perlfaq8.pod

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varies from system to system--see L<passwd> for specifics) and use
pwd_mkdb(8) to install it (see L<pwd_mkdb> for more details).

=head2 How do I set the time and date?

Assuming you're running under sufficient permissions, you should be
able to set the system-wide date and time by running the date(1)
program.  (There is no way to set the time and date on a per-process
basis.)  This mechanism will work for Unix, MS-DOS, Windows, and NT;
the VMS equivalent is C<set time>.

However, if all you want to do is change your time zone, you can
probably get away with setting an environment variable:

	$ENV{TZ} = "MST7MDT";		   # unixish
	$ENV{'SYS$TIMEZONE_DIFFERENTIAL'}="-5" # vms
	system "trn comp.lang.perl.misc";

=head2 How can I sleep() or alarm() for under a second?
X<Time::HiRes> X<BSD::Itimer> X<sleep> X<select>

If you want finer granularity than the 1 second that the C<sleep()>
function provides, the easiest way is to use the C<select()> function as
documented in L<perlfunc/"select">.  Try the C<Time::HiRes> and
the C<BSD::Itimer> modules (available from CPAN, and starting from
Perl 5.8 C<Time::HiRes> is part of the standard distribution).

=head2 How can I measure time under a second?
X<Time::HiRes> X<BSD::Itimer> X<sleep> X<select>

(contributed by brian d foy)

The C<Time::HiRes> module (part of the standard distribution as of
Perl 5.8) measures time with the C<gettimeofday()> system call, which
returns the time in microseconds since the epoch. If you can't install
C<Time::HiRes> for older Perls and you are on a Unixish system, you
may be able to call C<gettimeofday(2)> directly. See
L<perlfunc/syscall>.

=head2 How can I do an atexit() or setjmp()/longjmp()? (Exception handling)

Release 5 of Perl added the END block, which can be used to simulate
atexit().  Each package's END block is called when the program or
thread ends (see L<perlmod> manpage for more details).

For example, you can use this to make sure your filter program
managed to finish its output without filling up the disk:

	END {
		close(STDOUT) || die "stdout close failed: $!";
	}

The END block isn't called when untrapped signals kill the program,
though, so if you use END blocks you should also use

	use sigtrap qw(die normal-signals);

Perl's exception-handling mechanism is its eval() operator.  You can
use eval() as setjmp and die() as longjmp.  For details of this, see
the section on signals, especially the time-out handler for a blocking
flock() in L<perlipc/"Signals"> or the section on "Signals" in
the Camel Book.

If exception handling is all you're interested in, try the
exceptions.pl library (part of the standard perl distribution).

If you want the atexit() syntax (and an rmexit() as well), try the
AtExit module available from CPAN.

=head2 Why doesn't my sockets program work under System V (Solaris)?  What does the error message "Protocol not supported" mean?

Some Sys-V based systems, notably Solaris 2.X, redefined some of the
standard socket constants.  Since these were constant across all
architectures, they were often hardwired into perl code.  The proper
way to deal with this is to "use Socket" to get the correct values.

Note that even though SunOS and Solaris are binary compatible, these
values are different.  Go figure.

=head2 How can I call my system's unique C functions from Perl?

In most cases, you write an external module to do it--see the answer
to "Where can I learn about linking C with Perl? [h2xs, xsubpp]".
However, if the function is a system call, and your system supports
syscall(), you can use the syscall function (documented in
L<perlfunc>).

Remember to check the modules that came with your distribution, and
CPAN as well--someone may already have written a module to do it. On
Windows, try Win32::API.  On Macs, try Mac::Carbon.  If no module
has an interface to the C function, you can inline a bit of C in your
Perl source with Inline::C.

=head2 Where do I get the include files to do ioctl() or syscall()?

