have the following methods:

    TIESCALAR classname, LIST
    DESTROY this
    FETCH this,
    STORE this, value

Unlike dbmopen(), the tie() function will not use or require a module
for you--you need to do that explicitly yourself.  See L<DB_File>
or the F<Config> module for interesting tie() implementations.

For further details see L<perltie>, L<tied VARIABLE>.

=item tied VARIABLE

Returns a reference to the object underlying VARIABLE (the same value
that was originally returned by the tie() call which bound the variable
to a package.)  Returns the undefined value if VARIABLE isn't tied to a
package.

=item time

Returns the number of non-leap seconds since whatever time the system
considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
and 00:00:00 UTC, January 1, 1970 for most other systems).
Suitable for feeding to gmtime() and localtime().

=item 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;

=item tr///

The transliteration operator.  Same as y///. See L<perlop>.

=item truncate FILEHANDLE,LENGTH

=item truncate EXPR,LENGTH

Truncates the file opened on FILEHANDLE, or named by EXPR, to the
specified length.  Produces a fatal error if truncate isn't implemented
on your system.  Returns TRUE if successful, the undefined value
otherwise.

=item uc EXPR

=item 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 C<use locale> in force.  See L<perllocale>.

If EXPR is omitted, uses $_.

=item ucfirst EXPR

=item ucfirst

Returns the value of EXPR with the first character uppercased.  This is
the internal function implementing the \u escape in double-quoted strings.
Respects current LC_CTYPE locale if C<use locale> in force.  See L<perllocale>.

If EXPR is omitted, uses $_.

=item umask EXPR

=item umask

Sets the umask for the process to EXPR and returns the previous value.
If EXPR is omitted, merely returns the current umask.  If umask(2) is
not implemented on your system, returns C<undef>.  Remember that a
umask is a number, usually given in octal; it is I<not> a string of octal
digits.  See also L</oct>, if all you have is a string.

=item undef EXPR

=item 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 C<undef $hash{$key}>
will probably not do what you expect on most predefined variables or
DBM list values, so don't do that; see L<delete>.)  Always returns the
undefined value.  You can omit the EXPR, in which case nothing is
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

=item unlink LIST

=item 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 delete directories unless you are superuser and
the B<-U> flag is supplied to Perl.  Even if these conditions are
met, be warned that unlinking a directory can inflict damage on your
filesystem.  Use rmdir instead.

If LIST is omitted, uses $_.

=item unpack TEMPLATE,EXPR

Unpack does the reverse of pack: it takes a string representing a
structure and expands it out into a list value, returning the array
value.  (In a scalar context, it returns merely the first value
produced.)  The TEMPLATE has the same format as in the pack function.
Here's a subroutine that does substring:

    sub substr {
        local($what,$where,$howmuch) = @_;
        unpack("x$where a$howmuch", $what);
    }

and then there's

    sub ordinal { unpack("c",$_[0]); } # same as ord()

In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
themselves.  Default is a 16-bit checksum.  For example, the following
computes the same number as the System V sum program:

    while (<>) {
        $checksum += unpack("%16C*", $_);
    }
    $checksum %= 65536;

The following efficiently counts the number of set bits in a bit vector:

    $setbits = unpack("%32b*", $selectmask);

=item untie VARIABLE

Breaks the binding between a variable and a package.  (See tie().)

=item unshift ARRAY,LIST

Does the opposite of a C<shift>.  Or the opposite of a C<push>,
depending on how you look at it.  Prepends list to the front of the
array, and returns the new number of elements in the array.

    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.

=item use Module LIST

=item use Module

=item use Module VERSION LIST

=item 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 I<must> be a bareword.

If the first argument to C<use> is a number, it is treated as a version
number instead of a module name.  If the version of the Perl interpreter
is less than VERSION, then an error message is printed and Perl exits
immediately.  This is often useful if you need to check the current
Perl version before C<use>ing library modules which 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 L<Exporter>.  If no import
method can be found then the error is currently silently ignored.  This
may change to a fatal error in a future version.

If you don't want your namespace altered, explicitly supply an empty list:

    use Module ();

That is exactly equivalent to

    BEGIN { require Module; }

If the VERSION argument is present between Module and LIST, then the
C<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.  (Note that there is not a
comma after VERSION!)

Because this is a wide-open interface, pragmas (compiler directives)
are also implemented this way.  Currently implemented pragmas are:

    use integer;
    use diagnostics;
    use sigtrap qw(SEGV BUS);
    use strict  qw(subs vars refs);
    use subs    qw(afunc blurfl);

These pseudo-modules import semantics into the current block scope, 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 C<unimport Module LIST> instead of C<import>.

    no integer;
    no strict 'refs';

If no unimport method can be found the call fails with a fatal error.

See L<perlmod> for a list of standard modules and pragmas.

=item 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 modification time of each file is set
to the current time.  This code has the same effect as the "touch"
command if the files already exist:

    #!/usr/bin/perl
    $now = time;
    utime $now, $now, @ARGV;

=item 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, but it is the same order as
either the keys() or each() function would produce on the same hash.
As a side effect, it resets HASH's iterator.  See also keys(), each(),
and sort().

