printf prints to standard output, which is line-buffered by default when reading from a tty on UNIX variants. This can lead to strange problems for those who try to use it to track which parts of a program are being executed, as demonstrated by the following program with an infinite loop:

  #include <stdio.h>
  int main(int c, char **v) {
    printf("Hello\nJello");
    while(1) ;
    return 0;
  }