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<html><head><title>Sending and Receiving Signals (Learning Perl, 3rd Edition)</title><link rel="stylesheet" type="text/css" href="../style/style1.css" /><meta name="DC.Creator" content="Randal L. Schwartz and Tom Phoenix" /><meta name="DC.Format" content="text/xml" scheme="MIME" /><meta name="DC.Language" content="en-US" /><meta name="DC.Publisher" content="O'Reilly & Associates, Inc." /><meta name="DC.Source" scheme="ISBN" content="0596001320L" /><meta name="DC.Subject.Keyword" content="stuff" /><meta name="DC.Title" content="Learning Perl, 3rd Edition" /><meta name="DC.Type" content="Text.Monograph" /></head><body bgcolor="#ffffff"><img alt="Book Home" border="0" src="gifs/smbanner.gif" usemap="#banner-map" /><map name="banner-map"><area shape="rect" coords="1,-2,616,66" href="index.htm" alt="Learning Perl, 3rd Edition" /><area shape="rect" coords="629,-11,726,25" href="jobjects/fsearch.htm" alt="Search this book" /></map><div class="navbar"><table width="684" border="0"><tr><td align="left" valign="top" width="228"><a href="ch14_06.htm"><img alt="Previous" border="0" src="../gifs/txtpreva.gif" /></a></td><td align="center" valign="top" width="228"><a href="index.htm"></a></td><td align="right" valign="top" width="228"><a href="ch14_08.htm"><img alt="Next" border="0" src="../gifs/txtnexta.gif" /></a></td></tr></table></div><h2 class="sect1">14.7. Sending and Receiving Signals</h2><p><a name="INDEX-988" /> <a name="INDEX-989" />A Unix signal is a tinymessage sent to a process. It can't say much; it's like acar horn honking -- does that honk you hear mean "lookout -- the bridge collapsed" or "the light haschanged -- get going" or "stopdriving -- you've got a baby on the roof" or"hello, world"? Well, fortunately, Unix signals are alittle easier to interpret than that, because there's adifferent one for each of these situations.<a href="#FOOTNOTE-325">[325]</a></p><blockquote class="footnote"> <a name="FOOTNOTE-325" /><p>[325]Well, notexactly these situations, but analogous Unix-like ones. For these,the signals are <tt class="literal">SIGHUP</tt>,<tt class="literal">SIGCONT</tt>, <tt class="literal">SIGINT</tt>, and the fake<tt class="literal">SIGZERO</tt> (signal number zero).</p> </blockquote><p>Different signals are identified by a name (such as<tt class="literal">SIGINT</tt><a name="INDEX-990" />,meaning "interrupt signal") and a corresponding smallinteger (in the range from 1 to 16, 1 to 32, or 1 to 63, depending onyour Unix flavor). Signals are typically sent when a significantevent happens, such as pressing the interrupt character (typicallyControl-C) on the terminal, which sends a <tt class="literal">SIGINT</tt>to all the processes attached to that terminal.<a href="#FOOTNOTE-326">[326]</a> Some signals are sent automatically by thesystem, but they can also come from another process.</p><blockquote class="footnote"> <a name="FOOTNOTE-326" /><p>[326]Andyou thought that pressing Control-C stopped your program. Actually,it simply sends the <tt class="literal">SIGINT</tt> signal, and that stopsthe program by default. As we'll see later in this chapter, youcan make a program that does something different when<tt class="literal">SIGINT</tt> comes in, rather than stopping atonce.</p> </blockquote><p>You can send signals from your Perl process to another process, butyou have to know the target's process ID number. How to figurethat out is a bit complicated,<a href="#FOOTNOTE-327">[327]</a> but let's sayyou know that you want to send a <tt class="literal">SIGINT</tt> to process4201. That's easy enough:</p><blockquote class="footnote"> <a name="FOOTNOTE-327" /><p>[327]Usually you have theprocess ID because it's a child process you produced with<tt class="literal">fork</tt>, or you found it in a file or from anexternal program. Using an external program can be difficult andproblematic, which is why many long-running programs save their owncurrent process ID into a file, usually described in theprogram's documentation.</p> </blockquote><blockquote><pre class="code">kill 2, 4201 or die "Cannot signal 4201 with SIGINT: $!";</pre></blockquote><p>It's named "kill" because one of the primarypurposes of signals is to stop a process that's gone on longenough. You can also use the string <tt class="literal">'INT'</tt> in placeof the <tt class="literal">2</tt> there, because signal number<tt class="literal">2</tt> is <tt class="literal">SIGINT</tt>. If the process nolonger exists,<a href="#FOOTNOTE-328">[328]</a>you'll get a false return value, so you can also use thistechnique to see whether a process is still alive. A special signalnumber of <tt class="literal">0</tt> says "just check to see whetherI <em class="emphasis">could</em> send a signal if I wanted to, but Idon't want to, so don't actually send anything." Soa process probe might look like:</p><blockquote class="footnote"> <a name="FOOTNOTE-328" /><p>[328]Sending a signal will also fail ifyou're not the superuser and it's someone else'sprocess. It would be rude to send <tt class="literal">SIGINT</tt> tosomeone else's programs, anyway.</p> </blockquote><blockquote><pre class="code">unless (kill 0, $pid) { warn "$pid has gone away!";}</pre></blockquote><p>Perhaps a little more interesting than sending signals is catchingsignals. Why might you want to do this? Well, suppose you have aprogram that creates files in<em class="filename"> /tmp</em>, and younormally delete those files at the end of the program. If someonepresses Control-C during the execution, that leaves trash in<em class="filename">/tmp</em>, a very unpolite thing to do. To fix this,create a signal handler that takes care of the cleanup:</p><blockquote><pre class="code">my $temp_directory = "/tmp/myprog.