SEMOP(2)                    BSD System Calls Manual                   SEMOP(2)

NAME
     semop -- atomic array of operations on a semaphore set

SYNOPSIS
     ##include <>
     ##include <>
     ##include <>

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

DESCRIPTION
     The semop() system call atomically performs the array of operations indi-
     cated by array on the semaphore set indicated by semid.  The length of
     array is indicated by nops.  Each operation is encoded in a struct
     sembuf, which is defined as follows:

     struct sembuf {
             ushort semnum;        /* semaphore # */
             short   semop;         /* semaphore operation */
             short   semflg;        /* operation flags */
     };

     For each element in array, semop and semflg determine an operation to
     be performed on semaphore number semnum in the set.  The values SEMUNDO
     and IPCNOWAIT may be OR'ed into the semflg member in order to modify
     the behavior of the given operation.

     The operation performed depends as follows on the value of semop:

     ]o   When semop is positive and the process has alter permission, the
         semaphore's value is incremented by semop's value.  If SEMUNDO is
         specified, the semaphore's adjust on exit value is decremented by
         semop's value.  A positive value for semop generally corresponds to
         a process releasing a resource associated with the semaphore.

     ]o   The behavior when semop is negative and the process has alter per-
         mission, depends on the current value of the semaphore:

         ]o   If the current value of the semaphore is greater than or equal to
             the absolute value of semop, then the value is decremented by
             the absolute value of semop.  If SEMUNDO is specified, the sem-
             aphore's adjust on exit value is incremented by the absolute
             value of semop.

         ]o   If the current value of the semaphore is less than the absolute
             value of semop, one of the following happens:

             ]o   If IPCNOWAIT was specified, then semop() returns immediately
                 with a return value of EAGAIN.

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

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

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

                 ]o   The semaphore's value is greater than or equal to the
                     absolute value of semop.  When this condition becomes
                     true, the semaphore's value is decremented by the abso-
                     lute value of semop, the semaphore's adjust on exit
                     value is incremented by the absolute value of semop.

         A negative value for semop generally means that a process is waiting
         for a resource to become available.

     ]o   When semop is zero and the process has read permission, one of the
         following will occur:

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

         ]o   If IPCNOWAIT was specified, then semop() returns immediately
             with a return value of EAGAIN.

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

             ]o   Some other process removes the semaphore with the IPCRMID
                 option of semctl(2).  In this case, semop() returns immedi-
                 ately with a return value of EIDRM.

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

             ]o   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
     The semop() function returns the value 0 if successful; otherwise the
     value -1 is returned and the global variable errno is set to indicate the
     error.

ERORS
     The semop() system call will fail if:

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

     [EACES]           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 IPCNOWAIT was speci-
                        fied.

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

     [EFBIG]            semnum 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 SEMUNDO pool [SEMNU] is full.

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

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

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

BSD                           September 22, 1995                           BSD