Semop: When decreasing a set of semaphores are all decremented at once or does it block on first failure?
No updates happen until all updates can proceed as a unit.
The POSIX specification could be clearer about this point, although it does say that semop is atomic.
On Linux, semop(3) in glibc is a simple wrapper around semop(2). The semop(2) manpage in turn says
The set of operations contained in
sopsis performed in array order, and atomically, that is, the operations are performed either as a complete unit, or not at all.
The HP-UX semop(2) manpage is even clearer:
Semaphore array operations are atomic in that none of the semaphore operations are performed until blocking conditions on all of the semaphores in the array have been removed.