@node perlfaq8, perlfaq9, perlfaq7, perlfaq
@unnumberedsec NAME
perlfaq8 - System Interaction ($Revision: 1.21 $, $Date: 1997/04/24 22:44:19 $)

@unnumberedsec DESCRIPTION
This section of the Perl FAQ covers questions involving operating
system interaction.  This involves interprocess communication (IPC),
control over the user-interface (keyboard, screen and pointing
devices), and most anything else not related to data manipulation.

Read the FAQs and documentation specific to the port of perl to your
operating system (eg, @samp{perlvms} in this node, @samp{perlplan9} in this node, ...).  These should
contain more detailed information on the vagaries of your perl.

@unnumberedsubsec How do I find out which operating system I@'m running under?
The $^O variable ($OSTYPE if you use English) contains the operating
system that your perl binary was built for.

@unnumberedsubsec How come exec() doesn@'t return?
Because that@'s what it does: it replaces your currently running
program with a different one.  If you want to keep going (as is
probably the case if you@'re asking this question) use system()
instead.

@unnumberedsubsec How do I do fancy stuff with the keyboard/screen/mouse?
How you access/control keyboards, screens, and pointing devices
("mice") is system-dependent.  Try the following modules:

@table @asis
@item Keyboard

@example
Term::Cap               Standard perl distribution
Term::ReadKey           CPAN
Term::ReadLine::Gnu     CPAN
Term::ReadLine::Perl    CPAN
Term::Screen            CPAN
@end example

@itemx Screen

@example
Term::Cap               Standard perl distribution
Curses                  CPAN
Term::ANSIColor         CPAN
@end example

@itemx Mouse

@example
Tk                      CPAN
@end example

@end table
@unnumberedsubsec How do I ask the user for a password?
(This question has nothing to do with the web.  See a different
FAQ for that.)

There@'s an example of this in @samp{crypt}, @xref{perlfunc,Perlfunc},).  First, you put
the terminal into "no echo" mode, then just read the password
normally.  You may do this with an old-style ioctl() function, POSIX
terminal control (see @samp{POSIX} in this node, and Chapter 7 of the Camel), or a call
to the @strong{stty} program, with varying degrees of portability.

You can also do this for most systems using the Term::ReadKey module
from CPAN, which is easier to use and in theory more portable.

@unnumberedsubsec How do I read and write the serial port?
This depends on which operating system your program is running on.  In
the case of Unix, the serial ports will be accessible through files in
/dev; on other systems, the devices names will doubtless differ.
Several problem areas common to all device interaction are the
following

@table @asis
@item lockfiles
Your system may use lockfiles to control multiple access.  Make sure
you follow the correct protocol.  Unpredictable behaviour can result
from multiple processes reading from one device.

@item open mode
If you expect to use both read and write operations on the device,
you@'ll have to open it for update (see @samp{"open"}, @xref{perlfunc,Perlfunc}, for
details).  You may wish to open it without running the risk of
blocking by using sysopen() and @code{O_RDWR|O_NDELAY|O_NOCTTY} from the
Fcntl module (part of the standard perl distribution).  See
@samp{"sysopen"}, @xref{perlfunc,Perlfunc}, for more on this approach.

@item end of line
Some devices will be expecting a "\r" at the end of each line rather
than a "\n".  In some ports of perl, "\r" and "\n" are different from
their usual (Unix) ASCII values of "\012" and "\015".  You may have to
give the numeric values you want directly, using octal ("\015"), hex
("0x0D"), or as a control-character specification ("\cM").


@example
print DEV "atv1\012";   # wrong, for some devices
print DEV "atv1\015";   # right, for some devices
@end example

Even though with normal text files, a "\n" will do the trick, there is
still no unified scheme for terminating a line that is portable
between Unix, DOS/Win, and Macintosh, except to terminate @emph{ALL} line
ends with "\015\012", and strip what you don@'t need from the output.
This applies especially to socket I/O and autoflushing, discussed
next.

@item flushing output
If you expect characters to get to your device when you print() them,
you@'ll want to autoflush that filehandle, as in the older


@example
use FileHandle;
DEV->autoflush(1);
@end example

and the newer


@example
use IO::Handle;
DEV->autoflush(1);
@end example

You can use select() and the $| variable to control autoflushing
(see @samp{$|}, @xref{perlvar,Perlvar}, and @samp{select}, @xref{perlfunc,Perlfunc},):


@example
$oldh = select(DEV);
$| = 1;
select($oldh);
@end example

You@'ll also see code that does this without a temporary variable, as in


@example
select((select(DEV), $| = 1)[0]);
@end example

As mentioned in the previous item, this still doesn@'t work when using
socket I/O between Unix and Macintosh.  You@'ll need to hardcode your
line terminators, in that case.

