47.8. Semaphore Undo Values

Suppose that, having adjusted the value of a semaphore (e.g., decreased the semaphore value so that it is now 0), a process then terminates, either deliberately or accidentally. By default, the semaphore’s value is left unchanged. This may constitute a problem for other processes using the semaphore, since they may be blocked waiting on that semaphore—that is, waiting for the now-terminated process to undo the change it made.

To avoid such problems, we can employ the SEM_UNDO flag when changing the value of a semaphore via semop(). When this flag is specified, the kernel records the effect of the semaphore operation, and then undoes the operation if the process terminates. The undo happens regardless of whether the process terminates normally or abnormally.