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<html><head><title>The Perl Debugger (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="The Perl Debugger"><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="ch19_02.htm"><img src="../gifs/txtpreva.gif" alt="Previous" border="0"></a></td><td align="center" valign="top" width="171"><a href="part3.htm">Part 3: Perl as Technology</a></td><td align="right" valign="top" width="172"><a href="ch20_02.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0"></a></td></tr></table></div><hr width="515" align="left"><!-- SECTION BODY --><h1 class="chapter">Chapter 20. The Perl Debugger</h1><div class="htmltoc"><h4 class="tochead">Contents:</h4><p><a href="ch20_01.htm">Using the Debugger</a><br><a href="ch20_02.htm">Debugger Commands</a><br><a href="ch20_03.htm">Debugger Customization</a><br><a href="ch20_04.htm">Unattended Execution</a><br><a href="ch20_05.htm">Debugger Support</a><br><a href="ch20_06.htm">The Perl Profiler</a><br></p></div><p><a name="INDEX-3498"></a><a name="INDEX-3499"></a><a name="INDEX-3500"></a><a name="INDEX-3501"></a><a name="INDEX-3502"></a><a name="INDEX-3503"></a></p><p>First of all, have you tried the <tt class="literal">use warnings</tt> pragma?<a name="INDEX-3504"></a></p><p><a name="INDEX-3505"></a>If you invoke Perl with the <span class="option">-d</span> switch, your programwill be run inside the Perl debugger. This works like an interactivePerl environment, prompting for debugger commands that let you examinesource code, set breakpoints, dump out your function-call stack,change the values of variables, and so on. Any command not recognizedby the debugger is directly executed (using <tt class="literal">eval</tt>)as Perl code in the package of the code currently being debugged. (Thedebugger uses the <tt class="literal">DB</tt> package for its own stateinformation, to avoid trampling yours.) This is so wonderfullyconvenient that people often fire up the debugger just to test outPerl constructs interactively. In that case, it doesn't matter whatprogram you tell Perl to debug, so we'll choose one without muchmeaning:<blockquote><pre class="programlisting"><tt class="computeroutput">%</tt><tt class="userinput"><b> perl -de 42</b></tt></pre></blockquote>In Perl, the debugger is not a program completely separate from theone being debugged, the way it usually is in a typical programmingenvironment. Instead, the <span class="option">-d</span> flag tells the compilerto insert source information into the parse trees it's about to handoff to the interpreter. That means your code must first compilecorrectly for the debugger to work on it. If that is successful, theintrepreter preloads a special Perl library file containing thedebugger itself.<blockquote><pre class="programlisting"><tt class="computeroutput">%</tt><tt class="userinput"><b> perl -d /path/to/program</b></tt></pre></blockquote>The program will halt immediately before the first run-time executablestatement (but see the section <a href="ch20_01.htm#ch20-sect-ud">Section 20.1, "Using the Debugger"</a> regardingcompile-time statements) and ask you to enter a debugger command.Whenever the debugger halts and shows you a line of code, it displaysthe line that it's <em class="emphasis">about</em> to execute, not the one just executed.</p><p>As the debugger encounters a line, it first checks for a breakpoint,prints it (if the debugger is in trace mode), performs any actions(created with the <tt class="literal">a</tt> command described later in "DebuggerCommands"), and finally prompts the user if a breakpoint is presentor if the debugger is in single-step mode. If not, it evaluates theline normally and continues to the next line.</p><h2 class="sect1">20.1. Using the Debugger</h2><p>The debugger prompt is something like:<blockquote><pre class="programlisting">DB<8></pre></blockquote>or even:<blockquote><pre class="programlisting">DB<<17>></pre></blockquote><a name="INDEX-3506"></a><a name="INDEX-3507"></a><a name="INDEX-3508"></a></p><p>where the number shows how many commands you've executed. A<em class="emphasis">csh</em>-like history mechanism allows you toaccess previous commands by number. For example,<tt class="literal">!17</tt> would repeat command number 17. The number ofangle brackets indicates the depth of the debugger. For example, youget more than one set of brackets if you're already at a breakpointand then print out the result of a function call that itself also hasa breakpoint.</p><p><a name="INDEX-3509"></a>If you want to enter a multiline command, such as a subroutinedefinition with several statements, you may escape the newline thatwould normally end the debugger command with a backslash. Here's anexample:<blockquote><pre class="programlisting">DB<1> <b class="emphasis-bold">for (1..3) { \</b> cont: <b class="emphasis-bold">print "ok\n"; \</b> cont: <b class="emphasis-bold">}</b> ok ok ok</pre></blockquote>Let's say you want to fire up the debugger on a little program ofyours (let's call it <em class="emphasis">camel_flea</em>)and stop it as soon as it gets down to a function named<tt class="literal">infested</tt>. Here's how you'd do that:<blockquote><pre class="programlisting"><tt class="computeroutput">%</tt><tt class="userinput"><b> perl -d camel_flea</b></tt>Loading DB routines from perl5db.pl version 1.07Editor support available.Enter h or `h h' for help, or `man perldebug' for more help.main::(camel_flea:2): pests('bactrian', 4); DB<1></pre></blockquote>The debugger halts your program right before the first run-timeexecutable statement (but see below about compile-time statements) andasks you to enter a command. Again, whenever the debugger stops toshow you a line of code, it displays the line it's<em class="emphasis">about</em> to execute, not the one it just executed.The line displayed may not look exactly like it did in your sourcefile, particularly if you've run it through any kind of preprocessor.