Historically, these would be generated by the h2ph tool, part of the
standard perl distribution.  This program converts cpp(1) directives
in C header files to files containing subroutine definitions, like
&SYS_getitimer, which you can use as arguments to your functions.
It doesn't work perfectly, but it usually gets most of the job done.
Simple files like F<errno.h>, F<syscall.h>, and F<socket.h> were fine,
but the hard ones like F<ioctl.h> nearly always need to hand-edited.
Here's how to install the *.ph files:

	1.  become super-user
	2.  cd /usr/include
	3.  h2ph *.h */*.h

If your system supports dynamic loading, for reasons of portability and
sanity you probably ought to use h2xs (also part of the standard perl
distribution).  This tool converts C header files to Perl extensions.
See L<perlxstut> for how to get started with h2xs.

If your system doesn't support dynamic loading, you still probably
ought to use h2xs.  See L<perlxstut> and L<ExtUtils::MakeMaker> for
more information (in brief, just use B<make perl> instead of a plain
B<make> to rebuild perl with a new static extension).

=head2 Why do setuid perl scripts complain about kernel problems?

Some operating systems have bugs in the kernel that make setuid
scripts inherently insecure.  Perl gives you a number of options
(described in L<perlsec>) to work around such systems.

=head2 How can I open a pipe both to and from a command?

The IPC::Open2 module (part of the standard perl distribution) is an
easy-to-use approach that internally uses pipe(), fork(), and exec() to do
the job.  Make sure you read the deadlock warnings in its documentation,
though (see L<IPC::Open2>).  See
L<perlipc/"Bidirectional Communication with Another Process"> and
L<perlipc/"Bidirectional Communication with Yourself">

You may also use the IPC::Open3 module (part of the standard perl
distribution), but be warned that it has a different order of
arguments from IPC::Open2 (see L<IPC::Open3>).

=head2 Why can't I get the output of a command with system()?

You're confusing the purpose of system() and backticks (``).  system()
runs a command and returns exit status information (as a 16 bit value:
the low 7 bits are the signal the process died from, if any, and
the high 8 bits are the actual exit value).  Backticks (``) run a
command and return what it sent to STDOUT.

	$exit_status   = system("mail-users");
	$output_string = `ls`;

=head2 How can I capture STDERR from an external command?

There are three basic ways of running external commands:

	system $cmd;		# using system()
	$output = `$cmd`;		# using backticks (``)
	open (PIPE, "cmd |");	# using open()

With system(), both STDOUT and STDERR will go the same place as the
script's STDOUT and STDERR, unless the system() command redirects them.
Backticks and open() read B<only> the STDOUT of your command.

You can also use the open3() function from IPC::Open3.  Benjamin
Goldberg provides some sample code:

To capture a program's STDOUT, but discard its STDERR:

	use IPC::Open3;
	use File::Spec;
	use Symbol qw(gensym);
	open(NULL, ">", File::Spec->devnull);
	my $pid = open3(gensym, \*PH, ">&NULL", "cmd");
	while( <PH> ) { }
	waitpid($pid, 0);

To capture a program's STDERR, but discard its STDOUT:

	use IPC::Open3;
	use File::Spec;
	use Symbol qw(gensym);
	open(NULL, ">", File::Spec->devnull);
	my $pid = open3(gensym, ">&NULL", \*PH, "cmd");
	while( <PH> ) { }
	waitpid($pid, 0);

To capture a program's STDERR, and let its STDOUT go to our own STDERR:

	use IPC::Open3;
	use Symbol qw(gensym);
	my $pid = open3(gensym, ">&STDERR", \*PH, "cmd");
	while( <PH> ) { }
	waitpid($pid, 0);

To read both a command's STDOUT and its STDERR separately, you can
redirect them to temp files, let the command run, then read the temp
files:

	use IPC::Open3;
	use Symbol qw(gensym);
	use IO::File;
	local *CATCHOUT = IO::File->new_tmpfile;
	local *CATCHERR = IO::File->new_tmpfile;
	my $pid = open3(gensym, ">&CATCHOUT", ">&CATCHERR", "cmd");
	waitpid($pid, 0);
	seek $_, 0, 0 for \*CATCHOUT, \*CATCHERR;
	while( <CATCHOUT> ) {}
	while( <CATCHERR> ) {}