$$"; # create files below heremkdir $temp_directory, 0700 or die "Cannot create $temp_directory: $!";sub clean_up { unlink glob "$temp_directory/*"; rmdir $temp_directory;}sub my_int_handler { &clean_up; die "interrupted, exiting...\n";}$SIG{'INT'} = 'my_int_handler';.. # Time passes, the program runs, creates some temporary. # files in the temp directory, maybe somone presses Control-C.# Now it's the end of normal execution&clean_up;</pre></blockquote><p>The assignment into the special <tt class="literal">%SIG</tt> hashactivates the handler (until revoked). The key is the name of thesignal (without the constant SIG prefix), and the value is astring<a href="#FOOTNOTE-329">[329]</a> naming thesubroutine, without the ampersand. From then on, if a<tt class="literal">SIGINT</tt> comes along, Perl stops whatever it'sdoing and jumps immediately to the subroutine. Our subroutine cleansup the <a name="INDEX-991" />temp files and then exits. (And if nobodypresses Control-C, we'll still call<tt class="literal">&clean_up</tt> at the end of normal execution.)</p><blockquote class="footnote"> <a name="FOOTNOTE-329" /><p>[329]The value can also be a subroutine reference,but we're not doing those here.</p> </blockquote><p>If the subroutine returns rather than exiting, execution resumesright where it was interrupted. This can be useful if the interruptneeds to actually interrupt something rather than causing it to stop.For example, suppose processing each line of a file takes a fewseconds, which is pretty slow, and you want to abort the overallprocessing when an interrupt is processed, but not in the middle ofprocessing a line. Just set a flag in the interrupt procedure, andcheck it at the end of each line's processing:</p><blockquote><pre class="code">my $int_count;sub my_int_handler { $int_count++ }$SIG{'INT'} = 'my_int_handler';...$int_count = 0;while (<SOMEFILE>) { ... some processing that takes a few seconds ... if ($int_count) { # interrupt was seen! print "[processing interrupted...]\n"; last; }}</pre></blockquote><p>Now as each line is processed, the value of<tt class="literal">$int_count</tt> will be <tt class="literal">0</tt> if no onehas pressed Control-C, and so the loop continues to the next item.However, if an interrupt comes in, the interrupt handler incrementsthe <tt class="literal">$int_count</tt> flag, breaking out of the loop whenchecked at the end.</p><p>So, you can either set a flag or break out of the program, and thatcovers most of what you'll need from catching signals. Thecurrent implementation of signal handlers is not entirely withoutfaults,<a href="#FOOTNOTE-330">[330]</a> however, sokeep the stuff you're doing in there to an absolute minimum, oryour program may end up blowing up sometime when you least expectit.<a name="INDEX-992" /><a name="INDEX-993" /></p><blockquote class="footnote"> <a name="FOOTNOTE-330" /><p>[330]This is one of the top items on the Perldevelopers' list of things to be fixed, so we expect reliablesignal handling to be one of the first items on the new feature listfor Perl 6. The problem is that a signal may come in at any time,even when Perl isn't ready for one. If Perl is (for example) inthe middle of allocating some memory when a signal comes in, thesignal handler can accidentally try to allocate some memoryand -- your program is dead. You can't control when yourPerl code will allocate memory, but XSUB code (usually written in C)can safely handle signals. See the Perl documentation for moreinformation about this advanced topic.</p> </blockquote><hr width="684" align="left" /><div class="navbar"><table width="684" border="0"><tr><td align="left" valign="top" width="228"><a href="ch14_06.htm"><img alt="Previous" border="0" src="../gifs/txtpreva.gif" /></a></td><td align="center" valign="top" width="228"><a href="index.htm"><img alt="Home" border="0" src="../gifs/txthome.gif" /></a></td><td align="right" valign="top" width="228"><a href="ch14_08.htm"><img alt="Next" border="0" src="../gifs/txtnexta.gif" /></a></td></tr><tr><td align="left" valign="top" width="228">14.6. Getting Down and Dirty with Fork</td><td align="center" valign="top" width="228"><a href="index/index.htm"><img alt="Book Index" border="0" src="../gifs/index.gif" /></a></td><td align="right" valign="top" width="228">14.8. Exercises</td></tr></table></div><hr width="684" align="left" /><img alt="Library Navigation Links" border="0" src="../gifs/navbar.gif" usemap="#library-map" /><p><p><font size="-1"><a href="copyrght.htm">Copyright © 2002</a> O'Reilly & Associates. All rights reserved.</font></p><map name="library-map"><area shape="rect" coords="1,0,85,94" href="../index.htm"><area shape="rect" coords="86,1,178,103" href="../lwp/index.htm"><area shape="rect" coords="180,0,265,103" href="../lperl/index.htm"><area shape="rect" coords="267,0,353,105" href="../perlnut/index.htm"><area shape="rect" coords="354,1,446,115" href="../prog/index.htm"><area shape="rect" coords="448,0,526,132" href="../tk/index.htm"><area shape="rect" coords="528,1,615,119" href="../cookbook/index.htm"><area shape="rect" coords="617,0,690,135" href="../pxml/index.htm"></map></body></html>⌨️ 快捷键说明
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