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21.1.5.1 Support for DLLs without Debugging Symbols

Very often on windows, some of the DLLs that your program relies on do not include symbolic debugging information (for example, kernel32.dll). When GDB 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 GDB as “minimal symbols”.

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. It is also possible to force GDB to load a particular DLL before starting the executable — see the shared library information in Files, or the dll-symbols command in Cygwin Native. Currently, explicitly loading symbols from a DLL with no debugging information will cause the symbol names to be duplicated in GDB’s lookup table, which may adversely affect symbol lookup performance.

21.1.5.2 DLL Name Prefixes

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 KERNEL32!CreateFileA. The plain name is also entered into the symbol table, so CreateFileA 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.

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 GDB are case-sensitive this may cause some confusion. If in doubt, try the info functions and info variables commands or even maint print msymbols (see Symbols). Here’s an example:

(gdb) info function CreateFileA
All functions matching regular expression "CreateFileA":

Non-debugging symbols:
0x77e885f4  CreateFileA
0x77e885f4  KERNEL32!CreateFileA
(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...]

21.1.5.3 Working with Minimal Symbols

Symbols extracted from a DLL’s export table do not contain very much type information. All that GDB 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.

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:

(gdb) print 'cygwin1!__argv'
$1 = 268572168
(gdb) x 'cygwin1!__argv'
0x10021610:      "\230y\""

And two possible solutions:

(gdb) print ((char **)'cygwin1!__argv')[0]
$2 = 0x22fd98 "/cygdrive/c/mydirectory/myprogram"
(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"

Setting a break point within a DLL is possible even before the program starts execution. However, under these circumstances, GDB 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:

(gdb) break *&'python22!PyOS_Readline'
Breakpoint 1 at 0x1e04eff0

The author of these extensions is not entirely convinced that setting a break point within a shared DLL like kernel32.dll is completely safe.


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