=item vec EXPR,OFFSET,BITS

Treats the string in EXPR as a vector of unsigned integers, and
returns the value of the bit field specified by OFFSET.  BITS specifies
the number of bits that are reserved for each entry in the bit
vector.  This must be a power of two from 1 to 32. vec() may also be
assigned to, in which case parentheses are needed to give the expression
the correct precedence as in

    vec($image, $max_x * $x + $y, 8) = 3;

Vectors created with vec() can also be manipulated with the logical
operators |, &, and ^, which will assume a bit vector operation is
desired when both operands are strings.

The following code will build up an ASCII string saying 'PerlPerlPerl'.
The comments show the string after each step. Note that this code works
in the same way on big-endian or little-endian machines.

    my $foo = '';
    vec($foo,  0, 32) = 0x5065726C;     # 'Perl'
    vec($foo,  2, 16) = 0x5065;         # 'PerlPe'
    vec($foo,  3, 16) = 0x726C;         # 'PerlPerl'
    vec($foo,  8,  8) = 0x50;           # 'PerlPerlP'
    vec($foo,  9,  8) = 0x65;           # 'PerlPerlPe'
    vec($foo, 20,  4) = 2;              # 'PerlPerlPe'   . "\x02"
    vec($foo, 21,  4) = 7;              # 'PerlPerlPer'  # 'r' is "\x72"
    vec($foo, 45,  2) = 3;              # 'PerlPerlPer'  . "\x0c"
    vec($foo, 93,  1) = 1;              # 'PerlPerlPer'  . "\x2c"
    vec($foo, 94,  1) = 1;              # 'PerlPerlPerl' # 'l' is "\x6c"

To transform a bit vector into a string or array of 0's and 1's, use these:

    $bits = unpack("b*", $vector);
    @bits = split(//, unpack("b*", $vector));

If you know the exact length in bits, it can be used in place of the *.

=item wait

Waits for a child process to terminate and returns the pid of the
deceased process, or -1 if there are no child processes.  The status is
returned in C<$?>.

=item waitpid PID,FLAGS

Waits for a particular child process to terminate and returns the pid
of the deceased process, or -1 if there is no such child process.  The
status is returned in C<$?>.  If you say

    use POSIX ":sys_wait_h";
    ...
    waitpid(-1,&WNOHANG);

then you can do a non-blocking wait for any process.  Non-blocking wait
is available on machines supporting either the waitpid(2) or
wait4(2) system calls.  However, waiting for a particular pid with
FLAGS of 0 is implemented everywhere.  (Perl emulates the system call
by remembering the status values of processes that have exited but have
not been harvested by the Perl script yet.)

=item wantarray

Returns TRUE if the context of the currently executing subroutine is
looking for a list value.  Returns FALSE if the context is looking
for a scalar.  Returns the undefined value if the context is looking
for no value (void context).

    return unless defined wantarray;    # don't bother doing more
    my @a = complex_calculation();
    return wantarray ? @a : "@a";

=item warn LIST

Produces a message on STDERR just like die(), but doesn't exit or throw
an exception.

If LIST is empty and $@ already contains a value (typically from a
previous eval) that value is used after appending "\t...caught"
to $@. This is useful for staying almost, but not entirely similar to
die().

If $@ is empty then the string "Warning: Something's wrong" is used.

No message is printed if there is a C<$SIG{__WARN__}> handler
installed.  It is the handler's responsibility to deal with the message
as it sees fit (like, for instance, converting it into a die()).  Most
handlers must therefore make arrangements to actually display the
warnings that they are not prepared to deal with, by calling warn()
again in the handler.  Note that this is quite safe and will not
produce an endless loop, since C<__WARN__> hooks are not called from
inside one.

You will find this behavior is slightly different from that of
C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
instead call die() again to change it).

Using a C<__WARN__> handler provides a powerful way to silence all
warnings (even the so-called mandatory ones).  An example:

    # wipe out *all* compile-time warnings
    BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
    my $foo = 10;
    my $foo = 20;          # no warning about duplicate my $foo,
                           # but hey, you asked for it!
    # no compile-time or run-time warnings before here
    $DOWARN = 1;

    # run-time warnings enabled after here
    warn "\$foo is alive and $foo!";     # does show up

See L<perlvar> for details on setting C<%SIG> entries, and for more
examples.

=item write FILEHANDLE

=item write EXPR

=item write

Writes a formatted record (possibly multi-line) to the specified file,
using the format associated with that file.  By default the format for
a file is the one having the same name as the filehandle, but the
format for the current output channel (see the select() function) may be set
explicitly by assigning the name of the format to the C<$~> variable.

Top of form processing is handled automatically:  if there is
insufficient room on the current page for the formatted record, the
page is advanced by writing a form feed, a special top-of-page format
is used to format the new page header, and then the record is written.
By default the top-of-page format is the name of the filehandle with
"_TOP" appended, but it may be dynamically set to the format of your
choice by assigning the name to the C<$^> variable while the filehandle is
selected.  The number of lines remaining on the current page is in
variable C<$->, which can be set to 0 to force a new page.

If FILEHANDLE is unspecified, output goes to the current default output
channel, which starts out as STDOUT but may be changed by the
C<select> operator.  If the FILEHANDLE is an EXPR, then the expression
is evaluated and the resulting string is used to look up the name of
the FILEHANDLE at run time.  For more on formats, see L<perlform>.

Note that write is I<NOT> the opposite of read.  Unfortunately.

=item y///

The transliteration operator.  Same as tr///.  See L<perlop>.

=back