</p><p>Now, you'd like to stop as soon as your program gets to the <tt class="literal">infested</tt>function, so you establish a breakpoint there like so:<blockquote><pre class="programlisting">DB<1> <b class="emphasis-bold">b infested</b>DB<2> <b class="emphasis-bold">c</b></pre></blockquote>The debugger now continues until it hits that function, at which pointit says this:<blockquote><pre class="programlisting">main::infested(camel_flea:8): my $bugs = int rand(3);</pre></blockquote></p><p><a name="INDEX-3510"></a>To look at a "window" of source code around the breakpoint, usethe <tt class="literal">w</tt> command:<blockquote><pre class="programlisting">DB<2> <b class="emphasis-bold">w</b>5 }67 sub infested {8==>b my $bugs = int rand(3);9: our $Master;10: contaminate($Master);11: warn "needs wash"12 if $Master && $Master->isa("Human");1314: print "got $bugs\n";DB<2></pre></blockquote><a name="INDEX-3511"></a>As you see by the <tt class="literal">==></tt> marker, your current lineis line 8, and by the <tt class="literal">b</tt> there, you know it has abreakpoint on it. If you'` had set an action, there also would alsohave been an <tt class="literal">a</tt> there. The line numbers with colonsare breakable; the rest are not.</p><p><a name="INDEX-3512"></a><a name="INDEX-3513"></a>To see who called whom, ask for a stack backtrace using the <tt class="userinput"><b>T</b></tt> command:<blockquote><pre class="programlisting">DB<2> <b class="emphasis-bold">T</b>$ = main::infested called from file `Ambulation.pm' line 4@ = Ambulation::legs(1, 2, 3, 4) called from file `camel_flea' line 5. = main::pests('bactrian', 4) called from file `camel_flea' line 2</pre></blockquote><a name="INDEX-3514"></a><a name="INDEX-3515"></a><a name="INDEX-3516"></a><a name="INDEX-3517"></a><a name="INDEX-3518"></a><a name="INDEX-3519"></a></p><p>The initial character (<tt class="literal">$</tt>, <tt class="literal">@</tt>, or<tt class="literal">.</tt>) tells whether the function was called in ascalar, list, or void context, respectively. There are three linesbecause you were three functions deep when you ran the stackbacktrace. Here's what each line means:</p><ul><li><p>The first line says you were in the function<tt class="literal">main::infested</tt> when you ran the stack trace. Ittells you the function was called in a scalar context from line 4 ofthe file <em class="emphasis">Ambulation.pm</em>. It also shows that itwas called without any arguments whatsoever, meaning it was called as<tt class="literal">&infested</tt> instead of the normal way, as<tt class="literal">infested()</tt>.</p></li><li><p>The second line shows that the function<tt class="literal">Ambulation::legs</tt> was called in list context fromline number 5 of the <em class="emphasis">camel_flea</em>file, with those four arguments.</p></li><li><p>The third line shows that <tt class="literal">main::pests</tt> was called invoid context from line 2 of<em class="emphasis">camel_flea</em>.</p></li></ul><a name="INDEX-3520"></a><a name="INDEX-3521"></a><a name="INDEX-3522"></a><p>If you have compile-phase executable statements such as code from<tt class="literal">BEGIN</tt> and <tt class="literal">CHECK</tt> blocks or<tt class="literal">use</tt> statements, these will <em class="emphasis">not</em>ordinarily be stopped by the debugger, although<tt class="literal">require</tt>s and <tt class="literal">INIT</tt> blocks will,since they happen after the transition to run phase (see <a href="ch18_01.htm">Chapter 18, "Compiling"</a>). Compile-phase statements can betraced with the <tt class="literal">AutoTrace</tt> option set in<tt class="literal">PERLDB_OPTS</tt>.<a name="INDEX-3523"></a></p><p>You can exert a little control over the Perl debugger fromwithin your Perl program itself. You might do this, for example,to set an automatic breakpoint at a certain subroutine whenever aparticular program is run under the debugger. From your own Perlcode, however, you can transfer control back to the debugger usingthe following statement, which is harmless if the debugger is notrunning:<blockquote><pre class="programlisting">$DB::single = 1;</pre></blockquote>If you set <tt class="literal">$DB::single</tt> to 2, it's equivalent to the<tt class="literal">n</tt> command, whereas a value of 1 emulates the<tt class="literal">s</tt> command. The <tt class="literal">$DB::trace</tt>variable should be set to 1 to simulate the <tt class="literal">t</tt>command.</p><p><a name="INDEX-3524"></a>Another way to debug a module is to set breakpoint on <em class="emphasis">load</em>ing:<blockquote><pre class="programlisting">DB<7> <b class="emphasis-bold">b load c:/perl/lib/Carp.pm</b>Will stop on load of `c:/perl/lib/Carp.pm'.</pre></blockquote>and then restart the debugger using the <tt class="literal">R</tt> command.For finer control, you can use the <tt class="literal">b compilesubname</tt> to stop as soon as possible after a particularsubroutine is compiled.</p><a name="INDEX-3663"></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="ch19_02.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="ch20_02.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0"></a></td></tr><tr><td align="left" valign="top" width="172">19.2. Environment Variables</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">20.2. Debugger Commands</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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