Event Counters and Sequencers(3)

Synopsis

cc [ flag ... ] file ... -lunify -levents -lpvm3 -lgpvm3 -lthread -ll -lsocket -lnsl -lw -lposix4 -lucb [ library ... ]

#include "unify.h"

Description

Event counters and sequencers are abstract data types that provide scalable, efficient synchronization for distributed programs. Event counters allow Create_Event(3), Destroy_Event(3), Advance(3), Advance_N(3), Await(3), Await_with_Timeout(3), and Read_Count(3) operations. Conceptually, the data part of an event counter is just an integer count. When an event counter is created, count is initialized to 0. Advance(3) increments count. Advance_N(3) increments count by n. Await(3) takes an integer as a parameter and blocks until count becomes greater than or equal to the integer it is passed. Await_with_Timeout(3) takes an integer and a timeval structure containing a timeout as parameters and blocks until count becomes greater than or equal to the integer it is passed or timeout is expired. Returns the count when unblocking occured. Read_Count(3) returns a number that is not greater than count.

Event counters alone are not general enough to implement all interesting forms of synchronization. Sequencers are needed to provide serialization. That is they can be used to order operations. Create_Sequencer(3), Destroy_Sequencer(3), and Get_Ticket(3) are used to operate on sequencers. Each sequencer has a count variable which is initialized to zero. When a process calls Get_Ticket(3), the sequencer atomically increments count and returns the new count to the calling process.

Examples

The following code shows how to implement a simple semaphore using event counters and sequencers without error checking. Application programs should check return codes to verify correct operation.

typdef struct semtype_t
{
  int event_id;
  int seq_id;
} SemType;

void SemaphoreInit(SemType *sem, unsigned int count)
{
  sem->event = Create_Event();
  sem->seq = Create_Sequencer();
  while(count >= 0) {
    Advance(sem->event);
    count--;
  }
}

void SemaphoreDestroy(SemType *sem)
{
  DestroyEvent(sem->event);
  DestorySequencer(sem->seq);
}

void SemaphoreSignal(SemType *sem)
{
  Advance(sem->event);
}

void SemaphoreWait(SemType *sem)
{
  Await(GetTicket(sem->seq));
}

See Also

Advance(3), Advance_N(3), Await(3), Await_with_Timeout(3), Create_Event(3), Destroy_Event(3), Read_Count(3), Create_Sequencer(3), Destroy_Sequencer(3), Get_Ticket(3), unifyintro(3)