=head1 NAME
-perlfaq8 - System Interaction ($Revision: 1.25 $, $Date: 1998/07/05 15:07:20 $)
+perlfaq8 - System Interaction ($Revision: 1.39 $, $Date: 1999/05/23 18:37:57 $)
=head1 DESCRIPTION
=head2 How do I find out which operating system I'm running under?
-The $^O variable ($OSNAME if you use English) contains the operating
-system that your perl binary was built for.
+The $^O variable ($OSNAME if you use English) contains an indication of
+the name of the operating system (not its release number) that your perl
+binary was built for.
=head2 How come exec() doesn't return?
=head2 How do I read just one key without waiting for a return key?
Controlling input buffering is a remarkably system-dependent matter.
-If most systems, you can just use the B<stty> command as shown in
+On many systems, you can just use the B<stty> command as shown in
L<perlfunc/getc>, but as you see, that's already getting you into
portability snags.
=head2 How do I clear the screen?
-If you only have to so infrequently, use C<system>:
+If you only have do so infrequently, use C<system>:
system("clear");
}
}
-
=head2 How do I decode encrypted password files?
You spend lots and lots of money on dedicated hardware, but this is
theory provide (read-only) access to entries in the shadow password
file. To change the file, make a new shadow password file (the format
varies from system to system - see L<passwd(5)> for specifics) and use
-pwd_mkdb(8) to install it (see L<pwd_mkdb(5)> for more details).
+pwd_mkdb(8) to install it (see L<pwd_mkdb(8)> for more details).
=head2 How do I set the time and date?
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 your 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:
+If your 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:
require 'sys/syscall.ph';
$done = $start = pack($TIMEVAL_T, ());
- syscall( &SYS_gettimeofday, $start, 0)) != -1
+ syscall(&SYS_gettimeofday, $start, 0) != -1
or die "gettimeofday: $!";
##########################
=head2 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 (such as 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()
+Because the pipe open takes place in two steps: first Perl calls
+fork() to start a new process, then this new process calls exec() to
+run the program you really wanted to open. The first step reports
+success or failure to your process, so open() can only tell you
+whether the fork() succeeded or not.
+
+To find out if the exec() step 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 L<perlipc>.
+In some cases, even this won't work. 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. Now when you call wait(), you only learn whether or not the
+I<shell> could be successfully started. Best to avoid shell
+metacharacters.
+
On systems that follow the spawn() paradigm, open() I<might> do what
-you expect - unless perl uses a shell to start your command. In this
+you expect--unless perl uses a shell to start your command. In this
case the fork()/exec() description still applies.
=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 because backticks have a (potentially
-humungous) return value, and you're ignoring it. It's may also not be very
+humongous) 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:
system("cat /etc/termcap") == 0
or die "cat program failed!";
-Which will get the output quickly (as its generated, instead of only
+Which will get the output quickly (as it is generated, instead of only
at the end) and also check the return value.
system() also provides direct control over whether shell wildcard
}
Just as with system(), no shell escapes happen when you exec() a list.
+Further examples of this can be found in L<perlipc/"Safe Pipe Opens">.
-There are more examples of this L<perlipc/"Safe Pipe Opens">.
+Note that if you're stuck on Microsoft, no solution to this vexing issue
+is even possible. Even if Perl were to emulate fork(), you'd still
+be hosed, because Microsoft gives no argc/argv-style API. Their API
+always reparses from a single string, which is fundamentally wrong,
+but you're not likely to get the Gods of Redmond to acknowledge this
+and fix it for you.
=head2 Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)?
=item *
-Open /dev/tty and use the the TIOCNOTTY ioctl on it. See L<tty(4)>
+Open /dev/tty and use the TIOCNOTTY ioctl on it. See L<tty(4)>
for details. Or better yet, you can just use the POSIX::setsid()
function, so you don't have to worry about process groups.
=back
+The Proc::Daemon module, available from CPAN, provides a function to
+perform these actions for you.
+
=head2 How do I make my program run with sh and csh?
See the F<eg/nih> script (part of the perl source distribution).
use POSIX qw/getpgrp tcgetpgrp/;
open(TTY, "/dev/tty") or die $!;
- $tpgrp = tcgetpgrp(TTY);
+ $tpgrp = tcgetpgrp(fileno(*TTY));
$pgrp = getpgrp();
if ($tpgrp == $pgrp) {
print "foreground\n";
=head2 How do I timeout a slow event?
Use the alarm() function, probably in conjunction with a signal
-handler, as documented L<perlipc/"Signals"> and chapter 6 of the
+handler, as documented in L<perlipc/"Signals"> and chapter 6 of the
Camel. You may instead use the more flexible Sys::AlarmCall module
available from CPAN.
sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
or die "can't open /tmp/somefile: $!":
-=head2 How do I install a CPAN module?
+
+
+
+=head2 How do I install a module from CPAN?
-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:
+The easiest way is to have a module also named CPAN 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:
=over 4
another. Here are the deltas between the various inclusion constructs:
1) do $file is like eval `cat $file`, except the former:
- 1.1: searches @INC.
+ 1.1: searches @INC and updates %INC.
1.2: bequeaths an *unrelated* lexical scope on the eval'ed code.
2) require $file is like do $file, except the former:
use lib '/u/mydir/perl';
+This is almost the same as:
+
+ BEGIN {
+ unshift(@INC, '/u/mydir/perl');
+ }
+
+except that the lib module checks for machine-dependent subdirectories.
See Perl's L<lib> for more information.
=head2 How do I add the directory my program lives in to the module/library search path?
the PERLLIB environment variable
the PERL5LIB environment variable
- the perl -Idir commpand line flag
+ the perl -Idir command line flag
the use lib pragma, as in
use lib "$ENV{HOME}/myown_perllib";
dependent architectures. The lib.pm pragmatic module was first
included with the 5.002 release of Perl.
+=head2 What is socket.ph and where do I get it?
+
+It's a perl4-style file defining values for system networking
+constants. Sometimes it is built using h2ph when Perl is installed,
+but other times it is not. Modern programs C<use Socket;> instead.
+
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997, 1998 Tom Christiansen and Nathan Torkington.
+Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
All rights reserved.
When included as part of the Standard Version of Perl, or as part of
projects / p5sagit/p5-mst-13.2.git / blobdiff