perlfunc.pod

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access() family of system calls.  Also note that the C<-x> and C<-X> may
under this pragma return true even if there are no execute permission
bits set (nor any extra execute permission ACLs).  This strangeness is
due to the underlying system calls' definitions.  Read the
documentation for the C<filetest> pragma for more information.

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

The C<-T> and C<-B> switches work as follows.  The first block or so of the
file is examined for odd characters such as strange control codes or
characters with the high bit set.  If too many strange characters (>30%)
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 C<stat> or C<lstat> operators) are given
the special filehandle consisting of a solitary underline, then the stat
structure of the previous file test (or stat operator) is used, saving
a system call.  (This doesn't work with C<-t>, and you need to remember
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 C<$_>.

=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 the example in L<perlipc/"Sockets: Client/Server Communication">.

On systems that support a close-on-exec flag on files, the flag will
be set for the newly opened file descriptor, as determined by the
value of $^F.  See L<perlvar/$^F>.

=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 C<$_> 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 C<0> may be supplied to cancel the previous timer without
starting a new one.  The returned value is the amount of time remaining
on the previous timer.

For delays of finer granularity than one second, you may use Perl's
four-argument version of select() leaving the first three arguments
undefined, or you might be able to use the C<syscall> interface to
access setitimer(2) if your system supports it.  The Time::HiRes module
from CPAN may also prove useful.

It is usually a mistake to intermix C<alarm> and C<sleep> calls.
(C<sleep> may be internally implemented in your system with C<alarm>)

If you want to use C<alarm> to time out a system call you need to use an
C<eval>/C<die> pair.  You can't rely on the alarm causing the system call to
fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
restart system calls on some systems.  Using C<eval>/C<die> always works,
modulo the caveats given in L<perlipc/"Signals">.

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

=item atan2 Y,X

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

For the tangent operation, you may use the C<Math::Trig::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, DISCIPLINE

=item binmode FILEHANDLE

Arranges for FILEHANDLE to be read or written in "binary" or "text" mode
on systems where the run-time libraries distinguish between binary and
text files.  If FILEHANDLE is an expression, the value is taken as the
name of the filehandle.  DISCIPLINE can be either of C<":raw"> for
binary mode or C<":crlf"> for "text" mode.  If the DISCIPLINE is
omitted, it defaults to C<":raw">.

binmode() should be called after open() but before any I/O is done on
the filehandle.

On many systems binmode() currently has no effect, but in future, it
will be extended to support user-defined input and output disciplines.
On some systems binmode() is necessary when you're not working with a
text file.  For the sake of portability it is a good idea to always use
it when appropriate, and to never use it when it isn't appropriate.

In other words:  Regardless of platform, use binmode() on binary
files, and do not use binmode() on text files.

The C<open> pragma can be used to establish default disciplines.
See L<open>.

The operating system, device drivers, C libraries, and Perl run-time
system all work together to let the programmer treat a single
character (C<\n>) as the line terminator, irrespective of the external
representation.  On many operating systems, the native text file
representation matches the internal representation, but on some
platforms the external representation of C<\n> is made up of more than
one character.

Mac OS and all variants of Unix use a single character to end each line
in the external representation of text (even though that single
character is not necessarily the same across these platforms).
Consequently binmode() has no effect on these operating systems.  In
other systems like VMS, MS-DOS and the various flavors of MS-Windows
your program sees a C<\n> as a simple C<\cJ>, but what's stored in text
files are the two characters C<\cM\cJ>.  That means that, if you don't
use binmode() on these systems, C<\cM\cJ> sequences on disk will be
converted to C<\n> on input, and any C<\n> in your program will be
converted back to C<\cM\cJ> on output.  This is what you want for text
files, but it can be disastrous for binary files.

Another consequence of using binmode() (on some systems) is that
special end-of-file markers will be seen as part of the data stream.
For systems from the Microsoft family this means that if your binary
data contains C<\cZ>, the I/O subsystem will regard it as the end of
the file, unless you use binmode().

binmode() is not only important for readline() and print() operations,
but also when using read(), seek(), sysread(), syswrite() and tell()
(see L<perlport> for more details).  See the C<$/> and C<$\> variables
in L<perlvar> for how to manually set your input and output
line-termination sequences.

