X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlfaq8.pod;h=164d23529e1d966fa9d6b10cb11d84c6f23c24b5;hb=bea985324a3cd895f8e0a0c38e90ab3d09433415;hp=1df3b6ac0ab19299b40a9faf6ee4899dc12bb8fe;hpb=5b3eff12f7c4ea0bd1324f2fe0a16edec8764c93;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perlfaq8.pod b/pod/perlfaq8.pod index 1df3b6a..164d235 100644 --- a/pod/perlfaq8.pod +++ b/pod/perlfaq8.pod @@ -1,6 +1,6 @@ =head1 NAME -perlfaq8 - System Interaction ($Revision: 1.39 $, $Date: 1999/05/23 18:37:57 $) +perlfaq8 - System Interaction ($Revision: 1.17 $, $Date: 2003/01/26 17:44:04 $) =head1 DESCRIPTION @@ -77,7 +77,7 @@ Or like this: Controlling input buffering is a remarkably system-dependent matter. On many systems, you can just use the B command as shown in L, but as you see, that's already getting you into -portability snags. +portability snags. open(TTY, "+/dev/tty 2>&1"; @@ -188,14 +188,14 @@ positions, etc, you might wish to use Term::Cap module: =head2 How do I get the screen size? -If you have Term::ReadKey module installed from CPAN, +If you have Term::ReadKey module installed from CPAN, you can use it to fetch the width and height in characters and in pixels: use Term::ReadKey; ($wchar, $hchar, $wpixels, $hpixels) = GetTerminalSize(); -This is more portable than the raw C, but not as +This is more portable than the raw C, but not as illustrative: require 'sys/ioctl.ph'; @@ -275,7 +275,7 @@ next. If you expect characters to get to your device when you print() them, you'll want to autoflush that filehandle. You can use select() -and the C<$|> variable to control autoflushing (see L +and the C<$|> variable to control autoflushing (see L> and L, or L, ``How do I flush/unbuffer an output filehandle? Why must I do this?''): @@ -294,7 +294,7 @@ of code just because you're afraid of a little $| variable: DEV->autoflush(1); 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 +socket I/O between Unix and Macintosh. You'll need to hard code your line terminators, in that case. =item non-blocking input @@ -346,7 +346,12 @@ passwd(1), for example). =head2 How do I start a process in the background? -You could use +Several modules can start other processes that do not block +your Perl program. You can use IPC::Open3, Parallel::Jobs, +IPC::Run, and some of the POE modules. See CPAN for more +details. + +You could also use system("cmd &") @@ -375,10 +380,26 @@ not an issue with C. =item Zombies -You have to be prepared to "reap" the child process when it finishes +You have to be prepared to "reap" the child process when it finishes. $SIG{CHLD} = sub { wait }; + $SIG{CHLD} = 'IGNORE'; + +You can also use a double fork. You immediately wait() for your +first child, and the init daemon will wait() for your grandchild once +it exits. + + unless ($pid = fork) { + unless (fork) { + exec "what you really wanna do"; + die "exec failed!"; + } + exit 0; + } + waitpid($pid,0); + + See L for other examples of code to do this. Zombies are not an issue with C. @@ -424,8 +445,8 @@ If perl was installed correctly and your shadow library was written properly, the getpw*() functions described in L should in 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 for specifics) and use -pwd_mkdb(8) to install it (see L for more details). +varies from system to system--see L for specifics) and use +pwd_mkdb(8) to install it (see L for more details). =head2 How do I set the time and date? @@ -435,7 +456,7 @@ 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. -However, if all you want to do is change your timezone, you can +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 @@ -447,12 +468,14 @@ probably get away with setting an environment variable: 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 L. Try the Time::HiRes and -the BSD::Itimer modules (available from CPAN). +the BSD::Itimer modules (available from CPAN, and starting from +Perl 5.8 Time::HiRes is part of the standard distribution). =head2 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. +from CPAN, and starting from Perl 5.8 part of the standard distribution) +provides this functionality for some systems. 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 @@ -488,14 +511,14 @@ something like this: 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 manpage for more details). +thread ends (see L 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 @@ -533,7 +556,10 @@ syscall(), you can use the syscall function (documented in L). Remember to check the modules that came with your distribution, and -CPAN as well--someone may already have written a module to do it. +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()? @@ -571,8 +597,8 @@ scripts inherently insecure. Perl gives you a number of options 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). See -L and +though (see L). See +L and L You may also use the IPC::Open3 module (part of the standard perl @@ -602,6 +628,68 @@ 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 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 $pid = open3(gensym, \*PH, ">&NULL", "cmd"); + while( ) { } + waitpid($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 $pid = open3(gensym, ">&NULL", \*PH, "cmd"); + while( ) { } + waitpid($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 $pid = open3(gensym, ">&STDERR", \*PH, "cmd"); + while( ) { } + waitpid($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_tempfile; + local *CATCHERR = IO::File->new_tempfile; + my $pid = open3(gensym, ">&CATCHOUT", ">&CATCHERR", "cmd"); + waitpid($pid, 0); + seek $_, 0, 0 for \*CATCHOUT, \*CATCHERR; + while( ) {} + while( ) {} + +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_tempfile; + my $pid = open3(gensym, \*CATCHOUT, ">&CATCHERR", "cmd"); + while( ) {} + waitpid($pid, 0); + seek CATCHERR, 0, 0; + while( ) {} + +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"); @@ -632,9 +720,10 @@ STDOUT). Note that you I 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 . -To capture a command's STDERR and STDOUT together: +and pipe opens all use the Bourne shell are in the +F 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 @@ -677,50 +766,38 @@ 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? -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. - -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 could be successfully started...it's best to avoid shell -metacharacters. +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. -On systems that follow the spawn() paradigm, open() I do what -you expect--unless perl uses a shell to start your command. In this -case the fork()/exec() description still applies. +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 because backticks have a (potentially -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: +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 function is +another; it doesn't do this. - `cp file file.bak`; - -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. +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 haven't assigned the output anywhere, so it just wastes memory -(for a little while). You forgot to check C<$?> to see whether -the program even ran correctly, too. Even if you wrote +You forgot to check C<$?> to see whether the program even ran +correctly. Even if you wrote print `cat /etc/termcap`; @@ -737,11 +814,20 @@ processing may take place, whereas backticks do not. =head2 How can I call backticks without shell processing? -This is a bit tricky. Instead of writing +This is a bit tricky. You can't simply write the command +like this: @ok = `grep @opts '$search_string' @filenames`; -You have to do this: +As of Perl 5.8.0, you can use open() with multiple arguments. +Just like the list forms of system() and exec(), no shell +escapes happen. + + open( GREP, "-|", 'grep', @opts, $search_string, @filenames ); + chomp(@ok = ); + close GREP; + +You can also: my @ok = (); if (open(GREP, "-|")) { @@ -757,12 +843,9 @@ You have to do this: Just as with system(), no shell escapes happen when you exec() a list. Further examples of this can be found in L. -Note that if you're stuck on Microsoft, no solution to this vexing issue +Note that if you're use 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. +be stuck, because Microsoft does not have a 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)? @@ -809,7 +892,7 @@ 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.perl.com/CPAN/scripts/netstuff/telnet.emul.shar +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.. @@ -864,7 +947,7 @@ different process from the shell it was started from. Changes to a process are not reflected in its parent--only in any 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. +comp.unix.questions FAQ for details. =back @@ -885,7 +968,7 @@ module for other solutions. =item * -Open /dev/tty and use the TIOCNOTTY ioctl on it. See L +Open /dev/tty and use the TIOCNOTTY ioctl on it. See L for details. Or better yet, you can just use the POSIX::setsid() function, so you don't have to worry about process groups. @@ -938,6 +1021,9 @@ handler, as documented in L and the section on ``Signals'' in the Camel. You may instead use the more flexible Sys::AlarmCall module available from CPAN. +The alarm() function is not implemented on all versions of Windows. +Check the documentation for your specific version of Perl. + =head2 How do I set CPU limits? Use the BSD::Resource module from CPAN. @@ -946,14 +1032,19 @@ Use the BSD::Resource module from CPAN. Use the reaper code from L to call wait() when a SIGCHLD is received, or else use the double-fork technique described -in L. +in L. =head2 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/DBD . -A lot of information on this can be found at -http://www.symbolstone.org/technology/perl/DBI/ +The DBI module provides an abstract interface to most database +servers and types, including Oracle, DB2, Sybase, mysql, Postgresql, +ODBC, and flat files. The DBI module accesses each database type +through a database driver, or DBD. You can see a complete list of +available drivers on CPAN: http://www.cpan.org/modules/by-module/DBD/ . +You can read more about DBI on http://dbi.perl.org . + +Other modules provide more specific access: Win32::ODBC, Alzabo, iodbc, +and others found on CPAN Search: http://search.cpan.org . =head2 How do I make a system() exit on control-C? @@ -962,7 +1053,7 @@ sample code) and then have a signal handler for the INT signal that passes the signal on to the subprocess. Or you can check for it: $rc = system($cmd); - if ($rc & 127) { die "signal death" } + if ($rc & 127) { die "signal death" } =head2 How do I open a file without blocking? @@ -978,9 +1069,17 @@ sysopen(): =head2 How do I install a module from CPAN? 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: +This module comes with perl version 5.004 and later. + + $ perl -MCPAN -e shell + + cpan shell -- CPAN exploration and modules installation (v1.59_54) + ReadLine support enabled + + cpan> install Some::Module + +To manually install the CPAN module, or any well-behaved CPAN module +for that matter, follow these steps: =over 4 @@ -1086,15 +1185,11 @@ but other times it is not. Modern programs C instead. =head1 AUTHOR AND COPYRIGHT -Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington. +Copyright (c) 1997-2003 Tom Christiansen and Nathan Torkington. All rights reserved. -When included as part of the Standard Version of Perl, or as part of -its complete documentation whether printed or otherwise, this work -may be distributed only under the terms of Perl's Artistic License. -Any distribution of this file or derivatives thereof I -of that package require that special arrangements be made with -copyright holder. +This documentation is free; you can redistribute it and/or modify it +under the same terms as Perl itself. Irrespective of its distribution, all code examples in this file are hereby placed into the public domain. You are permitted and