But there's no real need for *both* to be tempfiles... the following
should work just as well, without deadlocking:

	use IPC::Open3;
	use Symbol qw(gensym);
	use IO::File;
	local *CATCHERR = IO::File->new_tmpfile;
	my $pid = open3(gensym, \*CATCHOUT, ">&CATCHERR", "cmd");
	while( <CATCHOUT> ) {}
	waitpid($pid, 0);
	seek CATCHERR, 0, 0;
	while( <CATCHERR> ) {}

And it'll be faster, too, since we can begin processing the program's
stdout immediately, rather than waiting for the program to finish.

With any of these, you can change file descriptors before the call:

	open(STDOUT, ">logfile");
	system("ls");

or you can use Bourne shell file-descriptor redirection:

	$output = `$cmd 2>some_file`;
	open (PIPE, "cmd 2>some_file |");

You can also use file-descriptor redirection to make STDERR a
duplicate of STDOUT:

	$output = `$cmd 2>&1`;
	open (PIPE, "cmd 2>&1 |");

Note that you I<cannot> simply open STDERR to be a dup of STDOUT
in your Perl program and avoid calling the shell to do the redirection.
This doesn't work:

	open(STDERR, ">&STDOUT");
	$alloutput = `cmd args`;  # stderr still escapes

This fails because the open() makes STDERR go to where STDOUT was
going at the time of the open().  The backticks then make STDOUT go to
a string, but don't change STDERR (which still goes to the old
STDOUT).

Note that you I<must> use Bourne shell (sh(1)) redirection syntax in
backticks, not csh(1)!  Details on why Perl's system() and backtick
and pipe opens all use the Bourne shell are in the
F<versus/csh.whynot> article in the "Far More Than You Ever Wanted To
Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz .  To
capture a command's STDERR and STDOUT together:

	$output = `cmd 2>&1`;                       # either with backticks
	$pid = open(PH, "cmd 2>&1 |");              # or with an open pipe
	while (<PH>) { }                            #    plus a read

To capture a command's STDOUT but discard its STDERR:

	$output = `cmd 2>/dev/null`;                # either with backticks
	$pid = open(PH, "cmd 2>/dev/null |");       # or with an open pipe
	while (<PH>) { }                            #    plus a read

To capture a command's STDERR but discard its STDOUT:

	$output = `cmd 2>&1 1>/dev/null`;           # either with backticks
	$pid = open(PH, "cmd 2>&1 1>/dev/null |");  # or with an open pipe
	while (<PH>) { }                            #    plus a read

To exchange a command's STDOUT and STDERR in order to capture the STDERR
but leave its STDOUT to come out our old STDERR:

	$output = `cmd 3>&1 1>&2 2>&3 3>&-`;        # either with backticks
	$pid = open(PH, "cmd 3>&1 1>&2 2>&3 3>&-|");# or with an open pipe
	while (<PH>) { }                            #    plus a read

To read both a command's STDOUT and its STDERR separately, it's easiest
to redirect them separately to files, and then read from those files
when the program is done:

	system("program args 1>program.stdout 2>program.stderr");

Ordering is important in all these examples.  That's because the shell
processes file descriptor redirections in strictly left to right order.

	system("prog args 1>tmpfile 2>&1");
	system("prog args 2>&1 1>tmpfile");

The first command sends both standard out and standard error to the
temporary file.  The second command sends only the old standard output
there, and the old standard error shows up on the old standard out.

=head2 Why doesn't open() return an error when a pipe open fails?

If the second argument to a piped open() contains shell
metacharacters, perl fork()s, then exec()s a shell to decode the
metacharacters and eventually run the desired program.  If the program
couldn't be run, it's the shell that gets the message, not Perl. All
your Perl program can find out is whether the shell itself could be
successfully started.  You can still capture the shell's STDERR and
check it for error messages.  See L<"How can I capture STDERR from an
external command?"> elsewhere in this document, or use the
IPC::Open3 module.

