Implement SHM event passing interface

[grandrew]

in order for helper to work we need to implement event passing interface.

implement multiple subscribers interface

[grandrew]

use condition variables and send events:

• http://stackoverflow.com/a/11473266
• https://computing.llnl.gov/tutorials/pthreads/#ConditionVariables
• http://stackoverflow.com/a/2783161
• http://pubs.opengroup.org/onlinepubs/009695399/functions/pthread_mutexattr_getpshared.html

pthread_mutex_lock(&count_mutex);
  while (count<COUNT_LIMIT) {
    pthread_cond_wait(&count_threshold_cv, &count_mutex);
    printf("watch_count(): thread %ld Condition signal received.\n", my_id);
    count += 125;
    printf("watch_count(): thread %ld count now = %d.\n", my_id, count);
    }
  pthread_mutex_unlock(&count_mutex);

etc.

a complete example:

/********************************************************
 * An example source module to accompany...
 *
 * "Using POSIX Threads: Programming with Pthreads"
 *     by Brad nichols, Dick Buttlar, Jackie Farrell
 *     O'Reilly & Associates, Inc.
 *
 ********************************************************
 * process_shared_mutex.c -- 
 *
 * Demonstrating the shared process mutex
 */

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/wait.h>

#include <pthread.h>

#ifndef _POSIX_THREAD_PROCESS_SHARED
#error This system does not support process shared mutex
#endif

int   shared_mem_id;
int   *shared_mem_ptr;

pthread_mutexattr_t mutex_shared_attr;
pthread_mutex_t *mptr;

extern int
main(void)
{
  pid_t  child_pid;
  int  status, rtn;

  /* initialize shared memory segment */
  if ((shared_mem_id = shmget(IPC_PRIVATE, 1*sizeof(pthread_mutex_t), 0660)) < 0)
    perror("shmget"), exit(1) ;

  if ((shared_mem_ptr = (int *)shmat(shared_mem_id, (void *)0, 0)) == NULL)
    perror("shmat"), exit(1);

  mptr = (pthread_mutex_t *)shared_mem_ptr;

  if (( rtn = pthread_mutexattr_init(&mutex_shared_attr)) != 0) 
    fprintf(stderr,"pthreas_mutexattr_init: %s",strerror(rtn)),exit(1);

  if (( rtn = pthread_mutexattr_setpshared(&mutex_shared_attr,PTHREAD_PROCESS_SHARED)) != 0)
    fprintf(stderr,"pthread_mutexattr_setpshared %s",strerror(rtn)),exit(1);

  if (( rtn = pthread_mutex_init(mptr, &mutex_shared_attr)) != 0)
    fprintf(stderr,"pthread_mutex_init %s",strerror(rtn)), exit(1);

  if ((child_pid = fork()) == 0) {
    /* first child */
    if ((rtn = pthread_mutex_lock(mptr)) != 0)
      fprintf(stderr,"child:pthread_mutex_lock %s",strerror(rtn)),exit(1);
    sleep(1);
    if ((rtn = pthread_mutex_unlock(mptr)) != 0)
      fprintf(stderr,"child:pthread_unmutex_lock %s",strerror(rtn)),exit(1);
    exit(0);
  } else {

    /* parent */ 
    sleep(1); 
    if ((rtn = pthread_mutex_lock(mptr)) != 0)
      fprintf(stderr,"parent:pthread_mutex_lock %d",strerror(rtn)),exit(1);
    if ((rtn = pthread_mutex_unlock(mptr)) != 0)
      fprintf(stderr,"child:pthread_unmutex_lock %d",strerror(rtn)),exit(1);
    wait(&status);
  }

  return 0;
}

[grandrew]

semaphores are perfectly suitable for the task of implementing a conditional variable so we will just add a semaphore for event notification.

[grandrew]

works and tested