DragonFly On-Line Manual Pages

SEMOP(2)                 DragonFly System Calls Manual                SEMOP(2)

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[], unsigned nops);

DESCRIPTION
     Semop() 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:

     o   When sem_op is positive, 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.

     o   The behavior when sem_op is negative 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 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.

         o   If the current value of the semaphore is less than sem_op's
             value, one of the following happens:

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

             o   If some other process has removed the semaphore with the
                 IPC_RMID option of semctl(), then semop() returns immediately
                 with a return value of EINVAL.

             o   Otherwise, the calling process is put to sleep until 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, and 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.

     o   When sem_op is zero, the process waits for the semaphore's value to
         become zero.  If it is already zero, the call to semop() can return
         immediately.  Otherwise, the calling process is put to sleep until
         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.

ENVIRONMENT
     The XSI Interprocess Communication family of functions is also available
     as an implementation in userspace.  To use it, the sysvipcd(8) daemon has
     to be running.

     If the USR_SYSVIPC variable is set in a process' environment, the process
     and its children will use the userspace implementation.

ERRORS
     Semop() will fail if:

     [EINVAL]           No semaphore set corresponds to semid.

     [EACCES]           Permission denied due to mismatch between operation
                        and mode of semaphore set.

     [EAGAIN]           The semaphore's value was less than sem_op, and
                        IPC_NOWAIT was specified.

     [E2BIG]            Too many operations were specified.

     [EFBIG]            sem_num was not in the range of valid semaphores for
                        the set.

SEE ALSO
     semctl(2), semget(2)

AUTHORS
     The DragonFly specific userspace implementation (see ENVIRONMENT) was
     written by Larisa Grigore.

DragonFly 4.3                   January 4, 2014                  DragonFly 4.3