shared_memory.cpp

/* m.cpp */

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/wait.h>
#include <sys/mman.h>
#include <pthread.h>

int main()
{
   pthread_mutexattr_t attributes;

   pthread_mutexattr_init(&attributes);
   pthread_mutexattr_setpshared(&attributes, PTHREAD_PROCESS_SHARED);

   int handle = shm_open("/shm", O_CREAT | O_RDWR, 0777);
   ftruncate(handle, 2048*sizeof(int));
   char *memory = (char *)mmap(0, 2048*sizeof(int), PROT_READ | PROT_WRITE,
                       MAP_SHARED, handle, 0);

   // mutex share
   pthread_mutex_t *mutex = (pthread_mutex_t*)memory;
   pthread_mutex_init(mutex, &attributes);

   pthread_mutexattr_destroy(&attributes);

   // variable share
   int *count = (int*)(memory + sizeof(pthread_mutex_t));
   *count = 0;
   printf("Initial count = %d\n", *count);

   int value_returned_from_child = 0;
   pid_t pid = fork();
   if(pid == 0) {
      pthread_mutex_lock(mutex);
      (*count)++;
      printf("Child process increased the count to %d\n", *count);
      pthread_mutex_unlock(mutex);
      value_returned_from_child = 99;
   }
   else {
      int status;
      // waiting for the child process to finish
      waitpid(pid, &status, 0);
      value_returned_from_child = WEXITSTATUS(status);
      printf("value_returned_from_child = %d\n", value_returned_from_child);
      pthread_mutex_lock(mutex);
      (*count)++;
      printf("Parent process increased the count to %d\n", *count);
      pthread_mutex_unlock(mutex);
   }

   munmap(memory, 2048*sizeof(int));
   shm_unlink("/shm");
   return value_returned_from_child;
}