MSET(2)                         Domain/OS SysV                         MSET(2)



NAME
     mset, mclear - set and clear semaphores for processes mapped in virtual
     memory

SYNOPSIS
     #include <sys/mman.h>

     value = mset(sem, wait)
     int value, wait;
     semaphore *sem;

     mclear(sem)
     semaphore *sem;

DESCRIPTION
     Domain/OS SysV provides primitives for synchronizing processes in virtual
     memory using semaphores in shared memory.  Semaphores must lie within a
     MAP_SHARED region with at least the modes PROT_READ and PROT_WRITE (see
     mmap(2)).  The MAP_HASSEMAPHORE flag must have been specified when the
     region was created.

     mset indivisibly tests and sets the semaphore sem.  sem must have been
     initialized to zero before the first call to mset.  If sem is 0 (false)
     at the time of the call to mset, the process will acquire the lock and
     mset returns true immediately.

     The kernel does not check for deadlocks.  mset does not block any signals
     while the semaphore is held.  The application must ensure that locked
     semaphores are cleaned up correctly in the event the process terminates
     abnormally.  If a signal is caught while the process is blocked in mset,
     mset is always restarted.

     If sem is true at the time of the call to mset and wait is false, mset
     returns false.  If wait is true, the "want" flag is set and the test-
     and-set is retried.  If the lock is still unavailable, mset relinquishes
     the processor until notified that it should retry.

     mclear indivisibly tests and clears the semaphore sem . If the "want"
     flag is false at the time of the call to mclear, mclear returns true
     immediately.  If the "want" flag is true at the time of the call to
     mclear, it arranges for waiting processes to retry before returning.

NOTES
     This implementation is based upon a proposal by McKusick and Karels, and
     upon other sources.  Its details may change in a future release.

SEE ALSO
     McKusick, M.K. and Karels, M.J.  A New Virtual Memory Implementation for
          Berkeley UNIX.  Berkeley, California:  Department of Electrical
          Engineering and Computer Science, University of California, 1986.
     madvise(2), mmap(2), mprotect(2).