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Asked: 2026-06-03T09:05:32+00:00

I have this C file ( sample.c ): #include <stdio.h> #define M 42 #define

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I have this C file (sample.c):

#include <stdio.h>

#define M 42
#define ADD(x) (M + x)

int main ()
{
  printf("%d\n", M);
  printf("%d\n", ADD(2));
  return 0;
}

which I compile with:

gcc -O0 -Wall -g3 sample.c -o sample

Then debug with

gdb ./sample

Output:

GNU gdb (Gentoo 7.3.1 p2) 7.3.1
Copyright (C) 2011 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 "x86_64-pc-linux-gnu".
For bug reporting instructions, please see:
<http://bugs.gentoo.org/>...
Reading symbols from /tmp/sample...done.

(gdb) macro list

(gdb) macro expand ADD(2)
expands to: ADD(2)

(gdb) print M
No symbol "M" in current context.

(gdb) q

This used to work. I need this to work, because I am using libraries which #define names for hardware peripherals and memory addresses.

This seems to be in direct contradiction of the behavior that is shown on the Sourceware GDB site).

What am I doing wrong?

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1 Answer

  1. Editorial Team

    It looks like the macros need to be "brought in scope" one way or another. If you follow exactly the examples in the page you link to, they work as advertised (at least they do for me).

    Example (t.c is your source file):

    $ gcc -O0 -g3 t.c

$ gdb ./a.out

GNU gdb (Gentoo 7.3.1 p2) 7.3.1
...
Reading symbols from .../a.out...done.
(gdb) info macro ADD
The symbol `ADD' has no definition as a C/C++ preprocessor macro
at <user-defined>:-1
             // Macros not loaded yet
(gdb) list main

1    #include <stdio.h>
2    #define M 42
3    #define ADD(x) (M + x)
4    int main ()
5    {
6      printf("%d\n", M);
7      printf("%d\n", ADD(2));
8      return 0;
9    }

(gdb) info macro ADD

Defined at /home/foo/tmp/t.c:3
#define ADD(x) (M + x)
             // Macros "in scope"/loaded

(gdb) macro expand ADD(42)

expands to: (42 + 42)

(gdb) macro expand M

expands to: 42

(gdb) macro expand ADD(M)

expands to: (42 + 42)

    Or:

    $ gdb ./a.out

GNU gdb (Gentoo 7.3.1 p2) 7.3.1
...
Reading symbols from .../a.out...done.

(gdb) macro expand ADD(1)

expands to: ADD(1)
             // Macros not available yet

(gdb) break main

Breakpoint 1 at 0x400538: file t.c, line 6.

(gdb) r

Starting program: /home/foo/tmp/a.out
Breakpoint 1, main () at t.c:6
6      printf("%d\n", M);

(gdb) macro expand ADD(1)

expands to: (42 + 1)
             // Macros loaded
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