semid, struct sembuf
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, nsops is the
number of operations in this array. The sembuf
structures in the array contain the following members:
u_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 behavior of each operation is influenced by the flags set in sem_flg in the following way:
- In the case where the calling process would normally block, waiting for a
semaphore to reach a certain value,
IPC_NOWAITmakes the call return immediately, returning a value of -1 and setting errno to
- 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.
Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error.
semop() will fail if:
- There is no semaphore associated with semid.
- 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 nsops is too big. The maximum is
specified in MAX_SOPS in
- sem_num in one of the sem_buf structures is less than 0, or greater than the actual number of semaphores in the set specified by semid.
SEM_UNDOwas requested, and there is not enough space left in the kernel to store the undo information.
- The requested operation cannot immediately be performed, and
IPC_NOWAITwas set in sem_flg.
- sops points to an illegal address.