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<html><head><title>Fatal Errors with die (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="ch11_02.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="ch11_04.htm"><img alt="Next" border="0" src="../gifs/txtnexta.gif" /></a></td></tr></table></div><h2 class="sect1">11.3. Fatal Errors with die</h2><p><a name="INDEX-761" /> <a name="INDEX-762" />Let's<a name="INDEX-763" />step aside for a moment. We need some stuffthat isn't directly related to (or limited to) filehandles, butis more about getting out of a program earlier than normal.</p><p>When a fatal error happens inside Perl (for example, if you divide byzero, use an invalid regular expression, or call a subroutine thathasn't been declared) your program stops with an error messagetelling why.<a href="#FOOTNOTE-253">[253]</a>But this functionality is available to us with the<tt class="literal">die</tt> function, so we can make our own fatalerrors.</p><blockquote class="footnote"> <a name="FOOTNOTE-253" /><p>[253]Well, it does this by default, buterrors may be trapped with an <tt class="literal">eval</tt> block, aswe'll see in <a href="ch17_01.htm">Chapter 17, "Some Advanced Perl Techniques"</a>.</p> </blockquote><p>The <tt class="literal">die</tt> function prints out the<a name="INDEX-764" />message you give it (to the standarderror stream, where such messages should go) and makes sure that yourprogram exits with a nonzero exit status.</p><p>You may not have known it, but every program that runs on Unix (andmany other modern operating systems) has an <a name="INDEX-765" />exit status, telling whether it wassuccessful or not. Programs that run other programs (like the<i class="command">make</i><a name="INDEX-766" /> utility program) look at that exit statusto see that everything is running correctly. The exit status is justa single byte, so it can't say much; traditionally, it is zerofor success and a nonzero value for failure. Perhaps one means asyntax error in the command arguments, while two means that somethingwent wrong during processing and three means the configuration filecouldn't be found; the details differ from one command to thenext. But zero always means that everything worked. When the exitstatus shows failure, a<a name="INDEX-767" />program like <i class="command">make</i> knowsnot to go on to the next step.</p><p>So we could rewrite the previous example, perhaps something like this:</p><blockquote><pre class="code">unless (open LOG, ">>logfile") { die "Cannot create logfile: $!";}</pre></blockquote><p>If the <tt class="literal">open</tt> fails, <tt class="literal">die</tt> willterminate the program and tell us that it cannot create the logfile.But what's that <tt class="literal">$!</tt> in the message?That's the human-readable complaint from the system. Ingeneral, when the system refuses to do something we'verequested (like opening a file), it will give us a reason (perhaps"permission denied" or "file not found," inthis case). This is the string that you may have obtained with<tt class="literal">perror</tt> in C or a similar language. Thishuman-readable complaint message will be available in Perl'sspecial variable <tt class="literal">$!</tt>.<a href="#FOOTNOTE-254">[254]</a> It's a good idea to include<tt class="literal">$!</tt><a name="INDEX-768" /> in the message when it could help theuser to figure out what he or she did wrong. But if you use<tt class="literal">die</tt> to indicate an error that is not the failureof a system request, don't include <tt class="literal">$!</tt>, sinceit will generally hold an unrelated message left over from somethingPerl did internally. It will hold a useful value only immediatelyafter a <em class="emphasis">failed</em> system request. A successfulrequest won't leave anything useful there.</p><blockquote class="footnote"> <a name="FOOTNOTE-254" /><p>[254]On somenon-Unix operating systems, <tt class="literal">$!</tt>may say somethinglike <tt class="literal">error number 7</tt>, leaving it up to the user tolook that one up in the documentation. On Windows and VMS, thevariable <tt class="literal">$^E</tt> may have additional diagnosticinformation. </p> </blockquote><p>There's one more thing that <tt class="literal">die</tt> will dofor you: it will automatically append the Perl<a name="INDEX-769" />program name and <a name="INDEX-770" />line number<a href="#FOOTNOTE-255">[255]</a> tothe end of the message, so you can easily identify which<tt class="literal">die</tt> in your program is responsible for theuntimely exit. The error message from the previous code might looklike this, if <tt class="literal">$!</tt> contained the message<tt class="literal">permission denied</tt>:</p><blockquote class="footnote"> <a name="FOOTNOTE-255" /><p>[255]If the errorhappened while reading from a file, the error message will includethe "<a name="INDEX-771" />chunk number" (usually the linenumber) from the file and the name of the filehandle as well, sincethose are often useful in tracking down a bug.</p> </blockquote><blockquote><pre class="code">Cannot create logfile: permission denied at your_program line 1234.