@item non-blocking input
If you are doing a blocking read() or sysread(), you@'ll have to
arrange for an alarm handler to provide a timeout (see
@samp{alarm}, @xref{perlfunc,Perlfunc},).  If you have a non-blocking open, you@'ll likely
have a non-blocking read, which means you may have to use a 4-arg
select() to determine whether I/O is ready on that device (see
@samp{"select"}, @xref{perlfunc,Perlfunc}.

@end table
@unnumberedsubsec How do I decode encrypted password files?
You spend lots and lots of money on dedicated hardware, but this is
bound to get you talked about.

Seriously, you can@'t if they are Unix password files - the Unix
password system employs one-way encryption.  Programs like Crack can
forcibly (and intelligently) try to guess passwords, but don@'t (can@'t)
guarantee quick success.

If you@'re worried about users selecting bad passwords, you should
proactively check when they try to change their password (by modifying
passwd(1), for example).

@unnumberedsubsec How do I start a process in the background?
You could use


@example
system("cmd &")
@end example

or you could use fork as documented in @samp{"fork"}, @xref{perlfunc,Perlfunc}, with
further examples in @samp{"fork"}, @xref{perlipc,Perlipc}.  Some things to be aware of, if you@'re
on a Unix-like system:

@table @asis
@item STDIN, STDOUT and STDERR are shared
Both the main process and the backgrounded one (the "child" process)
share the same STDIN, STDOUT and STDERR filehandles.  If both try to
access them at once, strange things can happen.  You may want to close
or reopen these for the child.  You can get around this with
opening a pipe (see @samp{"open"}, @xref{perlfunc,Perlfunc},) but on some systems this
means that the child process cannot outlive the parent.

@item Signals
You@'ll have to catch the SIGCHLD signal, and possibly SIGPIPE too.
SIGCHLD is sent when the backgrounded process finishes.  SIGPIPE is
sent when you write to a filehandle whose child process has closed (an
untrapped SIGPIPE can cause your program to silently die).  This is
not an issue with @code{system("cmd&")}.

@item Zombies
You have to be prepared to "reap" the child process when it finishes


@example
$SIG@{CHLD@} = sub @{ wait @};
@end example

See @samp{"Signals"}, @xref{perlipc,Perlipc}, for other examples of code to do this.
Zombies are not an issue with @code{system("prog &")}.

@end table
@unnumberedsubsec How do I trap control characters/signals?
You don@'t actually "trap" a control character.  Instead, that
character generates a signal, which you then trap.  Signals are
documented in @samp{"Signals"}, @xref{perlipc,Perlipc}, and chapter 6 of the Camel.

Be warned that very few C libraries are re-entrant.  Therefore, if you
attempt to print() in a handler that got invoked during another stdio
operation your internal structures will likely be in an
inconsistent state, and your program will dump core.  You can
sometimes avoid this by using syswrite() instead of print().

Unless you@'re exceedingly careful, the only safe things to do inside a
signal handler are: set a variable and exit.  And in the first case,
you should only set a variable in such a way that malloc() is not
called (eg, by setting a variable that already has a value).

For example:


@example
$Interrupted = 0;       # to ensure it has a value
$SIG@{INT@} = sub @{
    $Interrupted++;
        syswrite(STDERR, "ouch\n", 5);
@}
@end example

However, because syscalls restart by default, you@'ll find that if
you@'re in a "slow" call, such as <FH>, read(), connect(), or
wait(), that the only way to terminate them is by "longjumping" out;
that is, by raising an exception.  See the time-out handler for a
blocking flock() in @samp{"Signals"}, @xref{perlipc,Perlipc}, or chapter 6 of the Camel.

@unnumberedsubsec How do I modify the shadow password file on a Unix system?
If perl was installed correctly, the getpw*() functions described in
@xref{perlfunc,Perlfunc}, provide (read-only) access to the shadow password file.
To change the file, make a new shadow password file (the format varies
from system to system - see @samp{passwd(5)} in this node for specifics) and use
pwd_mkdb(8) to install it (see @samp{pwd_mkdb(5)} in this node for more details).

@unnumberedsubsec 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 @code{set time}.

