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<html><head><title>System V IPC (Programming Perl)</title><!-- STYLESHEET --><link rel="stylesheet" type="text/css" href="../style/style1.css"><!-- METADATA --><!--Dublin Core Metadata--><meta name="DC.Creator" content=""><meta name="DC.Date" content=""><meta name="DC.Format" content="text/xml" scheme="MIME"><meta name="DC.Generator" content="XSLT stylesheet, xt by James Clark"><meta name="DC.Identifier" content=""><meta name="DC.Language" content="en-US"><meta name="DC.Publisher" content="O'Reilly & Associates, Inc."><meta name="DC.Source" content="" scheme="ISBN"><meta name="DC.Subject.Keyword" content=""><meta name="DC.Title" content="System V IPC"><meta name="DC.Type" content="Text.Monograph"></head><body><!-- START OF BODY --><!-- TOP BANNER --><img src="gifs/smbanner.gif" usemap="#banner-map" border="0" alt="Book Home"><map name="banner-map"><AREA SHAPE="RECT" COORDS="0,0,466,71" HREF="index.htm" ALT="Programming Perl"><AREA SHAPE="RECT" COORDS="467,0,514,18" HREF="jobjects/fsearch.htm" ALT="Search this book"></map><!-- TOP NAV BAR --><div class="navbar"><table width="515" border="0"><tr><td align="left" valign="top" width="172"><a href="ch16_03.htm"><img src="../gifs/txtpreva.gif" alt="Previous" border="0"></a></td><td align="center" valign="top" width="171"><a href="ch16_01.htm">Chapter 16: Interprocess Communication</a></td><td align="right" valign="top" width="172"><a href="ch16_05.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0"></a></td></tr></table></div><hr width="515" align="left"><!-- SECTION BODY --><h2 class="sect1">16.4. System V IPC</h2><p><a name="INDEX-3048"></a><a name="INDEX-3049"></a>Everyone hates System V IPC. It's slower than paper tape, carvesout insidious little namespaces completely unrelated to the filesystem, uses human-hostilenumbers to name its objects, and is constantly losing track of its ownmind. Every so often, your sysadmin has to go on a search-and-destroymission to hunt down these lost SysV IPC objects with<em class="emphasis">ipcs</em>(1) and kill them with<em class="emphasis">ipcrm</em>(1), hopefully before the system runsout of memory.</p><p><a name="INDEX-3050"></a><a name="INDEX-3051"></a><a name="INDEX-3052"></a><a name="INDEX-3053"></a><a name="INDEX-3054"></a>Despite all this pain, ancient SysV IPC still has a few valid uses.The three kinds of IPC objects are shared memory, semaphores, andmessages. For message passing, sockets are the preferred mechanismsthese days, and they're a lot more portable, too. For simple uses ofsemaphores, the filesystem tends to get used. As for sharedmemory--well, now there's a problem for you. If you have it, the moremodern <em class="emphasis">mmap</em>(2) syscall fits the bill,<a href="#FOOTNOTE-11">[11]</a> but thequality of theimplementation varies from system to system. It also requires a bit ofcare to avoid letting Perl reallocate your strings from where<em class="emphasis">mmap</em>(2) put them. But when programmers look into using <em class="emphasis">mmap</em>(2),they hear these incoherent mumbles from the resident wizards about howit suffers from dodgy cache coherency issues on systems withoutsomething called a "unified buffer cache"--or maybe it was a"flycatcher unibus"--and, figuring the devil they know is better thanthe one they don't, run quickly back to the SysV IPC they know and hatefor all their shared memory needs.</p><blockquote class="footnote"><a name="FOOTNOTE-11"></a><p>[11]There's even an <tt class="literal">Mmap</tt> module on CPAN.</p></blockquote><p><a name="INDEX-3055"></a><a name="INDEX-3056"></a>Here's a little program that demonstrates controlled access to ashared memory buffer by a brood of sibling processes. SysV IPCobjects can also be shared among <em class="emphasis">unrelated</em> processes on the samecomputer, but then you have to figure out how they're goingto find each other. To mediate safe access, we'll create a semaphoreper piece.<a href="#FOOTNOTE-12">[12]</a><a name="INDEX-3057"></a><a name="INDEX-3058"></a></p><blockquote class="footnote"><a name="FOOTNOTE-12"></a><p>[12]It would be more realistic to create a pair of semaphoresfor each bit of shared memory, one for reading and the other forwriting, and in fact, that's what the <tt class="literal">IPC::Shareable</tt> module on CPAN does.But we're trying to keep things simple here. It's worth admitting,though, that with a couple of semaphores, you could then make useof pretty much the only redeeming feature of SysV IPC: you couldperform atomic operations on entire sets of semaphores as one unit,which is occasionally useful.</p></blockquote><p>Every time you want to get or put a new value into the shared memory,you have to go through the semaphore first. This can get prettytedious, so we'll wrap access in an object class. <tt class="literal">IPC::Shareable</tt>goes one step further, wrapping its object class in a<tt class="literal">tie</tt> interface.</p><p>This program runs until you interrupt it with a Control-C or equivalent:<blockquote><pre class="programlisting">#!/usr/bin/perl -wuse v5.6.0; # or betteruse strict;use sigtrap qw(die INT TERM HUP QUIT);my $PROGENY = shift(@ARGV) || 3;eval { main() }; # see DESTROY below for whydie if $@ && $@ !