Description

   The semop function performs operations on semaphores in the
   semaphore set specified by semid. These operations are supplied
   in a user-defined array of semaphore operation sembuf structures
   specified by sops. Each sembuf structure includes the following
   member variables:

   struct sembuf {            /* semaphore operation structure */
     unsigned short sem_num;  /* semaphore number */
              short sem_op;   /* semaphore operation */
              short sem_flg;  /* operation flags SEM_UNDO and IPC_NOWAIT */

   Each semaphore operation specified by the sem_op variable is
   performed on the corresponding semaphore specified by the semid
   function argument and the sem_num variable.

   The sem_op variable specifies one of three semaphore operations:

   1. If sem_op is a negative integer and the calling process has
      change permission, one of the following occurs:

      o  If semval (see <sem.h>) is greater than or equal to the
         absolute value of sem_op, the absolute value of sem_op
         is subtracted from semval. Also, if (sem_flg &SEM_UNDO)
         is non-zero, the absolute value of sem_op is added to the
         calling process' semadj value for the specified semaphore.

      o  If semval is less than the absolute value of sem_op
         and (sem_flg &IPC_NOWAIT) is nonzero, semop returns
         immediately.

      o  If semval is less than the absolute value of sem_op and
         (sem_flg &IPC_NOWAIT) is 0, semop increments the semncnt
         associated with the specified semaphore and suspends
         execution of the calling thread until one of the following
         conditions occurs:

         -  The value of semval becomes greater than or equal to the
            absolute value of sem_op. When this occurs, the value
            of semncnt associated with the specified semaphore is
            decremented, the absolute value of sem_op is subtracted
            from semval and, if (sem_flg &SEM_UNDO) is nonzero,
            the absolute value of sem_op is added to the calling
            process' semadj value for the specified semaphore.

         -  The semid for which the calling thread is awaiting
            action is removed from the system. When this occurs,
            errno is set equal to EIDRM and -1 is returned.

         -  The calling thread receives a signal that is to be
            intercepted. When this occurs, the value of semncnt
            associated with the specified semaphore is decremented,
            and the calling thread is resumes execution in the
            manner prescribed in sigaction.

   2. If sem_op is a positive integer and the calling process has
      change permission, the value of sem_op is added to semval
      and, if (sem_flg &SEM_UNDO) is nonzero, the value of sem_op
      is subtracted from the calling process' semadj value for the
      specified semaphore.

   3. If sem_op is 0 and the calling process has read permission,
      one of the following occurs:

      o  If semval is 0, semop returns immediately.

      o  If semval is nonzero and (sem_flg &IPC_NOWAIT) is nonzero,
         semop returns immediately.

      o  If semval is nonzero and (sem_flg &IPC_NOWAIT) is 0, semop
         increments the semzcnt associated with the specified
         semaphore and suspends execution of the calling thread
         until one of the following occurs:

         -  The value of semval becomes 0, at which time the value
            of semzcnt associated with the specified semaphore is
            decremented.

         -  The semid for which the calling thread is awaiting
            action is removed from the system. When this occurs,
            errno is set equal to EIDRM and -1 is returned.

         -  The calling thread receives a signal that is to be
            intercepted. When this occurs, the value of semzcnt
            associated with the specified semaphore is decremented,
            and the calling thread resumes execution in the manner
            prescribed in sigaction.

   On successful completion, the value of sempid for each semaphore
   specified in the array pointed to by sops is set equal to the
   process ID of the calling process.