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


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

@unnumberedsubsec How can I sleep() or alarm() for under a second?
If you want finer granularity than the 1 second that the sleep()
function provides, the easiest way is to use the select() function as
documented in @samp{"select"}, @xref{perlfunc,Perlfunc}.  If your system has itimers and
syscall() support, you can check out the old example in
http://www.perl.com/CPAN/doc/misc/ancient/tutorial/eg/itimers.pl .

@unnumberedsubsec How can I measure time under a second?
In general, you may not be able to.  The Time::HiRes module (available
from CPAN) provides this functionality for some systems.

In general, you may not be able to.  But if you system supports both the
syscall() function in Perl as well as a system call like gettimeofday(2),
then you may be able to do something like this:


@example
require @'sys/syscall.ph@';

$TIMEVAL_T = "LL";

$done = $start = pack($TIMEVAL_T, ());

syscall( &SYS_gettimeofday, $start, 0)) != -1
           or die "gettimeofday: $!";

##########################
# DO YOUR OPERATION HERE #
##########################

syscall( &SYS_gettimeofday, $done, 0) != -1
       or die "gettimeofday: $!";

@@start = unpack($TIMEVAL_T, $start);
@@done  = unpack($TIMEVAL_T, $done);

# fix microseconds
for ($done[1], $start[1]) @{ $_ /= 1_000_000 @}

$delta_time = sprintf "%.4f", ($done[0]  + $done[1]  )
                                        -
                             ($start[0] + $start[1] );
@end example

@unnumberedsubsec 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 @xref{perlmod,Perlmod}, manpage for more details).  It isn@'t
called when untrapped signals kill the program, though, so if you use
END blocks you should also use


@example
use sigtrap qw(die normal-signals);
@end example

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 @samp{"Signals"}, @xref{perlipc,Perlipc}, and chapter 6 of the Camel.

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.

@unnumberedsubsec 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.

@unnumberedsubsec 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
@xref{perlfunc,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.

@unnumberedsubsec 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 @file{errno.h}, @file{syscall.h}, and @file{socket.h} were fine,
but the hard ones like @file{ioctl.h} nearly always need to hand-edited.
Here@'s how to install the *.ph files:


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

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 @xref{perlxstut,Perlxstut}, for how to get started with h2xs.

If your system doesn@'t support dynamic loading, you still probably
ought to use h2xs.  See @xref{perlxstut,Perlxstut}, and @samp{ExtUtils::MakeMaker} in this node for
more information (in brief, just use @strong{make perl} instead of a plain
make to rebuild perl with a new static extension).

@unnumberedsubsec 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 @xref{perlsec,Perlsec},) to work around such systems.

@unnumberedsubsec 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 @samp{IPC::Open2} in this node).

@unnumberedsubsec 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 8 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.


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

@unnumberedsubsec How can I capture STDERR from an external command?
There are three basic ways of running external commands:


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

With system(), both STDOUT and STDERR will go the same place as the
script@'s versions of these, unless the command redirects them.
Backticks and open() read @strong{only} the STDOUT of your command.

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


@example
open(STDOUT, ">logfile");
system("ls");
@end example

or you can use Bourne shell file-descriptor redirection:


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

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


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

Note that you @emph{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:


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

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 @emph{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
http://www.perl.com/CPAN/doc/FMTEYEWTK/versus/csh.whynot .

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 @samp{IPC::Open3} in this node).

@unnumberedsubsec Why doesn@'t open() return an error when a pipe open fails?
It does, but probably not how you expect it to.  On systems that
follow the standard fork()/exec() paradigm (eg, Unix), it works like
this: open() causes a fork().  In the parent, open() returns with the
process ID of the child.  The child exec()s the command to be piped
to/from.  The parent can@'t know whether the exec() was successful or
not - all it can return is whether the fork() succeeded or not.  To
find out if the command succeeded, you have to catch SIGCHLD and
wait() to get the exit status.  You should also catch SIGPIPE if
you@'re writing to the child -- you may not have found out the exec()
failed by the time you write.  This is documented in @xref{perlipc,Perlipc}.

On systems that follow the spawn() paradigm, open() @emph{might} do what
you expect - unless perl uses a shell to start your command. In this
case the fork()/exec() description still applies.

@unnumberedsubsec 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 because backticks have a (potentially
humungous) return value, and you@'re ignoring it.  It@'s may also not be very
efficient, because you have to read in all the lines of output, allocate
memory for them, and then throw it away.  Too often people are lulled
to writing:


@example
@`cp file file.bak@`;
@end example

And now they think "Hey, I@'ll just always use backticks to run programs."
Bad idea: backticks are for capturing a program@'s output; the system()
function is for running programs.

