Determine the expansion of a C macro

Question

Is there a way to determine the "final value" of a expanded C macro by inspecting the compiled object or by running some form of gcc -E on the .c or .h file?

test.h

#define AAA 1
#define BBB 10
#define CCC (AAA+BBB)

test.c

#include <stdio.h>
#include "test.h"

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

Thus is there someway to get the expanded value either:

#define CCC 11

or

#define CCC (1+10)

If compiled with

gcc -dM -E test.c | grep CCC

or

gcc -dD -E test.c | grep CCC

outputs

#define CCC (AAA+BBB)

which requires me to still know what AAA and BBB are.

Compiled with:

gcc -E test.c

Gives (skipping the boilerplate):

# 4 "test.c"
int main(){
printf("%d\n", (1+10));
return 0;
}

While its expanded CCC I've now lost the mapping back to CCC.

Edit: As it was unclear, what i'd like is someway to determine what CCC is (either 11 or 1+10 (as the gcc -E example showed it only inserted (1+10) not 11), preferably without altering the code itself. Printf was a bad idea to use in the MRE, what i actually had in mind was code like:

struct my_struct {
    int my_array[CCC]
    ... other stuff ...
}

The question is how big is my_array, so i can make a struct in another language (python via ctypes) and know how much space i need. I know for structs i can use pahole but was hoping to do this with only gcc and in the more general case (say a global array not in a struct).

I have read the question twice. Could not understand what you are asking. — Eugene Sh., Jul 03 '18 at 16:55
Is something like `printf("'%s" %d\n", #CCC, CCC);` OK to report the macro name and expansion? — chux - Reinstate Monica, Jul 03 '18 at 16:57
Code could use `_Static_assert()` to test the value at compile time to perhaps meet a higher level compliance objective `_Static_assert(CCC > 0 && CCC < 20, "CCC out of expected range"); ` — chux - Reinstate Monica, Jul 03 '18 at 16:59
So you want to see an incomplete expansion? It can be done using special tools. Eclipse CDT has a step-by-step expansion tool. — Eugene Sh., Jul 03 '18 at 17:05
Can somebody name the duplicate? I know that there is one but admit that I can't pinpoint it. — Yunnosch, Jul 03 '18 at 17:10
https://stackoverflow.com/questions/28932534/expand-c-c-function-macros-without-preprocessor — Gem Taylor, Jul 03 '18 at 17:22

score 3 · Accepted Answer · answered Jul 03 '18 at 17:44

At no point will the preprocessor ever create

#define CCC (1+10)

The expansion of CCC is always (AAA+BBB); it's just that the result of macro expansion is rescanned for more macros to expand, at which point AAA and BBB turn into 1 and 10, respectively.

Perhaps a clearer example of this is

#define AAA 1
#define CCC AAA
#define AAA "hello"

size_t x = sizeof CCC;

This code will expand to "hello", not 1. CCC always has a value of AAA; it's just that by the time size_t x = sizeof CCC; is processed, AAA itself will turn into "hello".

This example also demonstrates that macros can be redefined, so there may not even be a single answer to "what is the value of CCC?".

That's why there's no simple compiler invocation or switch; what you want simply doesn't exist.

That said, if you can use custom code, you can just run e.g.

#define AAA 1
#define BBB 10
#define CCC (AAA+BBB)
CCC

through gcc -P -E and the result will be just (1+10).

alk · Answer 2 · 2018-07-03T17:13:54.530

#include <stdio.h>

#define AAA 1
#define BBB 2
#define CCC (AAA+BBB)

#define STRINGIFY(x) #x
#define PASTE(x) STRINGIFY(x)


int main(void)
{
  printf("CCC = '%s'\n", PASTE(CCC));
}

prints

CCC = '(1+10)'

An alternative version printing the same:

#include <stdio.h>

#define AAA 1
#define BBB 2
#define CCC (AAA+BBB)

#define STRINGIFY(x) #x
#define INTERMEDIATE(x) #x " = '" STRINGIFY(x) "'"

int main(void)
{
  printf("%s\n", INTERMEDIATE(CCC));
}

Determine the expansion of a C macro

2 Answers2

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