SEMOP(2)

NAME
semop - atomic array of operations on a semaphore set

SYNOPSIS
#include <sys/sem.h>
int
semop(int semid, struct sembuf *sops, size_t nsops);

DESCRIPTION
The semop() system call atomically performs the array of operations indicated by sops on the semaphore set indicated by semid. The length of sops is indicated by nsops. 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 sops, 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 and the process has alter permission, 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 associated with the semaphore.

•   The behavior when sem_op is negative and the process has alter permission, 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 semaphore's adjust on exit value is incremented by the absolute value of sem_op.

    •   If the current value of the semaphore is less than the absolute value of sem_op, one of the following happens:

        •   If IPC_NOWAIT was specified, then semop() returns immediately with a return value of EAGAIN.

        •   Otherwise, the calling process is put to sleep until one of the following conditions is satisfied:

            •   Some other process removes the semaphore with the IPC_RMID option of semctl(2). In this case, semop() returns immediately with a return value of EIDRM.

            •   The process receives a signal that is to be caught. In this case, the process will resume execution as defined by sigaction(2).

            •   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, the semaphore's adjust on exit value is incremented 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.

•   When sem_op is zero and the process has read permission, one of the following will occur:

    •   If the current value of the semaphore is equal to zero then semop() can return immediately.

    •   If IPC_NOWAIT was specified, then semop() returns immediately with a return value of EAGAIN.

    •   Otherwise, the calling process is put to sleep until one of the following conditions is satisfied:

        •   Some other process removes the semaphore with the IPC_RMID option of semctl(2). In this case, semop() returns immediately with a return value of EIDRM.

        •   The process receives a signal that is to be caught. In this case, the process will resume execution as defined by sigaction(2).

        •   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 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.

RETURN VALUES

ERRORS
The semop() system call will fail if:

[E2BIG]
    Too many operations are specified. [SEMOPM]

[EACCES]
    Permission is denied, due to a mismatch between the operation and the mode of the semaphore set.

[EAGAIN]
    The semaphore's value would result in the process being put to sleep and IPC_NOWAIT is specified.

[EFBIG]
    sem_num is not in the range of valid semaphores for the set.

[EIDRM]
    The semaphore set is removed from the system.

[EINTR]
    The semop() system call is interrupted by a signal.

[EINVAL]
    No semaphore set corresponds to semid, or the process would exceed the system-defined limit for the number of per-process SEM_UNDO structures.

[ENOSPC]
    The system SEM_UNDO pool [SEMMNU] is full.

[ERANGE]
    The requested operation would cause either the semaphore's current value [SEMVMX] or its adjust-on-exit value [SEMAEM] to exceed the system-imposed limits.

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

The include files <sys/types.h> and <sys/ipc.h> are necessary.

SEE ALSO
semctl(2), semget(2), sigaction(2), compat(5)

BUGS
The semop() system call may block waiting for memory even if IPC_NOWAIT was specified.