~ /^Caught a SIG/;print "\nDone.\n";exit;sub main { my $mem = ShMem->alloc("Original Creation at " . localtime); my(@kids, $child); $SIG{CHLD} = 'IGNORE'; for (my $unborn = $PROGENY; $unborn > 0; $unborn--) { if ($child = fork) { print "$$ begat $child\n"; next; } die "cannot fork: $!" unless defined $child; eval { while (1) { $mem->lock(); $mem->poke("$$ " . localtime) unless $mem->peek =~ /^$$\b/o; $mem->unlock(); } }; die if $@ && $@ !~ /^Caught a SIG/; exit; # child death } while (1) { print "Buffer is ", $mem->get, "\n"; sleep 1; }}</pre></blockquote><a name="INDEX-3059"></a>And here's the <tt class="literal">ShMem</tt> package,which that program uses. You can just tack it on to the end of theprogram, or put it in its own file (with a "<tt class="literal">1;</tt>" atthe end) and <tt class="literal">require</tt> it from the main program.(The two IPC modules it uses in turn are found in the standard Perldistribution.)<blockquote><pre class="programlisting">package ShMem;use IPC::SysV qw(IPC_PRIVATE IPC_RMID IPC_CREAT S_IRWXU);use IPC::Semaphore;sub MAXBUF() { 2000 }sub alloc { # constructor method my $class = shift; my $value = @_ ? shift : ''; my $key = shmget(IPC_PRIVATE, MAXBUF, S_IRWXU) or die "shmget: $!"; my $sem = IPC::Semaphore->new(IPC_PRIVATE, 1, S_IRWXU | IPC_CREAT) or die "IPC::Semaphore->new: $!"; $sem->setval(0,1) or die "sem setval: $!"; my $self = bless { OWNER => $$, SHMKEY => $key, SEMA => $sem, } => $class; $self->put($value); return $self;}</pre></blockquote>Now for the fetch and store methods. The <tt class="literal">get</tt> and<tt class="literal">put</tt> methods lock the buffer, but<tt class="literal">peek</tt> and <tt class="literal">poke</tt> don't, so thelatter two should be used only while the object is manuallylocked--which you have to do when you want to retrieve an old valueand store back a modified version, all under the same lock. The demoprogram does this in its <tt class="literal">while (1)</tt> loop. Theentire transaction must occur under the same lock, or the testing andsetting wouldn't be atomic and might bomb.<blockquote><pre class="programlisting">sub get { my $self = shift; $self->lock; my $value = $self->peek(@_); $self->unlock; return $value;}sub peek { my $self = shift; shmread($self->{SHMKEY}, my $buff='', 0, MAXBUF) or die "shmread: $!"; substr($buff, index($buff, "\0")) = ''; return $buff;}sub put { my $self = shift; $self->lock; $self->poke(@_); $self->unlock;}sub poke { my($self,$msg) = @_; shmwrite($self->{SHMKEY}, $msg, 0, MAXBUF) or die "shmwrite: $!";}sub lock { my $self = shift; $self->{SEMA}->op(0,-1,0) or die "semop: $!";}sub unlock { my $self = shift; $self->{SEMA}->op(0,1,0) or die "semop: $!";}</pre></blockquote><a name="INDEX-3060"></a>Finally, the class needs a destructor so that when the object goesaway, we can manually deallocate the shared memory and the semaphorestored inside the object. Otherwise, they'll outlive their creator,and you'll have to resort to <em class="emphasis">ipcs</em> and <em class="emphasis">ipcrm</em> (or a sysadmin) toget rid of them. That's why we went through the elaborate wrappers inthe main program to convert signals into exceptions: itthat all destructors get run, SysV IPC objects get deallocated, andsysadmins get off our case.<blockquote><pre class="programlisting">sub DESTROY { my $self = shift; return unless $self->{OWNER} == $$; # avoid dup dealloc shmctl($self->{SHMKEY}, IPC_RMID, 0) or warn "shmctl RMID: $!"; $self->{SEMA}->remove() or warn "sema->remove: $!";}</pre></blockquote></p><a name="INDEX-3061"></a><a name="INDEX-3062"></a><!-- BOTTOM NAV BAR --><hr width="515" align="left"><div class="navbar"><table width="515" border="0"><tr><td align="left" valign="top" width="172"><a href="ch16_03.htm"><img src="../gifs/txtpreva.gif" alt="Previous" border="0"></a></td><td align="center" valign="top" width="171"><a href="index.htm"><img src="../gifs/txthome.gif" alt="Home" border="0"></a></td><td align="right" valign="top" width="172"><a href="ch16_05.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0"></a></td></tr><tr><td align="left" valign="top" width="172">16.3. Pipes</td><td align="center" valign="top" width="171"><a href="index/index.htm"><img src="../gifs/index.gif" alt="Book Index" border="0"></a></td><td align="right" valign="top" width="172">16.5. Sockets</td></tr></table></div><hr width="515" align="left"><!-- LIBRARY NAV BAR --><img src="../gifs/smnavbar.gif" usemap="#library-map" border="0" alt="Library Navigation Links"><p><font size="-1"><a href="copyrght.htm">Copyright © 2001</a> O'Reilly & Associates. All rights reserved.</font></p><map name="library-map"> <area shape="rect" coords="2,-1,79,99" href="../index.htm"><area shape="rect" coords="84,1,157,108" href="../perlnut/index.htm"><area shape="rect" coords="162,2,248,125" href="../prog/index.htm"><area shape="rect" coords="253,2,326,130" href="../advprog/index.htm"><area shape="rect" coords="332,1,407,112" href="../cookbook/index.htm"><area shape="rect" coords="414,2,523,103" href="../sysadmin/index.htm"></map><!-- END OF BODY --></body></html>⌨️ 快捷键说明
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