Consider this line:


@example
@`cat /etc/termcap@`;
@end example

You haven@'t assigned the output anywhere, so it just wastes memory
(for a little while).  Plus you forgot to check $? to see whether
the program even ran correctly.  Even if you wrote


@example
print @`cat /etc/termcap@`;
@end example

In most cases, this could and probably should be written as


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

Which will get the output quickly (as its generated, instead of only
at the end ) and also check the return value.

system() also provides direct control over whether shell wildcard
processing may take place, whereas backticks do not.

@unnumberedsubsec How can I call backticks without shell processing?
This is a bit tricky.  Instead of writing


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

You have to do this:


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

Just as with system(), no shell escapes happen when you exec() a list.

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

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


@example
$where = tell(LOG);
seek(LOG, $where, 0);
@end example

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

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

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

@end enumerate
@unnumberedsubsec 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.

@unnumberedsubsec 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.perl.com/CPAN/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:


@example
use IO::Socket;                 # new in 5.004
$handle = IO::Socket::INET->new(@'www.perl.com:80@')
            || 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
@}
close $handle;
exit;
@end example

@unnumberedsubsec How can I write expect in Perl?
Once upon a time, there was a library called chat2.pl (part of the
standard perl distribution), which never really got finished.  These
days, your best bet is to look at the Comm.pl library available from
CPAN.

@unnumberedsubsec Is there a way to hide perl@'s command line from programs such as "ps"?
First of all note that if you@'re doing this for security reasons (to
avoid people seeing passwords, for example) then you should rewrite
your program so that critical information is never given as an
argument.  Hiding the arguments won@'t make your program completely
secure.

To actually alter the visible command line, you can assign to the
variable $0 as documented in @xref{perlvar,Perlvar}.  This won@'t work on all
operating systems, though.  Daemon programs like sendmail place their
state there, as in:


@example
$0 = "orcus [accepting connections]";
@end example

@unnumberedsubsec I @{changed directory, modified my environment@} in a perl script.  How come the change disappeared when I exited the script?  How do I get my changes to be visible?
@table @asis
@item Unix
In the strictest sense, it can@'t be done -- the script executes as a
different process from the shell it was started from.  Changes to a
process are not reflected in its parent, only in its own children
created after the change.  There is shell magic that may allow you to
fake it by eval()ing the script@'s output in your shell; check out the
comp.unix.questions FAQ for details.

@item VMS
Change to %ENV persist after Perl exits, but directory changes do not.

@end table
@unnumberedsubsec How do I close a process@'s filehandle without waiting for it to complete?
Assuming your system supports such things, just send an appropriate signal
to the process (see @samp{"kill"}, @xref{perlfunc,Perlfunc}.  It@'s common to first send a TERM
signal, wait a little bit, and then send a KILL signal to finish it off.

@unnumberedsubsec How do I fork a daemon process?
If by daemon process you mean one that@'s detached (disassociated from
its tty), then the following process is reported to work on most
Unixish systems.  Non-Unix users should check their Your_OS::Process
module for other solutions.

@itemize @bullet
@item 
Open /dev/tty and use the the TIOCNOTTY ioctl on it.  See @samp{tty(4)} in this node
for details.

@item 
Change directory to /

@item 
Reopen STDIN, STDOUT, and STDERR so they@'re not connected to the old
tty.

@item 
Background yourself like this:


@example
fork && exit;
@end example

@end itemize
@unnumberedsubsec How do I make my program run with sh and csh?
See the @file{eg/nih} script (part of the perl source distribution).

@unnumberedsubsec How do I find out if I@'m running interactively or not?
Good question.  Sometimes @code{-t STDIN} and @code{-t STDOUT} can give clues,
sometimes not.


@example
if (-t STDIN && -t STDOUT) @{
        print "Now what? ";
@}
@end example

On POSIX systems, you can test whether your own process group matches
the current process group of your controlling terminal as follows:


@example
use POSIX qw/getpgrp tcgetpgrp/;
open(TTY, "/dev/tty") or die $!;
$tpgrp = tcgetpgrp(TTY);
$pgrp = getpgrp();
if ($tpgrp == $pgrp) @{
    print "foreground\n";
@} else @{
    print "background\n";
@}
@end example

@unnumberedsubsec How do I timeout a slow event?
Use the alarm() function, probably in conjunction with a signal
handler, as documented @samp{"Signals"}, @xref{perlipc,Perlipc}, and chapter 6 of the
Camel.  You may instead use the more flexible Sys::AlarmCall module
available from CPAN.

