VMS Help  —  CRTL  semop  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.
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