SEMOP(2) BSD System Calls Manual SEMOP(2)
semop -- semaphore operations
semop(int semid, struct sembuf *sops, size_t nsops);
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 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:
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 from the
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 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 pro-
cesses waiting on a semaphore to block forever, should the
process that has the semaphore locked terminate in a criti-
Upon successful completion, the value 0 is returned; otherwise the
value -1 is returned and the global variable errno is set to indicate the
semop() will fail if:
[EINVAL] There is no semaphore associated with semid.
[EIDRM] 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 semaphore set.
[E2BIG] The value of nsops is too big. The maximum is speci-
fied in 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 per-
formed, and IPC_NOWAIT was set in sem_flg.
[EFAULT] sops points to an illegal address.
BSD April 29, 2017 BSD