fprintf doubt

[sdkumar00]

The following program doesn't "seem" to print "hello-out". While using fflush(0) does print out "hello-out". Wish to know why it does not without the fflush.

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

[Banfa]

The standard output string is normally line buffered. That means the system is not constrained to actually output the characters printed unless a new line is printed or the stream is flushed.

You print no newlines so the system is completely within its rights to hold that data in an internal buffer until such time as it is required to output it such as when you call fflush.

std error does not have the same behaviour because, since it is designed for debug messages that you normally want to see immediately, it is not buffer at all normally. Whatever is printed to std error gets printed immediately.