没有返回语句的Printf返回值

Question

Tried this on 3 different systems: CentOS, Ubuntu and Windows(MinGW) ; so I don't think this is an anomaly (undefined behavior)

int hello(void)
{
        (void)printf("HELLO WORLD\n");
}

int main(void)
{
        printf("%d\n",hello());
        return 0;
}

Output is:

HELLO WORLD
12

It returns the correct value, even with different text. Any explanations?

Answer 1

Yes, it is undefined behaviour .

If the ending } is reached, without a return , using the returned value is UB.

From C11 , chapter §6.9.1, Function definitions

If the } that terminates a function is reached, and the value of the function call is used by the caller, the behavior is undefined.

You should not really try to judge the UB by seeing the result of UB. It is, well, undefined , after all.

Answer 2

When you build your code, you may get a warning:

warning: no return statement in function returning non-void [-Wreturn-type]

In function

int hello(void)
{
    (void)printf("HELLO WORLD\n");
}

Because of no return statement. That's why you will get Undefined Behavior

Answer 3

While everyone points out the entire business is undefined behaviour, only one person answered to the actual question. However, there question is even more wrong and for some reason nobody tackles it.

Tried this on 3 different systems: CentOS, Ubuntu and Windows(MinGW) ; so I don't think this is an anomaly (undefined behavior)

This reasoning is seriously flawed (no amount of systems you test can tell you whether this is defined or not). Not only that, your testing was clearly superficial. On my system compiling the code with -O2 suddenly makes it print 0, and likely same things will happen on your system.

The reason it could have worked by accident is that on your system return value is kept in EAX which was not modified after return to main() and the compiler emitted code which moves the value from EAX to the first argument of another printf.

As was noted earlier and demonstrated with -O2 failure, this cannot be relied upon.

Finally a sentence which is even more wrong:

It returns the correct value, even with different text. Any explanations?

What?

Let's take a look at the function in question:

int hello(void)
{
        (void)printf("HELLO WORLD\n");
}

The function has no return statement, so whatever result you get is not "the correct value". In particular, you did not ask the language to return the value returned by printf. It so happened that in your testing you got the value returned by printf, but that's not what you asked for, hence it was not "correct" in any sense. Strictly speaking one could wonder if the return value of the function which was called last is guaranteed to be returned in such a case, but it was explained and demonstrated that no.

Answer 4

12 is exactly the return value of the first printf. Probably, the result of printf("HELLO WORLD\\n"); will be left on EAX register.

So, when second printf expect a return value it is the last returned value of a function( EAX ).