@unnumberedsubsec How do I set CPU limits?
Use the BSD::Resource module from CPAN.

@unnumberedsubsec How do I avoid zombies on a Unix system?
Use the reaper code from @samp{"Signals"}, @xref{perlipc,Perlipc}, to call wait() when a
SIGCHLD is received, or else use the double-fork technique described
in @samp{fork}, @xref{perlfunc,Perlfunc}.

@unnumberedsubsec How do I use an SQL database?
There are a number of excellent interfaces to SQL databases.  See the
DBD::* modules available from
http://www.perl.com/CPAN/modules/dbperl/DBD .

@unnumberedsubsec How do I make a system() exit on control-C?
You can@'t.  You need to imitate the system() call (see @xref{perlipc,Perlipc}, for
sample code) and then have a signal handler for the INT signal that
passes the signal on to the subprocess.

@unnumberedsubsec How do I open a file without blocking?
If you@'re lucky enough to be using a system that supports
non-blocking reads (most Unixish systems do), you need only to use the
O_NDELAY or O_NONBLOCK flag from the Fcntl module in conjunction with
sysopen():


@example
use Fcntl;
sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
    or die "can@'t open /tmp/somefile: $!":
@end example

@unnumberedsubsec How do I install a CPAN module?
The easiest way is to have the CPAN module do it for you.  This module
comes with perl version 5.004 and later.  To manually install the CPAN
module, or any well-behaved CPAN module for that matter, follow these
steps:

@enumerate
@item 
Unpack the source into a temporary area.

@item 

@example
perl Makefile.PL
@end example

@item 

@example
make
@end example

@item 

@example
make test
@end example

@item 

@example
make install
@end example

@end enumerate
If your version of perl is compiled without dynamic loading, then you
just need to replace step 3 (make) with @strong{make perl} and you will
get a new @file{perl} binary with your extension linked in.

See @samp{ExtUtils::MakeMaker} in this node for more details on building extensions,
the question "How do I keep my own module/library directory?"

@unnumberedsubsec How do I keep my own module/library directory?
When you build modules, use the PREFIX option when generating
Makefiles:


@example
perl Makefile.PL PREFIX=/u/mydir/perl
@end example

then either set the PERL5LIB environment variable before you run
scripts that use the modules/libraries (see @xref{perlrun,Perlrun},) or say


@example
use lib @'/u/mydir/perl@';
@end example

See Perl@'s @samp{lib} in this node for more information.

@unnumberedsubsec How do I add the directory my program lives in to the module/library search path?

@example
use FindBin;
use lib "$FindBin:Bin";
use your_own_modules;
@end example

@unnumberedsubsec How do I add a directory to my include path at runtime?
Here are the suggested ways of modifying your include path:


@example
the PERLLIB environment variable
the PERL5LIB environment variable
the perl -Idir commpand line flag
the use lib pragma, as in
    use lib "$ENV@{HOME@}/myown_perllib";
@end example

The latter is particularly useful because it knows about machine
dependent architectures.  The lib.pm pragmatic module was first
included with the 5.002 release of Perl.

@unnumberedsec How do I get one key from the terminal at a time, under POSIX?

@example
#!/usr/bin/perl -w
use strict;
$| = 1;
for (1..4) @{
    my $got;
    print "gimme: ";
    $got = getone();
    print "--> $got\n";
@}
exit;

BEGIN @{
    use POSIX qw(:termios_h);

my ($term, $oterm, $echo, $noecho, $fd_stdin);

$fd_stdin = fileno(STDIN);

$term     = POSIX::Termios->new();
$term->getattr($fd_stdin);
$oterm     = $term->getlflag();

$echo     = ECHO | ECHOK | ICANON;
$noecho   = $oterm & ~$echo;

sub cbreak @{
    $term->setlflag($noecho);
    $term->setcc(VTIME, 1);
    $term->setattr($fd_stdin, TCSANOW);
@}

sub cooked @{
    $term->setlflag($oterm);
    $term->setcc(VTIME, 0);
    $term->setattr($fd_stdin, TCSANOW);
@}

sub getone @{
    my $key = @'@';
    cbreak();
    sysread(STDIN, $key, 1);
    cooked();
    return $key;
@}

@}
END @{ cooked() @}
@end example

@unnumberedsec AUTHOR AND COPYRIGHT
Copyright (c) 1997 Tom Christiansen and Nathan Torkington.
All rights reserved.  See @xref{perlfaq,Perlfaq}, for distribution information.
    END-of-perlfaq8.pod
echo x - perlfaq9.pod
sed @'s/^X//@' >perlfaq9.pod << @'END-of-perlfaq9.pod@'
=head1 NAME

perlfaq9 - Networking ($Revision: 1.17 $, $Date: 1997/04/24 22:44:29 $)

@unnumberedsec DESCRIPTION
This section deals with questions related to networking, the internet,
and a few on the web.

