gdb prints wrong values when modifying arguments

Question

System

Fresh install of codeblocks 12.11 + mingw pack.

• win7 64
• gcc 4.7.1
• gdb 7.5

Example code

Compiled wit

Answer 1

Sometimes the optimizer is smarter than the debugger. Try debugging unoptimized code, or step through disassembly and watch the HW registers directly rather than stepping through C source lines and watching the debugger's idea of what the variables are.

Answer 2

I searched on gcc bugzilla and found this bug report :

• Bug 54218 - Debug info for function parameters is incorrect when compiled with -O0"

Althoug the report is about gcc 4.8 and I'm using 4.7, I tried the proposed workaround and it works !

Compiling with -fvar-tracking allows GDB to print the correct value for b after assignment.

Answer 3

gcc does not generate debugging info correctly for values that are in registers -- either values that have been put in registers or values that start there due to the calling conventions. This is a long-standing problem with gcc since at least 4.0, and makes it tricky to debug things.