Standard I/O

• The standard C library provides standard input and output streams (stdin and stdout, respectively).
• The C library also provides stderr, the standard error stream. Programs should print warning and error messages to standard error instead of standard output.
• The fprintf function can be used to print to stderr, for example:

  fprintf (stderr, ("Error: ..."));
  

• Note that stdout is buffered. Data written to stdout is not sent to the console (or other device, if it's redirected) until the buffer fills, the program exits normally, or stdout is closed.You can explicitly flush the buffer by calling the following:

  fflush (stdout);
  

  In contrast, stderr is not buffered; data written to stderr goes directly to the console.
• For example, this loop does not print one period every second; instead, the periods are buffered, and a bunch of them are printed together when the buffer fills.

  while (1) {
  printf (".");
  sleep (1);
  }
  

  In this loop, however, the periods do appear once a second:

  while (1) {
  fprintf (stderr, ".");
  sleep (1);
  }