c - 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 with -g and no optimization.

#include <stdio.h>

void foo(int a, int b);

int main() {

    foo(400, 42);

    return 0;
}

void foo(int a, int b) {

    a = a - 10;
    b = a + 1;

    printf("y2 %d
", b);

}

Problem

I put a breakpoint on "void foo(int a, int b)" and I look value of b as I step through the 3 lines. Either with the codeblocks debugging features or with the gdb command line, the value of b is 42 instead of being 391. The console output is correct, 391.

GDB commands

C:DebugTest>"C:Program Files (x86)CodeBlocksMinGWingdb.exe"
GNU gdb (GDB) 7.5
Copyright (C) 2012 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "i686-pc-mingw32".
For bug reporting instructions, please see:  
<http://www.gnu.org/software/gdb/bugs/>.
(gdb) file bin/Debug/DebugTest.exe
Reading symbols from C:DebugTestinDebugDebugTest.exe...done.
(gdb) break foo
Breakpoint 1 at 0x401363: file C:DebugTestmain.c, line 14.
(gdb) run
Starting program: C:DebugTestinDebugDebugTest.exe
[New Thread 3596.0x658]

Breakpoint 1, foo (a=400, b=42) at C:DebugTestmain.c:14
14          a = a - 10;
(gdb) print b
$1 = 42
(gdb) step
15          b = a + 1;
(gdb) print b
$2 = 42
(gdb) step
17          printf("y2 %d
", b);
(gdb) print b
$3 = 42
(gdb)

Remarks

• When the same operations are done without a function, with a and b as local variables inside main, the debug output is correct.
• When compiled with gcc 4.4.1, the debug output is correct.

Any idea what could be wrong ? =)

Answer 1

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.