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
     ensure 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.