File locking provides a very simple yet incredibly useful mechanism for coordinating file accesses. ... Setting the lock consists of filling out a struct flock (declared in fcntl .h ) that describes the type of lock needed, open () ing the file with the matching mode, and calling fcntl () with the proper arguments, comme ?a: struct flock fl; int fd; fl.l_type = F_WRLCK; /* F_RDLCK, F_WRLCK, F_UNLCK */ fl.l_whence = SEEK_SET; /* SEEK_SET, SEEK_CUR, SEEK_END */ fl.l ... Locks rule. ...