The UNIX fork service is a function that creates a new process (child process), which is almost an exact copy of the calling process (parent process). Do not use the z/OS UNIX fork service while a DCB or ACB is open to a UNIX file. The fork service creates a child process that is a duplicate of the calling (parent) process; however, the service does not duplicate various MVS control blocks, which creates unpredictable results in the child process.