semop() provides a number of atomic operations on a set of semaphores. The semaphore set is specified by
semid,
sops is an array of semaphore operations, and
nsops is the number of operations in this array. The
sembuf structures in the array contain the following members:
unsigned short sem_num; /* semaphore # */
short sem_op; /* semaphore operation */
short sem_flg; /* operation flags */
Each operation (specified in
sem_op) is applied to semaphore number
sem_num in the set of semaphores specified by
semid. The value of
sem_op determines the action taken in the following way:
•
sem_op 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 sem_op. The absolute value of sem_op is then subtracted from the value of the semaphore, and the calling process continues. Negative values of sem_op are thus used to enter critical regions.
•
sem_op is greater than 0. Its value is added to the value of the specified semaphore. This is used to leave critical regions.
•
sem_op 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
sem_flg in the following way:
IPC_NOWAIT
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 errno to EAGAIN.
SEM_UNDO
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.