How to revoke SEM_UNDO of latest operation on semaphore?

问题

My problem: A,B - semaphores.

Process is loop:

1. waits for A >= 1, decrements A
2. does some computation
3. increments B.
4. back to 1.

Goal: After process terminates i need decrements of A be equal to increments of B.

So i figure out i will add flag SEM_UNDO in 1. but it undos every DEcrement ever made to A by that process. How to revoke / undo SEM_UNDO from 1. when process reaches 4.?

回答1:

ok, i was enlightened when thinking about problem as "undoing" SEM_UNDO rather then "revoking" it.

after calling semop() with operation +x with SEM_UNDO, you can later undo that SEM_UNDO by calling semop() again with two operations:

1. -x with SEM_UNDO
2. +x without SEM_UNDO

来源：https://stackoverflow.com/questions/21057454/how-to-revoke-sem-undo-of-latest-operation-on-semaphore