Each operation (specified in is applied to semaphore number in the set of semaphores specified by The value of determines the action taken in the following way: is less than 0. The current process is blocked until the value of the semaphore is greater than or equal to the absolute value of The absolute value of is then subtracted from the value of the semaphore, and the calling process continues. Negative values of are thus used to enter critical regions. is greater than 0. Its value is added to the value of the specified semaphore. This is used to leave critical regions. is equal to 0. The calling process is blocked until the value of the specified semaphore reaches 0.
The behaviour of each operation is influenced by the flags set in in the following way: In the case where the calling process would normally block, waiting for a semaphore to reach a certain value, IPC_NOWAIT makes the call return immediately, returning a value of -1 and setting to EAGAIN. Keep track of the changes that this call makes to the value of a semaphore, so that they can be undone when the calling process terminates. This is useful to prevent other processes waiting on a semaphore to block forever, should the process that has the semaphore locked terminate in a critical section.
The semaphore set was removed while the process was waiting for one of its semaphores to reach a certain value.
The calling process has no permission to access the specified semaphore set. The value of is too big. The maximum is specified in MAX_SOPS in <sys/sem.h> in one of the sem_buf structures is less than 0, or greater than the actual number of semaphores in the set specified by SEM_UNDO was requested, and there is not enough space left in the kernel to store the unfo information. The requested operation can not immediately be performed, and IPC_NOWAIT was set in points to an illegal address.