int functions with no return statement

Question

In a course I am taking on embedded systems, I program LPC1768s which are based on ARM using C.

The main functions are always of int return type. However, there are no return statements in the end like a return 0.

I believe this may have something to do with the infinite while loops that these programs use because the controller is never really able to reach the return statement. However, I thought that as this is C, there might be errors, though it turned out that I was wrong.

Just for reference, here is the main function of a program trying to use UART

int main (void) 
{
    SystemInit();

    GPIO_PinDirection(LED1,OUTPUT);        /* Configure the pins as Output to blink the Leds*/
    GPIO_PinDirection(LED2,OUTPUT);

    TIMER_Init(0,100000);                  /* Configure timer0 to generate 100ms(100000us) delay*/
    TIMER_Init(1,500000);                  /* Configure timer1 to generate 500ms(500000us) delay*/

    TIMER_AttachInterrupt(0,myTimerIsr_0);  /* myTimerIsr_0 will be called by TIMER0_IRQn */
    TIMER_AttachInterrupt(1,myTimerIsr_1);  /* myTimerIsr_1 will be called by TIMER1_IRQn */

    TIMER_Start(0);                         /* Start the Timers */
    TIMER_Start(1);

    while(1)
    {
        //do nothing
    }
}

Answer 1

It really doesn't matter in practice what main() says it will return in this instance. Unless you enable strict C in your compiler flags, the compiler probably won't care what return type you gave it (so long as it is a supported format in said compiler).

Why does main() historically return int? Because the return value is supposed to tell the caller if the program completed successfully.

In this case, it will never return and also should never return, since there is almost certainly nothing set up to handle the main() function returning. Thus the return value is completely irrelevant. If you want it to return void, then do that. It won't make any real difference. Your compiler might generate slightly different assembly code, which may even be slightly less/more efficient than the code it would generate if you used int.

A void return type is supposed to define a function that 'returns normally' but doesn't provide a result. In this case main() does not return normally, it in fact never returns. C11 does actually provide a standard for defining a function that doesn't return (_Noreturn). I wouldn't want to bet on your compiler actually supporting it though. If it did, then the most correct deceleration would be: _Noreturn void main(void).

C also specifically declares that execution reaching the end of main() without encountering a 'return' statement will return a value of 0. So it's not a syntax error to have no return statement in your main() function.

I don't think this is super relevant to the OPs question. But as pointed out in the comments, it's not strictly true that the definition doesn't matter. It wont change the logic of your program, but it certainly could have a tiny impact on how efficient the generated code is. If you are worried about this then you need to find out what your specific compiler generates for the various definitions of main() that it supports.

Answer 2

The C standard allows two forms of environments: freestanding and hosted.

• Freestanding systems refers to systems without a file managing OS, such as microcontroller applications, RTOS applications, applications that are the OS.
• Hosted systems refer to programs running on top of an OS: Linux, Windows etc.

Hosted systems, and only hosted systems, have a special requirement that they must support the standard form int main (void). So when we have the flood of PC programmers entering the branch of embedded systems programming, they bring religious dogmas about main with them, because they only know the hosted system requirements.

But since almost all embedded systems are freestanding systems, we can completely ignore hosted systems here on the E.E site. This is what the standard says:

ISO 9899:2018 5.1.2.1 Freestanding environment

In a freestanding environment (in which C program execution may take place without any benefit of an operating system), the name and type of the function called at program startup are implementation-defined.

Implementation-defined means that the compiler decides which forms that are allowed. The most common form is void main (void). Some systems don't even have main, others may name it differently. What's important to realize is that the compiler makes this decision, never the programmer.

The problem with using int main (void), if supported by the compiler, is that it almost certainly generates calling convention overhead. The reset vector and CRT will call main and push data onto the stack, that just sits there as dead space. Various dirty hacks (like manually resetting SP) can dodge this waste, but generally int main (void) is bad form in freestanding systems, since it is wasteful and senseless. What are you going to return to?

If there is a return 0 or not in the end doesn't matter. In standard C, int main(void) is a special function and if missing a return statement, it is equivalent to writing return 0. Only the int main (void) function has this special rule, not ordinary functions.

So what you should do is to check which forms of main() your compiler allows, then pick one without a return statement. For example, gcc supports the preferred form void main (void) if you compile with gcc -ffreestanding.

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For more information:

I wrote a detailed answer on Stack Overflow, addressing both freestanding and hosted systems, in all versions of the C and C++ standards (at the point when it was written): What should main() return in C and C++?

Answer 3

Suppose you were to put

return 0;

after the

while (1) {}

How would you expect it to change the behavior of your program?