diff options
Diffstat (limited to 'pod/perlfaq8.pod')
| -rw-r--r-- | pod/perlfaq8.pod | 60 |
1 files changed, 40 insertions, 20 deletions
diff --git a/pod/perlfaq8.pod b/pod/perlfaq8.pod index 31cf9fc4b3..7b3ac3edf4 100644 --- a/pod/perlfaq8.pod +++ b/pod/perlfaq8.pod @@ -1,6 +1,6 @@ =head1 NAME -perlfaq8 - System Interaction ($Revision: 1.26 $, $Date: 1998/08/05 12:20:28 $) +perlfaq8 - System Interaction ($Revision: 1.36 $, $Date: 1999/01/08 05:36:34 $) =head1 DESCRIPTION @@ -325,7 +325,6 @@ go bump in the night, finally came up with this: } } - =head2 How do I decode encrypted password files? You spend lots and lots of money on dedicated hardware, but this is @@ -449,12 +448,12 @@ http://www.perl.com/CPAN/doc/misc/ancient/tutorial/eg/itimers.pl . =head2 How can I measure time under a second? -The Time::HiRes module (available from CPAN) provides this -functionality for some systems. +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'; @@ -462,7 +461,7 @@ then you may be able to do something like this: $done = $start = pack($TIMEVAL_T, ()); - syscall( &SYS_gettimeofday, $start, 0)) != -1 + syscall( &SYS_gettimeofday, $start, 0) != -1 or die "gettimeofday: $!"; ########################## @@ -674,19 +673,26 @@ 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? -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? @@ -908,7 +914,7 @@ the current process group of your controlling terminal as follows: 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"; @@ -1034,6 +1040,13 @@ scripts that use the modules/libraries (see L<perlrun>) or say 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? @@ -1056,9 +1069,15 @@ 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. +=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 @@ -1073,3 +1092,4 @@ are hereby placed into the public domain. You are permitted and encouraged to use this code in your own programs for fun or for profit as you see fit. A simple comment in the code giving credit would be courteous but is not required. + |