Seeing expanded C macros

Question

If I want to expand a C macro, what are some good ways to do that (besides tracing it manually)?

For instance, GTK_WIDGET_SET_FLAGS, it uses a macro that

Answer 1

GCC -save-temps

The big advantage of this option over -E is that it is very easy to add it to any build script, without interfering much in the build itself.

When you do:

gcc -save-temps -c -o main.o main.c

main.c

#define INC 1

int myfunc(int i) {
    return i + INC;
}

and now, besides the normal output main.o, the current working directory also contains the following files:

• main.i is a contains the desired preprossessed file:

  # 1 "main.c"
  # 1 ""
  # 1 ""
  # 31 ""
  # 1 "/usr/include/stdc-predef.h" 1 3 4
  # 32 "" 2
  # 1 "main.c"
  
  
  int myfunc(int i) {
      return i + 1;
  }
  

• main.s is a bonus, and contains the desired generated assembly:

      .file   "main.c"
      .text
      .globl  myfunc
      .type   myfunc, @function
  myfunc:
  .LFB0:
      .cfi_startproc
      pushq   %rbp
      .cfi_def_cfa_offset 16
      .cfi_offset 6, -16
      movq    %rsp, %rbp
      .cfi_def_cfa_register 6
      movl    %edi, -4(%rbp)
      movl    -4(%rbp), %eax
      addl    $1, %eax
      popq    %rbp
      .cfi_def_cfa 7, 8
      ret
      .cfi_endproc
  .LFE0:
      .size   myfunc, .-myfunc
      .ident  "GCC: (Ubuntu 8.3.0-6ubuntu1) 8.3.0"
      .section    .note.GNU-stack,"",@progbits
  

If you want to do it for a large number of files, consider using instead:

 -save-temps=obj

which saves the intermediate files to the same directory as the -o object output instead of the current working directory, thus avoiding potential basename conflicts.

Another cool thing about this option is if you add -v:

gcc -save-temps -c -o main.o -v main.c

it actually shows the explicit files being used instead of ugly temporaries under /tmp, so it is easy to know exactly what is going on, which includes the preprocessing / compilation / assembly steps:

/usr/lib/gcc/x86_64-linux-gnu/8/cc1 -E -quiet -v -imultiarch x86_64-linux-gnu main.c -mtune=generic -march=x86-64 -fpch-preprocess -fstack-protector-strong -Wformat -Wformat-security -o main.i
/usr/lib/gcc/x86_64-linux-gnu/8/cc1 -fpreprocessed main.i -quiet -dumpbase main.c -mtune=generic -march=x86-64 -auxbase-strip main.o -version -fstack-protector-strong -Wformat -Wformat-security -o main.s
as -v --64 -o main.o main.s

Tested in Ubuntu 19.04 amd64, GCC 8.3.0.