printf flush at program exit

Question

I'm interested in knowing how the printf() function's flush works when the program exits.

Let's take the following code:

int main(int ac, char **av)
{
    printf("Hi");
    return 0;
}

In this case, how does printf() manage to flush its buffer to stdout?

I guess it's platform dependent, so let's take Linux.

It could be implemented using gcc's __attribute__((dtor)) but then the standard library would be compiler dependent. I assume this is not the way it works.

Any explanations or links to documentation is appreciated. Thank you.

Answer 1

When the program exits normally, the exit function has always performed a clean shutdown of the standard I/O library, this causes all buffered output data to be flushed.

Returning an integer value from the main function is equivalent to calling exit with the same value.So, return 0 has the same effect with exit(0)

If _Exit or _exit was called, the process will be terminated immediately, the IO won't be flushed.