=item bless REF,CLASSNAME

=item bless REF

This function tells the thingy referenced by REF that it is now an object
in the CLASSNAME package.  If CLASSNAME is omitted, the current package
is used.  Because a C<bless> is often the last thing in a constructor,
it returns the reference for convenience.  Always use the two-argument
version if the function doing the blessing might be inherited by a
derived class.  See L<perltoot> and L<perlobj> for more about the blessing
(and blessings) of objects.

Consider always blessing objects in CLASSNAMEs that are mixed case.
Namespaces with all lowercase names are considered reserved for
Perl pragmata.  Builtin types have all uppercase names, so to prevent
confusion, you may wish to avoid such package names as well.  Make sure
that CLASSNAME is a true value.

See L<perlmod/"Perl Modules">.

=item caller EXPR

=item caller

Returns the context of the current subroutine call.  In scalar context,
returns the caller's package name if there is a caller, that is, if
we're in a subroutine or C<eval> or C<require>, and the undefined value
otherwise.  In list context, returns

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

With EXPR, it returns some extra information that the debugger uses to
print a stack trace.  The value of EXPR indicates how many call frames
to go back before the current one.

    ($package, $filename, $line, $subroutine, $hasargs,
    $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);

Here $subroutine may be C<(eval)> if the frame is not a subroutine
call, but an C<eval>.  In such a case additional elements $evaltext and
C<$is_require> are set: C<$is_require> is true if the frame is created by a
C<require> or C<use> statement, $evaltext contains the text of the
C<eval EXPR> statement.  In particular, for an C<eval BLOCK> statement,
$filename is C<(eval)>, but $evaltext is undefined.  (Note also that
each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
frame.  C<$hasargs> is true if a new instance of C<@_> was set up for the
frame.  C<$hints> and C<$bitmask> contain pragmatic hints that the caller
was compiled with.  The C<$hints> and C<$bitmask> values are subject to
change between versions of Perl, and are not meant for external use.

Furthermore, when called from within the DB package, caller returns more
detailed information: it sets the list variable C<@DB::args> to be the
arguments with which the subroutine was invoked.

Be aware that the optimizer might have optimized call frames away before
C<caller> had a chance to get the information.  That means that C<caller(N)>
might not return information about the call frame you expect it do, for
C<< N > 1 >>.  In particular, C<@DB::args> might have information from the 
previous time C<caller> was called.

=item chdir EXPR

Changes the working directory to EXPR, if possible.  If EXPR is omitted,
changes to the directory specified by C<$ENV{HOME}>, if set; if not,
changes to the directory specified by C<$ENV{LOGDIR}>.  If neither is
set, C<chdir> does nothing.  It returns true upon success, false
otherwise.  See the example under C<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

You can also import the symbolic C<S_I*> constants from the Fcntl
module:

    use Fcntl ':mode';

    chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
    # This is identical to the chmod 0755 of the above example.

=item chomp VARIABLE

=item chomp LIST

=item chomp

This safer version of L</chop> removes any trailing string
that corresponds to the current value of C<$/> (also known as
$INPUT_RECORD_SEPARATOR in the C<English> module).  It returns the total
number of characters removed from all its arguments.  It's often used to
remove the newline from the end of an input record when you're worried
that the final record may be missing its newline.  When in paragraph
mode (C<$/ = "">), it removes all trailing newlines from the string.
When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
a reference to an integer or the like, see L<perlvar>) chomp() won't
remove anything.  
If VARIABLE is omitted, it chomps C<$_>.  Example:

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

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

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

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

If you chomp a list, each element is chomped, and the total number of
characters removed is returned.

=item chop VARIABLE

=item chop LIST

=item chop

Chops off the last character of a string and returns the character
chopped.  It is much more efficient than C<s/.$//s> because it neither
scans nor copies the string.  If VARIABLE is omitted, chops C<$_>.
If VARIABLE is a hash, it chops the hash's values, but not its keys.

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

If you chop a list, each element is chopped.  Only the value of the
last C<chop> is returned.

Note that C<chop> returns the last character.  To return all but the last
character, use C<substr($string, 0, -1)>.

=item chown LIST

Changes the owner (and group) of a list of files.  The first two
elements of the list must be the I<numeric> uid and gid, in that
order.  A value of -1 in either position is interpreted by most
systems to leave that value unchanged.  Returns the number of files
successfully changed.