[SOLVED] How to enforce the usage of return values in C

How to enforce the usage of return values in C

I'm searching for a compiler flag for gcc and if possible for clang and the Microsoft compilers as well, that triggers a warning (error with -Werror) if a non-void function is called without using the return value like this:

int test() {
    return 4;
}

int main(void) {
    test(); //should trigger a warning
    int number = test(); //shouldn't trigger the warning
    return 0;
}

If there is no such compiler flag, maybe some way to tell the clang static analyzer to complain about it.

EDIT: To clarify my original question: I actually meant using the return value, not only assigning it.

Solution

I never used it myself (do you really need it?), you can try

defining the function with warn_unused_result attribute
enabling -Wunused-result flag with gcc.

This will tell you about any unused value from the function return.

In case, any doubts, SEE IT LIVE or SEE IT LIVE AGAIN ^{Thanks to M.M for the link in the comment}

Or:

#include <stdio.h>

extern int func1(void) __attribute__((warn_unused_result));
extern int func2(void);

int main(void)
{
    func1();
    int rc1 = func1();
    int rc2 = func1();
    func2();
    printf("%d\n", rc1);
    return 0;
}

Compilation (GCC 5.1.0 on Mac OS X 10.10.5):

$ gcc -O3 -g -std=c11 -Wall -Wextra -Werror -c warn.c
warn.c: In function ‘main’:
warn.c:10:9: error: unused variable ‘rc2’ [-Werror=unused-variable]
     int rc2 = func1();
         ^
warn.c:8:5: error: ignoring return value of ‘func1’, declared with attribute warn_unused_result [-Werror=unused-result]
     func1();
     ^
cc1: all warnings being treated as errors
$