Check if Pre processor Directive is defined in C Programming [duplicate] - c

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!\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
^

Answer 2

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.

Answer 3

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

Answer 4

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