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

Solution 1:

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);

Solution 2:

For C++17 the answer to this question changes since we now have the [[nodiscard]] attribute. Covered in [dcl.attr.nodiscard]:

The attribute-token nodiscard may be applied to the declarator-id in a function declaration or to the declaration of a class or enumeration. It shall appear at most once in each attribute-list and no attribute-argument-clause shall be present.

and

[ Example:

struct [[nodiscard]] error_info { /* ... */ };
error_info enable_missile_safety_mode();
void launch_missiles();
void test_missiles() {
  enable_missile_safety_mode(); // warning encouraged
  launch_missiles();
}
error_info &foo();
void f() { foo(); }             // warning not encouraged: not a nodiscard call, because neither
                                // the (reference) return type nor the function is declared nodiscard

 — end example ]

So modifying your example (see it live):

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


int main()
{
  int i = 7;
  f(i); // now we obtain a diagnostic

  return 1;
}

We now obtain a diagnostic with both gcc and clang e.g.

warning: ignoring return value of function declared with 'nodiscard' attribute [-Wunused-result]
  f(i); // now we obtain a diagnostic
  ^ ~