c - How to catch undefined macro in preprocessor #if condition?

Question

Based on this question How to catch empty defined macro with gcc? I have another problem. How to catch undefined macro in preprocessor #if condition? Example code:

#include <stdio.h> 

int main()
{
#if ENABLE_SOMETHING == 1
    ... do something ..
#endif
 return 0;
}

Is there a way to catch error/warning when ENABLE_SOMETHING is not set using gcc compiler(maybe some flag)? Or maybe there are external tools which I can use?

---

I know than i can write something like this :

#ifndef ENABLE_SOMETHING
    #error "ENABLE_SOMETHING undefined!"
#endif

But I have a huge amount of different defines(ENABLE_STH1, ENABLE_STH2, ENALBE_STH3 ... etc.) in code and i don't want to fix this manually. I'm looking for some automatic solution for our project.

Answer 1

Is there a way to catch error/warning when ENABLE_SOMETHING is not set using gcc compiler(maybe some flag)?

With GCC you can use the -Wundef flag.

From the official documentation

-Wundef

Warn if an undefined identifier is evaluated in an #if directive. Such identifiers are replaced with zero.

EDIT:

For example, this C-code:

#include <stdio.h>

int main(void)
{
#if UNDEFINED_MACRO
  printf("hi mum!
");
#endif

  return 0;
}

... compiled with GCC and the -Wundef flag yields this:

$ gcc undef.c -Wundef
undef.c: In function 'main':
undef.c:5:5: warning: "UNDEFINED_MACRO" is not defined [-Wundef]
 #if UNDEFINED_MACRO
     ^