semid, struct sembuf
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,
nsops is the number of operations in this array. The
sembuf structures in the array contain the following
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
- 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
- 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:
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.
- In the case where the calling process would normally block, waiting for a
semaphore to reach a certain value,
makes the call return immediately, returning a value of -1 and setting
- 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
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
- 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.
was 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_NOWAIT was set in
- sops points to an illegal address.