Why does printf not flush after the call unless a newline is in the format string?

The stdout stream is line buffered by default, so will only display what’s in the buffer after it reaches a newline (or when it’s told to). You have a few options to print immediately:

• Print to stderrinstead using fprintf (stderr is unbuffered by default):

  fprintf(stderr, "I will be printed immediately");
  

• Flush stdout whenever you need it to using fflush:

  printf("Buffered, will be flushed");
  fflush(stdout); // Will now print everything in the stdout buffer
  

• Disable buffering on stdout by using setbuf:

  setbuf(stdout, NULL);
  

• Or use the more flexible setvbuf:

  setvbuf(stdout, NULL, _IONBF, 0);