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146 lines
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<title>Non-debug DLL Symbols - Debugging with GDB</title>
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<a name="Non-debug-DLL-Symbols"></a>
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<a name="Non_002ddebug-DLL-Symbols"></a>
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Up: <a rel="up" accesskey="u" href="Cygwin-Native.html#Cygwin-Native">Cygwin Native</a>
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<h5 class="subsubsection">21.1.4.1 Support for DLLs without Debugging Symbols</h5>
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<p><a name="index-DLLs-with-no-debugging-symbols-1524"></a><a name="index-Minimal-symbols-and-DLLs-1525"></a>
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Very often on windows, some of the DLLs that your program relies on do
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not include symbolic debugging information (for example,
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<samp><span class="file">kernel32.dll</span></samp>). When <span class="sc">gdb</span> doesn't recognize any debugging
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symbols in a DLL, it relies on the minimal amount of symbolic
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information contained in the DLL's export table. This section
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describes working with such symbols, known internally to <span class="sc">gdb</span> as
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“minimal symbols”.
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<p>Note that before the debugged program has started execution, no DLLs
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will have been loaded. The easiest way around this problem is simply to
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start the program — either by setting a breakpoint or letting the
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program run once to completion.
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<h5 class="subsubsection">21.1.4.2 DLL Name Prefixes</h5>
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<p>In keeping with the naming conventions used by the Microsoft debugging
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tools, DLL export symbols are made available with a prefix based on the
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DLL name, for instance <code>KERNEL32!CreateFileA</code>. The plain name is
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also entered into the symbol table, so <code>CreateFileA</code> is often
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sufficient. In some cases there will be name clashes within a program
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(particularly if the executable itself includes full debugging symbols)
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necessitating the use of the fully qualified name when referring to the
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contents of the DLL. Use single-quotes around the name to avoid the
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exclamation mark (“!”) being interpreted as a language operator.
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<p>Note that the internal name of the DLL may be all upper-case, even
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though the file name of the DLL is lower-case, or vice-versa. Since
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symbols within <span class="sc">gdb</span> are <em>case-sensitive</em> this may cause
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some confusion. If in doubt, try the <code>info functions</code> and
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<code>info variables</code> commands or even <code>maint print msymbols</code>
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(see <a href="Symbols.html#Symbols">Symbols</a>). Here's an example:
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<pre class="smallexample"> (gdb) info function CreateFileA
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All functions matching regular expression "CreateFileA":
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Non-debugging symbols:
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0x77e885f4 CreateFileA
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0x77e885f4 KERNEL32!CreateFileA
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</pre>
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<pre class="smallexample"> (gdb) info function !
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All functions matching regular expression "!":
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Non-debugging symbols:
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0x6100114c cygwin1!__assert
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0x61004034 cygwin1!_dll_crt0@0
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0x61004240 cygwin1!dll_crt0(per_process *)
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[etc...]
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</pre>
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<h5 class="subsubsection">21.1.4.3 Working with Minimal Symbols</h5>
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<p>Symbols extracted from a DLL's export table do not contain very much
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type information. All that <span class="sc">gdb</span> can do is guess whether a symbol
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refers to a function or variable depending on the linker section that
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contains the symbol. Also note that the actual contents of the memory
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contained in a DLL are not available unless the program is running. This
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means that you cannot examine the contents of a variable or disassemble
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a function within a DLL without a running program.
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<p>Variables are generally treated as pointers and dereferenced
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automatically. For this reason, it is often necessary to prefix a
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variable name with the address-of operator (“&”) and provide explicit
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type information in the command. Here's an example of the type of
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problem:
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<pre class="smallexample"> (gdb) print 'cygwin1!__argv'
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'cygwin1!__argv' has unknown type; cast it to its declared type
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</pre>
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<pre class="smallexample"> (gdb) x 'cygwin1!__argv'
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'cygwin1!__argv' has unknown type; cast it to its declared type
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</pre>
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<p>And two possible solutions:
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<pre class="smallexample"> (gdb) print ((char **)'cygwin1!__argv')[0]
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$2 = 0x22fd98 "/cygdrive/c/mydirectory/myprogram"
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</pre>
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<pre class="smallexample"> (gdb) x/2x &'cygwin1!__argv'
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0x610c0aa8 <cygwin1!__argv>: 0x10021608 0x00000000
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(gdb) x/x 0x10021608
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0x10021608: 0x0022fd98
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(gdb) x/s 0x0022fd98
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0x22fd98: "/cygdrive/c/mydirectory/myprogram"
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</pre>
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<p>Setting a break point within a DLL is possible even before the program
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starts execution. However, under these circumstances, <span class="sc">gdb</span> can't
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examine the initial instructions of the function in order to skip the
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function's frame set-up code. You can work around this by using “*&”
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to set the breakpoint at a raw memory address:
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<pre class="smallexample"> (gdb) break *&'python22!PyOS_Readline'
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Breakpoint 1 at 0x1e04eff0
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</pre>
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<p>The author of these extensions is not entirely convinced that setting a
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break point within a shared DLL like <samp><span class="file">kernel32.dll</span></samp> is completely
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safe.
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</body></html>
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