semop - semaphore operations
semop(int semid, struct sembuf *sops, size_t nsops);
semop() provides a number of atomic operations on a set of
The semaphore set is specified by semid. sops is an array
operations, nsops is the number of operations in this array.
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
sem_num in the set of semaphores specified by semid. The
value of sem_op
determines the action taken in the following way:
+o 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
value of the semaphore, and the calling process continues. Negative
values of sem_op are thus used to enter critical regions.
+o sem_op is greater than 0. Its value is added to the
value of the
specified semaphore. This is used to leave critical regions.
+o 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:
IPC_NOWAIT In the case where the calling process would
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
process terminates. This is useful to prevent
waiting on a semaphore to block forever,
process that has the semaphore locked terminate
in a critical
Upon successful completion, a value of 0 is returned. Otherwise, -1 is
returned and the global variable errno is set to indicate
semop() will fail if:
[EINVAL] 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
[EACCES] The calling process has no permission to access the specified
[E2BIG] The value of nsops is too big. The maximum is
MAX_SOPS in <sys/sem.h>.
[EFBIG] 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.
[ENOSPC] SEM_UNDO was requested, and there is not
enough space left
in the kernel to store the undo information.
[EAGAIN] The requested operation can not immediately be
and IPC_NOWAIT was set in sem_flg.
[EFAULT] sops points to an illegal address.
In case of a removed semaphore identifier, errno should be
set to EIDRM,
but OpenBSD does not define this error.
OpenBSD 3.6 August 17, 1995
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