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.

You will get a pre-processor error if macro is not defines in above example. Something like this error: 'MAX_N_LENGTH' undeclared — Vagish
– Vagish, Commented Feb 1, 2019 at 9:27
If MAX_N_LENGTH is undefined then the compiler will not compile the code. — Duck Dodgers
– Duck Dodgers, Commented Feb 1, 2019 at 9:27
Are you saying that you have undefined tokens used in your code, and it's compiling? I don't understand this question. — George
– George, Commented Feb 1, 2019 at 9:38
Yes, I use these tokens to enable/disable some parts of code in diffrent build configurations. But i want to have some guard for future errors if somebody will make new #if but will forgot to add #define (which happend). — s.paszko
– s.paszko, Commented Feb 1, 2019 at 9:40

Morten Jensen · Accepted Answer · 2024-07-12 17:34:39Z

11

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 (clang supports it as well).

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!\n");
#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
     ^

edited Jul 12, 2024 at 17:34

answered Feb 1, 2019 at 10:04

Morten Jensen

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2 Comments

s.paszko Over a year ago

Ok, so -Wundef together with -Werror will give me an error, won't it?

Morten Jensen Over a year ago

@s.paszko: Yes. -Werror converts warnings into errors that makes compilation fail, so that would emit an error if the compiler encountered an undefined macro

Blaze · Accepted Answer · 2019-02-01 09:30:42Z

0

Let's assume you have this code and it compiles, but you don't know if MAX_N_LENGTH is a macro, or if it's something else:

int main()
{
    int a = MAX_N_LENGTH; // MAX_N_LENGTH could also be an int declared somewhere else
    return 0;
}

You can check whether it actually is a macro like this:

#ifdef MAX_N_LENGTH
    printf("MAX_N_LENGTH is a macro.\n");
#else
    printf("MAX_N_LENGTH is NOT macro.\n");
#endif // MAX_N_LENGTH

Of course, if that ever is an issue, I'd rethink my naming conventions.

answered Feb 1, 2019 at 9:30

Blaze

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3 Comments

s.paszko Over a year ago

Thanks for question but i've gave wrong example. Question redited.

Blaze Over a year ago

The same still applies, you can use #ifdef ... #endif to get conditional compilation depending on whether the macro in question exists or not.

s.paszko Over a year ago

Ok, added info in question. I know this solution but i'm looking for some automatic solution.

David Collins · Accepted Answer · 2019-02-01 09:31:05Z

0

You could try something like the following:

#ifndef MAX_N_LENGTH
#warning "MAX_N_LENGTH is undefined"
    int array[16];
#else
    int array[MAX_N_LENGTH + 1];
#endif

answered Feb 1, 2019 at 9:31

David Collins

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1 Comment

s.paszko Over a year ago

Thanks for question but i've gave wrong example. Question redited.

chqrlie · Accepted Answer · 2019-02-01 09:51:20Z

0

You can test if a macro is defined in a #if preprocessor expression with defined(ENABLE_SOMETHING):

#if !defined(ENABLE_SOMETHING)
  #error ENABLE_SOMETHING is not defined
#endif

You can handle macros with an empty definition this way:

#if ENABLE_SOMETHING + 0 == 1
   /* ENABLE_SOMETHING is defined and not 0 or empty */
#endif

answered Feb 1, 2019 at 9:51

chqrlie

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