Arun Arun - 11 months ago 96
C++ Question

g++ How to get warning on ignoring function return value

lint produces some warning like:

foo.c XXX Warning 534: Ignoring return value of function bar()

From the lint manual

534 Ignoring return value of function

'Symbol' (compare with Location) A
function that returns a value is
called just for side effects as, for
example, in a statement by itself or
the left-hand side of a comma
operator. Try: (void) function(); to
call a function and ignore its return
value. See also the fvr, fvo and fdr
flags in §5.5 "Flag Options".

I want to get this warning, if there exists any, during compilation. Is there any option in gcc/g++ to achieve this? I had turned on
but that apparently did not detect this.

Answer Source

Thanks to WhirlWind and paxdiablo for the answer and comment. Here is my attempt to put the pieces together into a complete (?) answer.

-Wunused-result is the relevant gcc option. And it is turned on by default. Quoting from gcc warning options page:


Do not warn if a caller of a function marked with attribute warn_unused_result (see Variable Attributes) does not use its return value. The default is -Wunused-result

So, the solution is to apply the warn_unused_result attribute on the function.

Here is a full example. The contents of the file unused_result.c

int foo() { return 3; }

int bar() __attribute__((warn_unused_result));
int bar() { return 5; }

int main()
    bar();    /* line 9 */
    return 0;

and corresponding compilation result:

$gcc unused_result.c 
unused_result.c: In function ‘main’:
unused_result.c:9: warning: ignoring return value of ‘bar’, declared with attribute warn_unused_result

Note again that it is not necessary to have -Wunused-result since it is default. One may be tempted to explicitly mention it to communicate the intent. Though that is a noble intent, but after analyzing the situation, my choice, however, would be against that. Because, having -Wunused-result in the compile options may generate a false sense of security/satisfaction which is not true unless the all the functions in the code base are qualified with warn_unused_result.