</pre></blockquote><p>That's pretty helpful -- in fact, we always seem to wantmore information in our error messages than we put in the first timearound. If you don't want the line number and file revealed,make sure that the dying words have a newline on the end. That is,another way you could use <tt class="literal">die</tt> is in a line likethis, with a trailing<a name="INDEX-772" /> <a name="INDEX-773" />newline:</p><blockquote><pre class="code">die "Not enough arguments\n" if @ARGV < 2;</pre></blockquote><p>If there aren't at least two command-line arguments, thatprogram will say so and quit. It won't include the program nameand line number, since the line number is of no use to the user; thisis the user's error, after all. As a rule of thumb, put thenewline on messages that indicate a usage error and leave it off whenit the error might be something you want to track down duringdebugging.<a href="#FOOTNOTE-256">[256]</a></p><blockquote class="footnote"> <a name="FOOTNOTE-256" /><p>[256]The program's name is inPerl's special variable<tt class="literal">$0</tt><a name="INDEX-774" />, so you may wish to include that inthe string: <tt class="literal">"$0:Not enough arguments\n"</tt>. This isuseful if the program may be used in a pipeline or shell script, forexample, where it's not obvious which command is complaining.<tt class="literal">$0</tt> can be changed during the execution of theprogram, however. You might also want to look into the special<a name="INDEX-775" />_ _<tt class="literal">FILE</tt>_ _ and __<tt class="literal">LINE</tt>_ _tokens (or the<tt class="literal">caller</tt><a name="INDEX-776" /> function) to get the information thatis being left out by adding the newline, so you can print it in yourown choice of format.</p> </blockquote><p>When opening a file fails, though, there's an easier and morecommon way instead of the <tt class="literal">unless</tt> block:</p><blockquote><pre class="code">open LOG, ">>logfile" or die "Cannot create logfile: $!";</pre></blockquote><p>This uses the low-precedence short-circuit <a name="INDEX-777" /><tt class="literal">or</tt> operator that we sawin <a href="ch10_01.htm">Chapter 10, "More Control Structures"</a>. If the <tt class="literal">open</tt>succeeds, it returns true, and the <tt class="literal">or</tt> is done. Ifthe <tt class="literal">open</tt> fails, it returns false, and theshort-circuit <tt class="literal">or</tt> goes on to the right side anddies with the message. You can read this as if it were English:"Open this file, or die!" It may not be the battle crythat will win a war, but it's a good way to write code.</p><p>You should always check the return value of<tt class="literal">open</tt>, since the rest of the program is relyingupon its success. That's why we say that this is really theonly way to write <tt class="literal">open</tt> -- with <tt class="literal">ordie</tt> after it.<a href="#FOOTNOTE-257">[257]</a> Until you're ready to beextra tricky, you should simply think of this as the syntax for<tt class="literal">open</tt>. Typing <tt class="literal">or die</tt> and amessage takes only a moment when you're writing the program,but it can save hours, or possibly days of debugging time whensomething goes wrong.</p><blockquote class="footnote"> <a name="FOOTNOTE-257" /><p>[257]Older code may use thehigher-precedence <tt class="literal">||</tt> operator instead. The onlydifference is the precedence, but it's a big one! If the<tt class="literal">open</tt> is written without parentheses, thehigher-precedence operator will bind to the filename argument, not tothe return value -- so the return value of <tt class="literal">open</tt>isn't being checked after all. If you use the<tt class="literal">||</tt>, be sure to use the parentheses. Better yet,just use the low-precedence <tt class="literal">or</tt> as we'veshown here whenever you're writing <tt class="literal">ordie</tt>.</p> </blockquote><a name="lperl3-CHP-11-SECT-3.1" /><div class="sect2"><h3 class="sect2">11.3.1. Warning Messages with warn</h3><p>Just as <tt class="literal">die</tt> can indicate a fatal error that actslike one of Perl's builtin errors (like dividing by zero), youcan use the <tt class="literal">warn</tt><a name="INDEX-778" /><a name="INDEX-779" /> function to cause a warning that actslike one of Perl's builtin warnings (like using an<tt class="literal">undef</tt> value as if it were defined, when warningsare enabled).</p><p>The <tt class="literal">warn</tt> function works just like<tt class="literal">die</tt> does, except for that last step -- itdoesn't actually quit the program. But it adds the<a name="INDEX-780" />program name and <a name="INDEX-781" />line number if needed, and it prints themessage to standard error, just as <tt class="literal">die</tt>would.<a href="#FOOTNOTE-258">[258]</a></p><blockquote class="footnote"> <a name="FOOTNOTE-258" /><p>[258]Warnings can't be trapped with an<tt class="literal">eval</tt> block, like fatal errors can. But see thedocumentation for the _ _<tt class="literal">WARN</tt>_ _ pseudo-signal (inthe <em class="emphasis">perlvar</em> manpage) if you need to trap awarning.</p> </blockquote><p>And having talked about death and dire warnings, we now return you toyour regularly scheduled filehandle instructional material. Readon.<a name="INDEX-782" /><a name="INDEX-783" /></p></div><hr width="684" align="left" /><div class="navbar"><table width="684" border="0"><tr><td align="left" valign="top" width="228"><a href="ch11_02.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="ch11_04.htm"><img alt="Next" border="0" src="../gifs/txtnexta.gif" /></a></td></tr><tr><td align="left" valign="top" width="228">11.2. Opening a Filehandle</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">11.4. Using Filehandles</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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