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<html><head><title>Processes as Filehandles (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_04.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_06.htm"><img alt="Next" border="0" src="../gifs/txtnexta.gif" /></a></td></tr></table></div><h2 class="sect1">14.5. Processes as Filehandles</h2><p><a name="INDEX-977" /> <a name="INDEX-978" />So far, we've been looking at ways todeal with synchronous processes, where Perl stays in charge, launchesa command, (usually) waits for it to finish, then possibly grabs itsoutput. But Perl can also launch a child process that stays alive,communicating<a href="#FOOTNOTE-322">[322]</a> to Perl on an ongoing basis until the task is complete.</p><blockquote class="footnote"> <a name="FOOTNOTE-322" /><p>[322]Via pipes, or whatever your operatingsystem provides for simple interprocess communication.</p></blockquote><p>The syntax for launching a concurrent (parallel) child process is toput the command as the "filename" for an<tt class="literal">open</tt> call, and either precede the command orfollow the command with a vertical bar, which is the"pipe" character. For that reason, this is often called a<a name="INDEX-979" /><a name="INDEX-980" /><em class="firstterm">piped open</em>:</p><blockquote><pre class="code">open DATE, "date|" or die "cannot pipe from date: $!";open MAIL, "|mail merlyn" or die "cannot pipe to mail: $!";</pre></blockquote><p>In the first example, with the vertical bar on the right, the commandis launched with its standard output connected to the<tt class="literal">DATE</tt> filehandle opened for reading, similar to theway that the command <i class="command">date | your_program</i> wouldwork from the shell. In the second example, with the vertical bar onthe left, the command's standard input is connected to the<tt class="literal">MAIL</tt> filehandle opened for writing, similar towhat happens with the command <i class="command">your_program | mailmerlyn</i>. In either case, the command is now launched andcontinues independently of the Perl process.<a href="#FOOTNOTE-323">[323]</a></p><blockquote class="footnote"> <a name="FOOTNOTE-323" /><p>[323]If thePerl process exits before the command is complete, a commandthat's been reading will see end-of-file, while a commandthat's been writing will get a "broken pipe" errorsignal on the next write, by default.</p> </blockquote><p>The open fails if the child process cannot be created. If the commanditself does not exist or exits erroneously, this will (generally) notbe seen as an error when opening, but as an error when closing.We'll get to that in a moment.</p><p>For all intents and purposes, the rest of the program doesn'tknow, doesn't care, and would have to work pretty hard tofigure out that this is a filehandle opened on a process rather thanon a file. So, to get data from a filehandle opened for reading,we'll just do the normal read:</p><blockquote><pre class="code">my $now = <DATE>;</pre></blockquote><p>And to send data to the mail process (waiting for the body of amessage to deliver to <em class="emphasis">merlyn</em> on standard input),a simple print-with-a-filehandle will do:</p><blockquote><pre class="code">print MAIL "The time is now $now"; # presume $now ends in newline</pre></blockquote><p>In short, you can pretend that these filehandles are hooked up tomagical files, one that contains the output of the<i class="command">date</i> command, and one that will automatically bemailed by the <i class="command">mail</i> command.</p><p>If a process is connected to a filehandle that is open for reading,and then exits, the filehandle returns end-of-file, just like readingup to the end of a normal file. When you close a filehandle open forwriting to a process, the process will see end-of-file. So, to finishsending the email, close the handle:</p><blockquote><pre class="code">close MAIL;die "mail: non-zero exit of $?" if $?;</pre></blockquote><p>Closing a filehandle attached to a process waits for the process tocomplete, so that Perl can get the process's exit status. Theexit status is then available in the <tt class="literal">$?</tt> variable(reminiscent of the same variable in the Bourne Shell), and is thesame kind of number as the value returned by the<tt class="literal">system</tt> function: zero for success, nonzero forfailure. Each new exited process overwrites the previous valuethough, so save it quickly if you want it. (The <tt class="literal">$?</tt>variable also holds the exit status of the most recent<tt class="literal">system</tt> or backquoted command, if you'recurious.)</p><p>The processes are synchronized just like a pipelined command. If youtry to read and no data is available, the process is suspended(without consuming additional CPU time) until the sending program hasstarted speaking again. Similarly, if a writing process gets ahead ofthe reading process, the writing process is slowed down until thereader starts to catch up. There's a buffer (usually 4K bytesor so) in between so they don't have to stay precisely in lockstep.</p><p>Why use processes as filehandles? Well, it's the only easy wayto write to a process based on the results of a computation. But ifyou're just reading, backquotes are often much easier tomanage, unless you want to have the results as they come in.</p><p>For example, the Unix <i class="command">find</i><a name="INDEX-981" /> command locates files based ontheir attributes, and it can take quite a while if used on a fairlylarge number of files (such as starting from the<em class="emphasis">root</em> directory). You can put a<i class="command">find</i> command inside backquotes, but it'soften nicer to see the results as they are found:</p><blockquote><pre class="code">open F, "find / -atime +90 -size +1000 -print|" or die "fork: $!";while (<F>) { chomp; printf "%s size %dK last accessed on %s\n", $_, (1023 + -s $_)/1024, -A $_;}</pre></blockquote><p>The <i class="command">find</i> command here is looking for all the filesthat were not accessed within the past 90 days and that are largerthan 1000 blocks. (These are good candidates to be moved off tolonger-term storage.) While <i class="command">find</i> is searching andsearching, Perl can wait. As each file is found, Perl responds to theincoming name and displays some information about that file forfurther research. Had this been written with backquotes, we'dnot see any output until the <i class="command">find</i> commmand hadfinished, and it's comforting to see that it's actuallydoing the job even before it's<i class="command" /><a name="INDEX-982" /> done.<a name="INDEX-983" /> <a name="INDEX-984" /></p><hr width="684" align="left" /><div class="navbar"><table width="684" border="0"><tr><td align="left" valign="top" width="228"><a href="ch14_04.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_06.htm"><img alt="Next" border="0" src="../gifs/txtnexta.gif" /></a></td></tr><tr><td align="left" valign="top" width="228">14.4. Using Backquotes to Capture Output</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">14.6. Getting Down and Dirty with Fork</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. 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