@unnumberedsubsec My CGI script runs from the command line but not the browser.  Can you help me fix it?
Sure, but you probably can@'t afford our contracting rates :-)

Seriously, if you can demonstrate that you@'ve read the following FAQs
and that your problem isn@'t something simple that can be easily
answered, you@'ll probably receive a courteous and useful reply to your
question if you post it on comp.infosystems.www.authoring.cgi (if it@'s
something to do with HTTP, HTML, or the CGI protocols).  Questions that
appear to be Perl questions but are really CGI ones that are posted to
comp.lang.perl.misc may not be so well received.

The useful FAQs are:


@example
http://www.perl.com/perl/faq/idiots-guide.html
http://www3.pair.com/webthing/docs/cgi/faqs/cgifaq.shtml
http://www.perl.com/perl/faq/perl-cgi-faq.html
http://www-genome.wi.mit.edu/WWW/faqs/www-security-faq.html
http://www.boutell.com/faq/
@end example

@unnumberedsubsec How do I remove HTML from a string?
The most correct way (albeit not the fastest) is to use HTML::Parse
from CPAN (part of the libwww-perl distribution, which is a must-have
module for all web hackers).

Many folks attempt a simple-minded regular expression approach, like
C<s/<.*?>//g>, but that fails in many cases because the tags
may continue over line breaks, they may contain quoted angle-brackets,
or HTML comment may be present.  Plus folks forget to convert
entities, like @code{&lt;} for example.

Here@'s one "simple-minded" approach, that works for most files:


@example
#!/usr/bin/perl -p0777
s/<(?:[^>@'"]*|([@'"]).*?\1)*>//gs
@end example

If you want a more complete solution, see the 3-stage striphtml
program in
http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/striphtml.gz
.

@unnumberedsubsec How do I extract URLs?
A quick but imperfect approach is


@example
#!/usr/bin/perl -n00
# qxurl - tchrist@@perl.com
print "$2\n" while m@{
        < \s*
          A \s+ HREF \s* = \s* (["@']) (.*?) \1
        \s* >
@}gsix;
@end example

This version does not adjust relative URLs, understand alternate
bases, deal with HTML comments, deal with HREF and NAME attributes in
the same tag, or accept URLs themselves as arguments.  It also runs
about 100x faster than a more "complete" solution using the LWP suite
of modules, such as the
http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/xurl.gz
program.

@unnumberedsubsec How do I download a file from the user@'s machine?  How do I open a file on another machine?
In the context of an HTML form, you can use what@'s known as
@strong{multipart/form-data} encoding.  The CGI.pm module (available from
CPAN) supports this in the start_multipart_form() method, which isn@'t
the same as the startform() method.

@unnumberedsubsec How do I make a pop-up menu in HTML?
Use the B<<SELECT>> and B<<OPTION>> tags.  The CGI.pm
module (available from CPAN) supports this widget, as well as many
others, including some that it cleverly synthesizes on its own.

@unnumberedsubsec How do I fetch an HTML file?
One approach, if you have the lynx text-based HTML browser installed
on your system, is this:


@example
$html_code = @`lynx -source $url@`;
$text_data = @`lynx -dump $url@`;
@end example

The libwww-perl (LWP) modules from CPAN provide a more powerful way to
do this.  They work through proxies, and don@'t require lynx:


@example
# print HTML from a URL
use LWP::Simple;
getprint "http://www.sn.no/libwww-perl/";

# print ASCII from HTML from a URL
use LWP::Simple;
use HTML::Parse;
use HTML::FormatText;
my ($html, $ascii);
$html = get("http://www.perl.com/");
defined $html
    or die "Can@'t fetch HTML from http://www.perl.com/";
$ascii = HTML::FormatText->new->format(parse_html($html));
print $ascii;
@end example

@unnumberedsubsec how do I decode or create those %-encodings on the web?
Here@'s an example of decoding:


@example
$string = "http://altavista.digital.com/cgi-bin/query?pg=q&what=news&fmt=.&q=%2Bcgi-bin+%2Bperl.exe";
$string =~ s/%([a-fA-F0-9]@{2@})/chr(hex($1))/ge;
@end example

Encoding is a bit harder, because you can@'t just blindly change
all the non-alphanumunder character (@code{\W}) into their hex escapes.
It@'s important that characters with special meaning like @code{/} and @code{?}
@emph{not} be translated.  Probably the easiest way to get this right is
to avoid reinventing the wheel and just use the URI::Escape module,
which is part of the libwww-perl package (LWP) available from CPAN.

