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NAME

     semop - atomic array of operations on a semaphore set

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

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

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

DESCRIPTION

     The semop() system call atomically performs the array of operations
     indicated 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 {
             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 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.

ERRORS

     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 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

     semctl(2), semget(2), sigaction(2)

BUGS

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