If there are no shell metacharacters in the argument of open(), Perl
runs the command directly, without using the shell, and can correctly
report whether the command started.

=head2 What's wrong with using backticks in a void context?

Strictly speaking, nothing.  Stylistically speaking, it's not a good
way to write maintainable code.  Perl has several operators for
running external commands.  Backticks are one; they collect the output
from the command for use in your program.  The C<system> function is
another; it doesn't do this.

Writing backticks in your program sends a clear message to the readers
of your code that you wanted to collect the output of the command.
Why send a clear message that isn't true?

Consider this line:

	`cat /etc/termcap`;

You forgot to check C<$?> to see whether the program even ran
correctly.  Even if you wrote

	print `cat /etc/termcap`;

this code could and probably should be written as

	system("cat /etc/termcap") == 0
	or die "cat program failed!";

which will echo the cat command's output as it is generated, instead
of waiting until the program has completed to print it out. It also
checks the return value.

C<system> also provides direct control over whether shell wildcard
processing may take place, whereas backticks do not.

=head2 How can I call backticks without shell processing?

This is a bit tricky.  You can't simply write the command
like this:

	@ok = `grep @opts '$search_string' @filenames`;

As of Perl 5.8.0, you can use C<open()> with multiple arguments.
Just like the list forms of C<system()> and C<exec()>, no shell
escapes happen.

	open( GREP, "-|", 'grep', @opts, $search_string, @filenames );
	chomp(@ok = <GREP>);
	close GREP;

You can also:

	my @ok = ();
	if (open(GREP, "-|")) {
		while (<GREP>) {
			chomp;
			push(@ok, $_);
		}
		close GREP;
	} else {
		exec 'grep', @opts, $search_string, @filenames;
	}

Just as with C<system()>, no shell escapes happen when you C<exec()> a
list. Further examples of this can be found in L<perlipc/"Safe Pipe
Opens">.

Note that if you're using Windows, no solution to this vexing issue is
even possible.  Even if Perl were to emulate C<fork()>, you'd still be
stuck, because Windows does not have an argc/argv-style API.

=head2 Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)?

Some stdio's set error and eof flags that need clearing.  The
POSIX module defines clearerr() that you can use.  That is the
technically correct way to do it.  Here are some less reliable
workarounds:

=over 4

=item 1

Try keeping around the seekpointer and go there, like this:

	$where = tell(LOG);
	seek(LOG, $where, 0);

=item 2

If that doesn't work, try seeking to a different part of the file and
then back.

=item 3

If that doesn't work, try seeking to a different part of
the file, reading something, and then seeking back.

=item 4

If that doesn't work, give up on your stdio package and use sysread.

=back

=head2 How can I convert my shell script to perl?

Learn Perl and rewrite it.  Seriously, there's no simple converter.
Things that are awkward to do in the shell are easy to do in Perl, and
this very awkwardness is what would make a shell->perl converter
nigh-on impossible to write.  By rewriting it, you'll think about what
you're really trying to do, and hopefully will escape the shell's
pipeline datastream paradigm, which while convenient for some matters,
causes many inefficiencies.

=head2 Can I use perl to run a telnet or ftp session?

Try the Net::FTP, TCP::Client, and Net::Telnet modules (available from
CPAN).  http://www.cpan.org/scripts/netstuff/telnet.emul.shar
will also help for emulating the telnet protocol, but Net::Telnet is
quite probably easier to use..

If all you want to do is pretend to be telnet but don't need
the initial telnet handshaking, then the standard dual-process
approach will suffice:

	use IO::Socket;             # new in 5.004
	$handle = IO::Socket::INET->new('www.perl.com:80')
	    or die "can't connect to port 80 on www.perl.com: $!";
	$handle->autoflush(1);
	if (fork()) {               # XXX: undef means failure
	    select($handle);
	    print while <STDIN>;    # everything from stdin to socket
	} else {
	    print while <$handle>;  # everything from socket to stdout
	}