@unnumberedsubsec How do I redirect to another page?
Instead of sending back a @code{Content-Type} as the headers of your
reply, send back a @code{Location:} header.  Officially this should be a
@code{URI:} header, so the CGI.pm module (available from CPAN) sends back
both:


@example
Location: http://www.domain.com/newpage
URI: http://www.domain.com/newpage
@end example

Note that relative URLs in these headers can cause strange effects
because of "optimizations" that servers do.

@unnumberedsubsec How do I put a password on my web pages?
That depends.  You@'ll need to read the documentation for your web
server, or perhaps check some of the other FAQs referenced above.

@unnumberedsubsec How do I edit my .htpasswd and .htgroup files with Perl?
The HTTPD::UserAdmin and HTTPD::GroupAdmin modules provide a
consistent OO interface to these files, regardless of how they@'re
stored.  Databases may be text, dbm, Berkley DB or any database with a
DBI compatible driver.  HTTPD::UserAdmin supports files used by the
@`Basic@' and @`Digest@' authentication schemes.  Here@'s an example:


@example
use HTTPD::UserAdmin ();
HTTPD::UserAdmin
          ->new(DB => "/foo/.htpasswd")
          ->add($username => $password);
@end example

@unnumberedsubsec How do I make sure users can@'t enter values into a form that cause my CGI script to do bad things?
Read the CGI security FAQ, at
http://www-genome.wi.mit.edu/WWW/faqs/www-security-faq.html, and the
Perl/CGI FAQ at
http://www.perl.com/CPAN/doc/FAQs/cgi/perl-cgi-faq.html.

In brief: use tainting (see @xref{perlsec,Perlsec},), which makes sure that data
from outside your script (eg, CGI parameters) are never used in
eval or system calls.  In addition to tainting, never use the
single-argument form of system() or exec().  Instead, supply the
command and arguments as a list, which prevents shell globbing.

@unnumberedsubsec How do I parse an email header?
For a quick-and-dirty solution, try this solution derived
from page 222 of the 2nd edition of "Programming Perl":


@example
$/ = @'@';
$header = <MSG>;
$header =~ s/\n\s+/ /g;  # merge continuation lines
%head = ( UNIX_FROM_LINE, split /^([-\w]+):\s*/m, $header );
@end example

That solution doesn@'t do well if, for example, you@'re trying to
maintain all the Received lines.  A more complete approach is to use
the Mail::Header module from CPAN (part of the MailTools package).

@unnumberedsubsec How do I decode a CGI form?
A lot of people are tempted to code this up themselves, so you@'ve
probably all seen a lot of code involving @code{$ENV@{CONTENT_LENGTH@}} and
@code{$ENV@{QUERY_STRING@}}.  It@'s true that this can work, but there are
also a lot of versions of this floating around that are quite simply
broken!

Please do not be tempted to reinvent the wheel.  Instead, use the
CGI.pm or CGI_Lite.pm (available from CPAN), or if you@'re trapped in
the module-free land of perl1 .. perl4, you might look into cgi-lib.pl
(available from http://www.bio.cam.ac.uk/web/form.html).

@unnumberedsubsec How do I check a valid email address?
You can@'t.

Without sending mail to the address and seeing whether it bounces (and
even then you face the halting problem), you cannot determine whether
an email address is valid.  Even if you apply the email header
standard, you can have problems, because there are deliverable
addresses that aren@'t RFC-822 (the mail header standard) compliant,
and addresses that aren@'t deliverable which are compliant.

Many are tempted to try to eliminate many frequently-invalid email
addresses with a simple regexp, such as
@code{/^[\w.-]+\@@([\w.-]\.)+\w+$/}.  However, this also throws out many
valid ones, and says nothing about potential deliverability, so is not
suggested.  Instead, see
http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/ckaddr.gz ,
which actually checks against the full RFC spec (except for nested
comments), looks for addresses you may not wish to accept email to
(say, Bill Clinton or your postmaster), and then makes sure that the
hostname given can be looked up in DNS.  It@'s not fast, but it works.

