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<title>Debugging with GDB: Non-debug DLL Symbols</title>

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<a name="Non_002ddebug-DLL-Symbols"></a>
<div class="header">
<p>
Up: <a href="Cygwin-Native.html#Cygwin-Native" accesskey="u" rel="up">Cygwin Native</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="Support-for-DLLs-without-Debugging-Symbols"></a>
<h4 class="subsubsection">21.1.5.1 Support for DLLs without Debugging Symbols</h4>
<a name="index-DLLs-with-no-debugging-symbols"></a>
<a name="index-Minimal-symbols-and-DLLs"></a>

<p>Very often on windows, some of the DLLs that your program relies on do
not include symbolic debugging information (for example,
<samp>kernel32.dll</samp>).  When <small>GDB</small> doesn&rsquo;t recognize any debugging
symbols in a DLL, it relies on the minimal amount of symbolic
information contained in the DLL&rsquo;s export table.  This section
describes working with such symbols, known internally to <small>GDB</small> as
&ldquo;minimal symbols&rdquo;.
</p>
<p>Note that before the debugged program has started execution, no DLLs
will have been loaded.  The easiest way around this problem is simply to
start the program &mdash; either by setting a breakpoint or letting the
program run once to completion.
</p>
<a name="DLL-Name-Prefixes"></a>
<h4 class="subsubsection">21.1.5.2 DLL Name Prefixes</h4>

<p>In keeping with the naming conventions used by the Microsoft debugging
tools, DLL export symbols are made available with a prefix based on the
DLL name, for instance <code>KERNEL32!CreateFileA</code>.  The plain name is
also entered into the symbol table, so <code>CreateFileA</code> is often
sufficient.  In some cases there will be name clashes within a program
(particularly if the executable itself includes full debugging symbols)
necessitating the use of the fully qualified name when referring to the
contents of the DLL.  Use single-quotes around the name to avoid the
exclamation mark (&ldquo;!&rdquo;)  being interpreted as a language operator.
</p>
<p>Note that the internal name of the DLL may be all upper-case, even
though the file name of the DLL is lower-case, or vice-versa.  Since
symbols within <small>GDB</small> are <em>case-sensitive</em> this may cause
some confusion. If in doubt, try the <code>info functions</code> and
<code>info variables</code> commands or even <code>maint print msymbols</code>
(see <a href="Symbols.html#Symbols">Symbols</a>). Here&rsquo;s an example:
</p>
<div class="smallexample">
<pre class="smallexample">(gdb) info function CreateFileA
All functions matching regular expression &quot;CreateFileA&quot;:

Non-debugging symbols:
0x77e885f4  CreateFileA
0x77e885f4  KERNEL32!CreateFileA
</pre></div>

<div class="smallexample">
<pre class="smallexample">(gdb) info function !
All functions matching regular expression &quot;!&quot;:

Non-debugging symbols:
0x6100114c  cygwin1!__assert
0x61004034  cygwin1!_dll_crt0@0
0x61004240  cygwin1!dll_crt0(per_process *)
[etc...]
</pre></div>

<a name="Working-with-Minimal-Symbols"></a>
<h4 class="subsubsection">21.1.5.3 Working with Minimal Symbols</h4>

<p>Symbols extracted from a DLL&rsquo;s export table do not contain very much
type information. All that <small>GDB</small> can do is guess whether a symbol
refers to a function or variable depending on the linker section that
contains the symbol. Also note that the actual contents of the memory
contained in a DLL are not available unless the program is running. This
means that you cannot examine the contents of a variable or disassemble
a function within a DLL without a running program.
</p>
<p>Variables are generally treated as pointers and dereferenced
automatically. For this reason, it is often necessary to prefix a
variable name with the address-of operator (&ldquo;&amp;&rdquo;) and provide explicit
type information in the command. Here&rsquo;s an example of the type of
problem:
</p>
<div class="smallexample">
<pre class="smallexample">(gdb) print 'cygwin1!__argv'
$1 = 268572168
</pre></div>

<div class="smallexample">
<pre class="smallexample">(gdb) x 'cygwin1!__argv'
0x10021610:      &quot;\230y\&quot;&quot;
</pre></div>

<p>And two possible solutions:
</p>
<div class="smallexample">
<pre class="smallexample">(gdb) print ((char **)'cygwin1!__argv')[0]
$2 = 0x22fd98 &quot;/cygdrive/c/mydirectory/myprogram&quot;
</pre></div>

<div class="smallexample">
<pre class="smallexample">(gdb) x/2x &amp;'cygwin1!__argv'
0x610c0aa8 &lt;cygwin1!__argv&gt;:    0x10021608      0x00000000
(gdb) x/x 0x10021608
0x10021608:     0x0022fd98
(gdb) x/s 0x0022fd98
0x22fd98:        &quot;/cygdrive/c/mydirectory/myprogram&quot;
</pre></div>

<p>Setting a break point within a DLL is possible even before the program
starts execution. However, under these circumstances, <small>GDB</small> can&rsquo;t
examine the initial instructions of the function in order to skip the
function&rsquo;s frame set-up code. You can work around this by using &ldquo;*&amp;&rdquo;
to set the breakpoint at a raw memory address:
</p>
<div class="smallexample">
<pre class="smallexample">(gdb) break *&amp;'python22!PyOS_Readline'
Breakpoint 1 at 0x1e04eff0
</pre></div>

<p>The author of these extensions is not entirely convinced that setting a
break point within a shared DLL like <samp>kernel32.dll</samp> is completely
safe.
</p>
<hr>
<div class="header">
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Up: <a href="Cygwin-Native.html#Cygwin-Native" accesskey="u" rel="up">Cygwin Native</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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