How to enforce the usage of return values in C

Question

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.

That would cause lots of unnecessary warnings. For instance, every call to `printf()` would generate a complaint -- they return the number of characters printed, but no one really cares. — Barmar, Sep 04 '15 at 06:56
@Barmar, I could see where this might be handy for non-library functions though. When I add a return value to a previously void function, I have to change the rest of the signature just to hunt down all the places that use it. — Leeor, Sep 04 '15 at 07:01
Or you could use `grep`, or the IDE's built-in search facilities, like "where used". — user3386109, Sep 04 '15 at 07:07
There's a list of all GCC warning options here: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html — Barmar, Sep 04 '15 at 07:08
possible duplicate of [how to raise warning if return value is disregarded - gcc or static code check?](http://stackoverflow.com/questions/2042780/how-to-raise-warning-if-return-value-is-disregarded-gcc-or-static-code-check) — Andreas Fester, Sep 24 '15 at 08:27

score 12 · Accepted Answer · edited May 23 '17 at 12:30

12

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
$

edited May 23 '17 at 12:30

Community

1
1

answered Sep 04 '15 at 06:58

Sourav Ghosh

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@Andreas: What makes you think that? – Jonathan Leffler Sep 04 '15 at 07:03
3

This will warn if he assigns the result to a variable but doesn't use the variable. He wants a warning for when he doesn't assign the result to a variable. – Barmar Sep 04 '15 at 07:03
2

@Barmar: are you sure? The 'variable set but not used' warning is separate, I think. – Jonathan Leffler Sep 04 '15 at 07:05
@JonathanLeffler With OP's code, I do not get a warning even with `-Wunused-result` (gcc 4.8.4) – Andreas Fester Sep 04 '15 at 07:08
1

@M.M I was in the process of doing the same. Thanks Much, I'll add your link too in my answer. :) – Sourav Ghosh Sep 04 '15 at 07:12
Perfect, setting the attribute on a per function basis is even better than complaining about every unused result. – FSMaxB Sep 04 '15 at 07:25
@FSMaxB Yes, indeed. :) – Sourav Ghosh Sep 04 '15 at 07:26

Andreas Fester · Answer 2 · 2015-09-04T07:41:58.007

Some static code analyzers like splint can check for these kind of things:

$ splint check.c
Splint 3.1.2 --- 03 May 2009

check.c: (in function main)
check.c:6:5: Return value (type int) ignored: test()
  Result returned by function call is not used. If this is intended, can cast
  result to (void) to eliminate message. (Use -retvalint to inhibit warning)

Unlike @Sourav's answer, this does not require a specific __attribute__ annotation on the target function, but on the other hand possibly emits many warnings. Its usually possible to suppress the warnings for specific functions or function calls by using annotations (like /*@alt void@*/).

How to enforce the usage of return values in C

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