Complex example

Using the saved uid to get back to root later:

  setresuid( getuid(), getuid(), 0 );   /* become user, with saved uid */

  /* do something as user */
  system("/some/program");
  fd = open("/some/file", O_CREAT | O_TRUNC | O_EXCL, 0666);

  setresuid( 0, 0, 0 );                 /* get back our full root privs */

  /* do something as root */
  system("/usr/bin/id");

This method is prone to mistakes when you try to manipulate id's back and forth frequently.

(And using system is a big big no-no.)