why doesn't this c programme print the first printf statement?

Question

#include<stdio.h>
#include <unistd.h>
int main(){
      while(1)
      {

              fprintf(stdout,"hello-out");
              fprintf(stderr,"hello-err");
              sleep(1);
      }
      return 0;
}

On compiling this programme in gcc and on executing it only prints hello-err and not hello-out.Why is that so?Can someone please explain the reason behind it?

Answer 1

If you add a ' ' to your message it will (or should), ie. "hello-out ".

The reason being is that stdout is buffered in order to be more efficient, whereas stderr doesn't buffer it's output and is more appropriate for error messages and things that need to be printed immediately.

stdout will usually be flushed when:

• A newline ( ) is to be printed
• You read in from stdin
• fflush() is called on it

EDIT: The other thing I wanted to add before my computer crashed...twice...was that you can also use setbuf(stdout, NULL); to disable buffering of stdout. I've done that before when I've had to use write() (Unix) and didn't want my output to be buffered.