Sleep and output flush

I saw the snippet below somewhere online. The following program doesnt print "hello-out"开发者_StackOverflow中文版. The reason I feel is because before it is actually flushed to the console from the buffer,

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

Is my reason correct? If not what is the correct reason?

You should put \n at the end of the string to have it flushed, or use fflush to force it out.

The reason the following fix is because I do know that stdout is line-buffered by default, whereas stderr is not.

#include <stdio.h>
#include <unistd.h>

int main()
{
    while(1)
    {
        fprintf(stdout,"hello-out");
        fflush(stdout);
        // sleep(1);
        fprintf(stderr,"hello-err");
        sleep(1);
    }
    return 0;
}

Update another way to 'fix' it without changing the source code is by using e.g. stdbuf:

 stdbuf -o 0 ./test

which switches standard output to unbuffer mode as well. Interestingly, if you specify

 stdbuf -e L ./test

to make stderr linebuffered as well, no output will appear at all (until the first lineend gets printed)