semop, semtimedop - System V semaphore operations

#include <sys/sem.h>
int semop(int semid, struct sembuf *sops, size_t nsops);
int semtimedop(int semid, struct sembuf *sops, size_t nsops,
 const struct timespec *_Nullable timeout);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

    _GNU_SOURCE

Each semaphore in a System V semaphore set has the following associated values:

unsigned short  semval;   /* semaphore value */
unsigned short  semzcnt;  /* # waiting for zero */
unsigned short  semncnt;  /* # waiting for increase */
pid_t           sempid;   /* PID of process that last
                             modified the semaphore value */

semop() performs operations on selected semaphores in the set indicated by semid. Each of the nsops elements in the array pointed to by sops is a structure that specifies an operation to be performed on a single semaphore. The elements of this structure are of type struct sembuf, containing the following members:

unsigned short sem_num;  /* semaphore number */
short          sem_op;   /* semaphore operation */
short          sem_flg;  /* operation flags */

Flags recognized in sem_flg are IPC_NOWAIT and SEM_UNDO. If an operation specifies SEM_UNDO, it will be automatically undone when the process terminates.

The set of operations contained in sops is performed in array order, and atomically, that is, the operations are performed either as a complete unit, or not at all. The behavior of the system call if not all operations can be performed immediately depends on the presence of the IPC_NOWAIT flag in the individual sem_flg fields, as noted below.

Each operation is performed on the sem_num-th semaphore of the semaphore set, where the first semaphore of the set is numbered 0. There are three types of operation, distinguished by the value of sem_op.

If sem_op is a positive integer, the operation adds this value to the semaphore value (semval). Furthermore, if SEM_UNDO is specified for this operation, the system subtracts the value sem_op from the semaphore adjustment (semadj) value for this semaphore. This operation can always proceed—it never forces a thread to wait. The calling process must have alter permission on the semaphore set.

If sem_op is zero, the process must have read permission on the semaphore set. This is a "wait-for-zero" operation: if semval is zero, the operation can immediately proceed. Otherwise, if IPC_NOWAIT is specified in sem_flg, semop() fails with errno set to EAGAIN (and none of the operations in sops is performed). Otherwise, semzcnt (the count of threads waiting until this semaphore's value becomes zero) is incremented by one and the thread sleeps until one of the following occurs:

• semval becomes 0, at which time the value of semzcnt is decremented.

• The semaphore set is removed: semop() fails, with errno set to EIDRM.

• The calling thread catches a signal: the value of semzcnt is decremented and semop() fails, with errno set to EINTR.

If sem_op is less than zero, the process must have alter permission on the semaphore set. If semval is greater than or equal to the absolute value of sem_op, the operation can proceed immediately: the absolute value of sem_op is subtracted from semval, and, if SEM_UNDO is specified for this operation, the system adds the absolute value of sem_op to the semaphore adjustment (semadj) value for this semaphore. If the absolute value of sem_op is greater than semval, and IPC_NOWAIT is specified in sem_flg, semop() fails, with errno set to EAGAIN (and none of the operations in sops is performed). Otherwise, semncnt (the counter of threads waiting for this semaphore's value to increase) is incremented by one and the thread sleeps until one of the following occurs:

• semval becomes greater than or equal to the absolute value of sem_op: the operation now proceeds, as described above.

• The semaphore set is removed from the system: semop() fails, with errno set to EIDRM.

• The calling thread catches a signal: the value of semncnt is decremented and semop() fails, with errno set to EINTR.

On successful completion, the sempid value for each semaphore specified in the array pointed to by sops is set to the caller's process ID. In addition, the sem_otime is set to the current time.

semtimedop()

semtimedop() behaves identically to semop() except that in those cases where the calling thread would sleep, the duration of that sleep is limited by the amount of elapsed time specified by the timespec structure whose address is passed in the timeout argument. (This sleep interval will be rounded up to the system clock granularity, and kernel scheduling delays mean that the interval may overrun by a small amount.) If the specified time limit has been reached, semtimedop() fails with errno set to EAGAIN (and none of the operations in sops is performed). If the timeout argument is NULL, then semtimedop() behaves exactly like semop().

Note that if semtimedop() is interrupted by a signal, causing the call to fail with the error EINTR, the contents of timeout are left unchanged.

On success, semop() and semtimedop() return 0. On failure, they return -1, and set errno to indicate the error.

The following code segment uses semop() to atomically wait for the value of semaphore 0 to become zero, and then increment the semaphore value by one.

struct sembuf sops[2];
int semid;
/* Code to set semid omitted */
sops[0].sem_num = 0;        /* Operate on semaphore 0 */
sops[0].sem_op = 0;         /* Wait for value to equal 0 */
sops[0].sem_flg = 0;
sops[1].sem_num = 0;        /* Operate on semaphore 0 */
sops[1].sem_op = 1;         /* Increment value by one */
sops[1].sem_flg = 0;
if (semop(semid, sops, 2) == -1) {
    perror("semop");
    exit(EXIT_FAILURE);
}

A further example of the use of semop() can be found in shmop(2).

clone(2), semctl(2), semget(2), sigaction(2), capabilities(7), sem_overview(7), sysvipc(7), time(7)