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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> [<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’t recognize any debugging |
| symbols in a DLL, it relies on the minimal amount of symbolic |
| information contained in the DLL’s export table. This section |
| describes working with such symbols, known internally to <small>GDB</small> as |
| “minimal symbols”. |
| </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 — 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 (“!”) 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’s an example: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(gdb) info function CreateFileA |
| All functions matching regular expression "CreateFileA": |
| |
| Non-debugging symbols: |
| 0x77e885f4 CreateFileA |
| 0x77e885f4 KERNEL32!CreateFileA |
| </pre></div> |
| |
| <div class="smallexample"> |
| <pre class="smallexample">(gdb) info function ! |
| All functions matching regular expression "!": |
| |
| 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’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 (“&”) and provide explicit |
| type information in the command. Here’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: "\230y\"" |
| </pre></div> |
| |
| <p>And two possible solutions: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(gdb) print ((char **)'cygwin1!__argv')[0] |
| $2 = 0x22fd98 "/cygdrive/c/mydirectory/myprogram" |
| </pre></div> |
| |
| <div class="smallexample"> |
| <pre class="smallexample">(gdb) x/2x &'cygwin1!__argv' |
| 0x610c0aa8 <cygwin1!__argv>: 0x10021608 0x00000000 |
| (gdb) x/x 0x10021608 |
| 0x10021608: 0x0022fd98 |
| (gdb) x/s 0x0022fd98 |
| 0x22fd98: "/cygdrive/c/mydirectory/myprogram" |
| </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’t |
| examine the initial instructions of the function in order to skip the |
| function’s frame set-up code. You can work around this by using “*&” |
| to set the breakpoint at a raw memory address: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(gdb) break *&'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> |
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