Here@'s an alternative strategy used by many CGI script authors: Check
the email address with a simple regexp (such as the one above).  If
the regexp matched the address, accept the address.  If the regexp
didn@'t match the address, request confirmation from the user that the
email address they entered was correct.

@unnumberedsubsec How do I decode a MIME/BASE64 string?
The MIME-tools package (available from CPAN) handles this and a lot
more.  Decoding BASE64 becomes as simple as:


@example
use MIME::base64;
$decoded = decode_base64($encoded);
@end example

A more direct approach is to use the unpack() function@'s "u"
format after minor transliterations:


@example
tr#A-Za-z0-9+/##cd;                   # remove non-base64 chars
tr#A-Za-z0-9+/# -_#;                  # convert to uuencoded format
$len = pack("c", 32 + 0.75*length);   # compute length byte
print unpack("u", $len . $_);         # uudecode and print
@end example

@unnumberedsubsec How do I return the user@'s email address?
On systems that support getpwuid, the $< variable and the
Sys::Hostname module (which is part of the standard perl distribution),
you can probably try using something like this:


@example
use Sys::Hostname;
$address = sprintf(@'%s@@%s@', getpwuid($<), hostname);
@end example

Company policies on email address can mean that this generates addresses
that the company@'s email system will not accept, so you should ask for
users@' email addresses when this matters.  Furthermore, not all systems
on which Perl runs are so forthcoming with this information as is Unix.

The Mail::Util module from CPAN (part of the MailTools package) provides a
mailaddress() function that tries to guess the mail address of the user.
It makes a more intelligent guess than the code above, using information
given when the module was installed, but it could still be incorrect.
Again, the best way is often just to ask the user.

@unnumberedsubsec How do I send/read mail?
Sending mail: the Mail::Mailer module from CPAN (part of the MailTools
package) is UNIX-centric, while Mail::Internet uses Net::SMTP which is
not UNIX-centric.  Reading mail: use the Mail::Folder module from CPAN
(part of the MailFolder package) or the Mail::Internet module from
CPAN (also part of the MailTools package).


@example
# sending mail
 use Mail::Internet;
 use Mail::Header;
 # say which mail host to use
 $ENV@{SMTPHOSTS@} = @'mail.frii.com@';
 # create headers
 $header = new Mail::Header;
 $header->add(@'From@', @'gnat@@frii.com@');
 $header->add(@'Subject@', @'Testing@');
 $header->add(@'To@', @'gnat@@frii.com@');
 # create body
 $body = @'This is a test, ignore@';
 # create mail object
 $mail = new Mail::Internet(undef, Header => $header, Body => \[$body]);
 # send it
 $mail->smtpsend or die;
@end example

@unnumberedsubsec How do I find out my hostname/domainname/IP address?
A lot of code has historically cavalierly called the @code{@`hostname@`}
program.  While sometimes expedient, this isn@'t very portable.  It@'s
one of those tradeoffs of convenience versus portability.

The Sys::Hostname module (part of the standard perl distribution) will
give you the hostname after which you can find out the IP address
(assuming you have working DNS) with a gethostbyname() call.


@example
use Socket;
use Sys::Hostname;
my $host = hostname();
my $addr = inet_ntoa(scalar(gethostbyname($name)) || @'localhost@');
@end example

Probably the simplest way to learn your DNS domain name is to grok
it out of /etc/resolv.conf, at least under Unix.  Of course, this
assumes several things about your resolv.conf configuration, including
that it exists.

(We still need a good DNS domain name-learning method for non-Unix
systems.)

@unnumberedsubsec How do I fetch a news article or the active newsgroups?
Use the Net::NNTP or News::NNTPClient modules, both available from CPAN.
This can make tasks like fetching the newsgroup list as simple as:


@example
perl -MNews::NNTPClient
  -e @'print News::NNTPClient->new->list("newsgroups")@'
@end example

@unnumberedsubsec How do I fetch/put an FTP file?
LWP::Simple (available from CPAN) can fetch but not put.  Net::FTP (also
available from CPAN) is more complex but can put as well as fetch.

@unnumberedsubsec How can I do RPC in Perl?
A DCE::RPC module is being developed (but is not yet available), and
will be released as part of the DCE-Perl package (available from
CPAN).  No ONC::RPC module is known.

@unnumberedsec AUTHOR AND COPYRIGHT
Copyright (c) 1997 Tom Christiansen and Nathan Torkington.
All rights reserved.  See @xref{perlfaq,Perlfaq}, for distribution information.

