SEMOP(2) BSD System Calls Manual SEMOP(2)NAME
semop — atomic array of operations on a semaphore set
LIBRARY
Standard C Library (libc, -lc)
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:
· 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.
· The behavior when sem_op is negative depends on the current value of
the semaphore:
· 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 sem‐
aphore's adjust on exit value is incremented by the absolute
value of sem_op.
· If the current value of the semaphore is less than sem_op's
value, one of the following happens:
· If IPC_NOWAIT was specified, then semop() returns immediately
with a return value of EAGAIN.
· 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.
· Otherwise, the calling process is put to sleep until the sem‐
aphore'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 incremented by the abso‐
lute value of sem_op.
A negative value for sem_op generally means that a process is
waiting for a resource to become available.
· 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
resource is released if a process terminates unexpectedly.
RETURN VALUES
The semop() function returns the value 0 if successful; otherwise the
value -1 is returned and the global variable errno is set to indicate the
error.
ERRORSSemop() 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.
[E2BIG] Too many operations were specified.
[EFBIG] sem_num was not in the range of valid semaphores for
the set.
SEE ALSOsemctl(2), semget(2)BSD September 22, 1995 BSD