Drakosha Drakosha - 1 month ago 8
C Question

How to raise warning if return value is disregarded?

I'd like to see all the places in my code (C++) which disregard return value of a function. How can I do it - with gcc or static code analysis tool?

Bad code example:

int f(int z) {
return z + (z*2) + z/3 + z*z + 23;
}


int main()
{
int i = 7;
f(i); ///// <<----- here I disregard the return value

return 1;
}


Please note that:


  • it should work even if the function and its use are in different files

  • free static check tool


Answer

You want GCC's warn_unused_result attribute:

#define WARN_UNUSED __attribute__((warn_unused_result))

int WARN_UNUSED f(int z) {
    return z + (z*2) + z/3 + z*z + 23;
}

int main()
{
  int i = 7;
  f(i); ///// <<----- here i disregard the return value
  return 1;
}

Trying to compile this code produces:

$ gcc test.c
test.c: In function `main':
test.c:16: warning: ignoring return value of `f', declared with
attribute warn_unused_result

You can see this in use in the Linux kernel; they have a __must_check macro that does the same thing; looks like you need GCC 3.4 or greater for this to work. Then you will find that macro used in kernel header files:

unsigned long __must_check copy_to_user(void __user *to,
                                        const void *from, unsigned long n);
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