PURPOSE: To facilitate the exclusive control of access by allowing the access to a shared file only when the use of lock file is permitted.

CONSTITUTION: A program A maintains a lock file R in a read mode for the time t0Wt1, reads/writes files 1Wn, and rewrites the lock file at the time t1 after the final file is updated. During that period, a program B will not receive any read request even if it is issued to any of the files 1Wn, maintaining them in a read request wait state. At the time t2 after the program A releases the lock file R, the program B reads the lock file R immediately. Then the program B starts updating the files, reading/writing the file, then file 1, then file 2 and so on. During this period t2Wt3, the program A will be waiting for a read request even if a file read request is issued.