NAME

LIBRARY

SYNOPSIS

int
semop(int semid, struct sembuf *array, size_t nops);

DESCRIPTION

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 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 ensure that a resource is released if a process terminates unexpectedly.

RETURN VALUES

ERRORS

[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.

[EACCES]

Permission denied due to mismatch between operation and mode of semaphore set.

[EAGAIN]

The semaphore's value would have resulted in the process being put to sleep and IPC_NOWAIT was specified.

[E2BIG]

Too many operations were specified. [SEMOPM]

[EFBIG]

sem_num was not in the range of valid semaphores for the set.

[EIDRM]

The semaphore set was removed from the system.

[EINTR]

The semop() system call was interrupted by a signal.

[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.

SEE ALSO

BUGS