Tuesday, December 25, 2012

examine macro definition in gdb

When debugging a c++ application, I used to refer to source code to find out the actual definition of a macro. If the macro is not a simple one, I had to perform the expansion on a paper or use the 'gcc -E' command to find out what's the actual result. This is a tedious task. The gdb macro command helps examine the macro value, as long as the application under debugging contains information about preprocessor macros. This can be satisfied by passing -g3 option to the gcc compiler. As an example, we need to debug the application below
 1 #define FOO foo_value
 2 #define STR(val) "STR of "#val
 3 #define VAL STR(FOO)
 5 int main(int argc, const char *argv[])
 6 {
 7     const char* t = VAL;
 8 #undef VAL
 9 #define VAL "test" // define VAL to a different value 
10     const char* t2 = VAL;
12     return 0;
13 }
We compile the code with gcc -g3 command, and debug it in gdb. Then we can examine the actual value of VAL macro with macro exp command.
(gdb) macro exp VAL  // run when break on line 7
expands to: "STR of ""FOO"
(gdb) macro exp VAL  // run when break on line 10
expands to: "test"
It's worthy of note that the macro expansion is context awareness in gdb, so we can get different value when the application breaks on line 7 and 10.