call return inside GCC compound statement expressions

Question

Can I safely use return inside GCC compound statement expressions ?

For example I define macro

#define CHECK_FUNC_RESULT(func)    \
({                                 \
   int result = func();            \
   if (!result) return;            \
   result;                         \
})

and use it somewhere in code in such way:

int function1() 
{
   if (some_condition) 
     return 0;
   ...
   return 1; 
}

void function2()
{
   if(CHECK_FUNC_RESULT(function1)) {
      ... to do something
   }
}

Can I expect returning from function2 (on some_condition == true) without any undefined behavior ?

Tartary

Answer 1

It should work, but your if() statement will only be processed if function1 returns 1 (in your example: never). This looks like a dangerous construction, as an else would never be executed.

Gonyea

Please, can you be more specific ? – Tartary

if(CHECK_FUNC_RESULT(function1)) { ... to do something } else { ... do something else } In this case, the else will not be executed. Without looking at the macro definition, this is not obvious. Otherwise, I can't find any reference pointing to an undefined behaviour when there is a return statement within the if condition. So it should work as you intend it to work, but if someone else looks at the code it can be confusing. – Gonyea

But what about the topic of the question ? Is this undefined behavior or no ? – Tartary

As far as I can tell, this will not result in undefined behaviour. – Gonyea

Answer 2

A cleaner way to achieve what you want, and you can also pass any number of arguments to func (in this example 3 args):

#define RET_IF_ZERO(func, args) \
do {                            \
    int result = func args;     \
    if (!result)                \
        return;                 \
} while (0)

Example of usage:

void function1(int arg1, int arg2, int arg3)
{
    if (some_condition) 
        return 0;
    ...
    return 1;
}

void function2()
{
   RET_IF_ZERO(function1, (arg1, arg2, arg3)));

   ... do to something
}

Terce

Your answer does not correspond with topic of question – Tartary