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SEMOP(2) System Calls Manual SEMOP(2)

semopatomic array of operations on a semaphore set

library “libc”

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>

int
semop(int semid, struct sembuf array[], unsigned nops);

() 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 '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:

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 voluntarily 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.

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.

The XSI Interprocess Communication family of functions is also available as an implementation in userspace. To use it, the sysvipcd(8) daemon has to be running.

If the USR_SYSVIPC variable is set in a process' environment, the process and its children will use the userspace implementation.

Semop() will fail if:

[]
No semaphore set corresponds to semid.
[]
Permission denied due to mismatch between operation and mode of semaphore set.
[]
The semaphore's value was less than sem_op, and IPC_NOWAIT was specified.
[]
Too many operations were specified.
[]
sem_num was not in the range of valid semaphores for the set.

semctl(2), semget(2)

The DragonFly specific userspace implementation (see ENVIRONMENT) was written by Larisa Grigore.

January 4, 2014 DragonFly-5.6.1