SEMOP(2) BSD Programmer's Manual SEMOP(2)NAMEsemop - atomic array of operations on a semaphore set
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int
semop(int semid, struct sembuf array[], unsigned nops);
DESCRIPTIONSemop() atomically performs the array of operations indicated by array on
the semaphore set indicated by semid. The length of array is indicated by
nops. Each operation is encoded in a struct sembuf, which is defined as
follows:
struct sembuf {
u_short sem_num; /* semaphore # */
short sem_op; /* semaphore operation */
short sem_flg; /* operation flags */
};
For each element in array, sem_op and sem_flg determine an operation to
be performed on semaphore number sem_num in the set. The values SEM_UNDO
and IPC_NOWAIT may be OR'ed into the sem_flg member in order to modify
the behavior of the given operation.
The operation performed depends as follows on the value of sem_op:
o When sem_op is positive, the semaphore's value is incremented by
sem_op's value. If SEM_UNDO is specified, the semaphore's adjust on
exit value is decremented by sem_op's value. A positive value for
sem_op generally corresponds to a process releasing a resource asso-
ciated with the semaphore.
o The behavior when sem_op is negative depends on the current value of
the semaphore:
o If the current value of the semaphore is greater than or equal to
the absolute value of sem_op, then the value is decremented by
the absolute value of sem_op. If SEM_UNDO is specified, the
semaphore's adjust on exit value is incremented by the absolute
value of sem_op.
o If the current value of the semaphore is less than sem_op's value
one of the following happens:
o If IPC_NOWAIT was specified, then semop() returns immediately
with a return value of EAGAIN.
o If some other process has removed the semaphore with the
IPC_RMID option of semctl(), then semop() returns immediately
with a return value of EINVAL.
o Otherwise, the calling process is put to sleep until the
semaphore's value is greater than or equal to the absolute
value of sem_op. When this condition becomes true, the
semaphore's value is decremented by the absolute value of
sem_op, and the semaphore's adjust on exit value is incre-
mented by the absolute value of sem_op.
A negative value for sem_op generally means that a process is
waiting for a resource to become available.
o When sem_op is zero, the process waits for the semaphore's value to
become zero. If it is already zero, the call to semop() can return
immediately. Otherwise, the calling process is put to sleep until
the semaphore's value becomes zero.
For each semaphore a process has in use, the kernel maintains an `adjust
on exit' value, as alluded to earlier. When a process exits, either vol-
untarily or involuntarily, the adjust on exit value for each semaphore is
added to the semaphore's value. This can be used to insure that a re-
source is released if a process terminates unexpectedly.
RETURN VALUES
On success, semop() returns 0; otherwise, -1 is returned and errno is set
to indicate the error.
ERRORS
The semop() function will fail if:
[EINVAL] No semaphore set corresponds to semid.
[EACCES] Permission denied due to mismatch between operation and
mode of semaphore set.
[EAGAIN] The semaphore's value was less than sem_op, and IPC_NOWAIT
was specified.
[EINTR] An interrupt was received.
[E2BIG] Too many operations were specified.
[EFBIG] The sem_num argument was not in the range of valid
semaphores for the set.
SEE ALSOsemget(2), semctl(2)4th Berkeley Distribution July 3, 1996 2
