Why does the print to stdout not happen when using fprintf?

Question

I have this code:

#include <stdio.h>
#include <unistd.h>
int main()
{
        while(1)
        {
                fprintf(stdout,"hello-out");
                fprintf(stderr,"hello-err");
                sleep(1);
        }
        return 0;
}

The output is hello-err hello-err hello-err hello-err hello-err hello-err at 1 sec intervals. I want to know why hello-out never gets printed.

Answer 1

You need to fflush stdout because usually stdout is line buffered and you don't issue a new line character in your program.

            fprintf(stdout,"hello-out");
            fflush(stdout);

stderr is not fully buffered by default so you don't need to fflush it.

Answer 2

stdout is line-buffered by default, meaning that the buffer will be flushed at every end-of-line ('\n'). stderr is unbuffured, so every character is sent automatically without the need to flush.

You can confirm this by placing a \n at the end of the stdout output. That way both lines will be printed at 1 second intervals.