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| <html><head><meta http-equiv="Content-Type" content="text/html; charset=ANSI_X3.4-1968"><title>Debugging Cygwin Programs</title><link rel="stylesheet" type="text/css" href="docbook.css"><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot"><link rel="home" href="cygwin-ug-net.html" title="Cygwin User's Guide"><link rel="up" href="programming.html" title="Chapter 4. Programming with Cygwin"><link rel="prev" href="programming.html" title="Chapter 4. Programming with Cygwin"><link rel="next" href="dll.html" title="Building and Using DLLs"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Debugging Cygwin Programs</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="programming.html">Prev</a> </td><th width="60%" align="center">Chapter 4. Programming with Cygwin</th><td width="20%" align="right"> <a accesskey="n" href="dll.html">Next</a></td></tr></table><hr></div><div class="sect1"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="gdb"></a>Debugging Cygwin Programs</h2></div></div></div><p>When your program doesn't work right, it usually has a "bug" in |
| it, meaning there's something wrong with the program itself that is |
| causing unexpected results or crashes. Diagnosing these bugs and |
| fixing them is made easy by special tools called |
| <span class="emphasis"><em>debuggers</em></span>. In the case of Cygwin, the debugger |
| is GDB, which stands for "GNU DeBugger". This tool lets you run your |
| program in a controlled environment where you can investigate the |
| state of your program while it is running or after it crashes. |
| Crashing programs sometimes create "core" files. In Cygwin these are |
| regular text files that cannot be used directly by GDB. |
| </p><p>Before you can debug your program, you need to prepare your |
| program for debugging. What you need to do is add |
| <code class="literal">-g</code> to all the other flags you use when compiling |
| your sources to objects.</p><div class="example"><a name="gdb-g"></a><p class="title"><b>Example 4.4. Compiling with -g</b></p><div class="example-contents"><pre class="screen"> |
| <code class="prompt">bash$</code> gcc -g -O2 -c myapp.c |
| <code class="prompt">bash$</code> gcc -g myapp.c -o myapp |
| </pre></div></div><br class="example-break"><p>What this does is add extra information to the objects (they get |
| much bigger too) that tell the debugger about line numbers, variable |
| names, and other useful things. These extra symbols and debugging |
| information give your program enough information about the original |
| sources so that the debugger can make debugging much easier for |
| you.</p><p>To invoke GDB, simply type <span class="command"><strong>gdb myapp.exe</strong></span> at the |
| command prompt. It will display some text telling you about itself, |
| then <code class="literal">(gdb)</code> will appear to prompt you to enter |
| commands. Whenever you see this prompt, it means that gdb is waiting |
| for you to type in a command, like <span class="command"><strong>run</strong></span> or |
| <span class="command"><strong>help</strong></span>. Oh <code class="literal">:-)</code> type |
| <span class="command"><strong>help</strong></span> to get help on the commands you can type in, or |
| read the [<span class="citation">GDB User's Manual</span>] for a complete |
| description of GDB and how to use it.</p><p>If your program crashes and you're trying to figure out why it |
| crashed, the best thing to do is type <span class="command"><strong>run</strong></span> and let |
| your program run. After it crashes, you can type |
| <span class="command"><strong>where</strong></span> to find out where it crashed, or |
| <span class="command"><strong>info locals</strong></span> to see the values of all the local |
| variables. There's also a <span class="command"><strong>print</strong></span> that lets you look |
| at individual variables or what pointers point to.</p><p>If your program is doing something unexpected, you can use the |
| <span class="command"><strong>break</strong></span> command to tell gdb to stop your program when it |
| gets to a specific function or line number:</p><div class="example"><a name="gdb-break"></a><p class="title"><b>Example 4.5. "break" in gdb</b></p><div class="example-contents"><pre class="screen"> |
| <code class="prompt">(gdb)</code> break my_function |
| <code class="prompt">(gdb)</code> break 47 |
| </pre></div></div><br class="example-break"><p>Now, when you type <span class="command"><strong>run</strong></span> your program will stop |
| at that "breakpoint" and you can use the other gdb commands to look at |
| the state of your program at that point, modify variables, and |
| <span class="command"><strong>step</strong></span> through your program's statements one at a |
| time.</p><p>Note that you may specify additional arguments to the |
| <span class="command"><strong>run</strong></span> command to provide command-line arguments to |
| your program. These two cases are the same as far as your program is |
| concerned:</p><div class="example"><a name="gdb-cliargs"></a><p class="title"><b>Example 4.6. Debugging with command line arguments</b></p><div class="example-contents"><pre class="screen"> |
| <code class="prompt">bash$</code> myprog -t foo --queue 47 |
|
|
| <code class="prompt">bash$</code> gdb myprog |
| <code class="prompt">(gdb)</code> run -t foo --queue 47 |
| </pre></div></div><br class="example-break"></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="programming.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="programming.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="dll.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 4. Programming with Cygwin </td><td width="20%" align="center"><a accesskey="h" href="cygwin-ug-net.html">Home</a></td><td width="40%" align="right" valign="top"> Building and Using DLLs</td></tr></table></div></body></html> |
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