flock() (idea)

A C function that allows the program to request a shared or exclusive lock on a file. Returns 0 on sucess and -1 on failure.

  #define LOCK_SH 1       /* shared lock */
  #define LOCK_EX 2       /* exclusive lock */
  #define LOCK_NB 4       /* don't block when locking */
  #define LOCK_UN 8       /* unlock */
  int flock(
          int filedes,
          int operation );

(from the flock() man page)

Where filedes is a file descriptor returned by open() or fcntl() and operation is one of the constants defined in the above block.