/* Program to demonstrate shared memory IPC  */
/* Written by Carey Williamson  Jan 27, 2010 */

#include <sys/types.h>
#include <sys/stat.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>

#define DEBUG 1
#define SLOW 1

int main()
  {
    pid_t pid;
    int i, num = 0;
    int segmentid;
    void *memptr;

    /* create a shared memory region */
    segmentid = shmget(IPC_PRIVATE, 1024, IPC_CREAT | 0777);

    /* check return value */
    if( segmentid == -1 )
      printf("Failed to allocate shared memory segment!\n");
    else printf("Allocated a shared memory segment (%d)\n", segmentid);

    /* fork a child process */
    pid = fork();

    if( pid < 0 )
      {
	/* error occurred */
	fprintf(stderr, "Fork failed!\n");
	return 1;
      }
    else if( pid == 0 )
      {
	/* child process */
#ifdef DEBUG
	printf("\t\t\t\tI am the child! Here I go...\n");
#endif
	/* attach shared memory segment within process address space */
	memptr = (void *)shmat(segmentid, NULL, 0);

	/* check return value */
	if( memptr == (void *) -1 )
	  printf("\t\t\t\tChild failed to attach shared memory segment! %d\n", errno);
	else printf("\t\t\t\tChild process: memptr = %x\n", memptr);

#ifdef SLOW
        sleep(1);
#endif
	printf("\t\t\t\tChild process views msg = %s\n", memptr);
	sprintf(memptr, "Hello 1 from child");
	printf("\t\t\t\tChild process sets msg = %s\n", memptr);
#ifdef SLOW
        sleep(3);
#endif
	printf("\t\t\t\tChild process views msg = %s\n", memptr);
	sprintf(memptr, "Hello 2 from child");
	printf("\t\t\t\tChild process sets msg = %s\n", memptr);
#ifdef SLOW
        sleep(5);
#endif
	printf("\t\t\t\tChild process views msg = %s\n", memptr);
	sprintf(memptr, "Hello 3 from child");
	printf("\t\t\t\tChild process sets msg = %s\n", memptr);

#ifdef SLOW
        sleep(7);
#endif
	/* detach shared memory segment from process address space */
	shmdt(memptr);

	printf("\t\t\t\tThat was fun! All done now.\n");
      }
    else
      {
	/* parent process */
#ifdef DEBUG
	printf("I am the parent!\n");
#endif
	/* attach shared memory segment within process address space */
	memptr = (void *)shmat(segmentid, NULL, 0);

	/* check return value */
	if( memptr == (void *) -1 )
	  printf("Parent failed to attach shared memory segment! %d\n", errno);
	else printf("Parent process: memptr = %x\n", memptr);

#ifdef SLOW
        sleep(2);
#endif
	printf("Parent process views msg = %s\n", memptr);
	sprintf(memptr, "Hello 1 from parent");
	printf("Parent process sets msg = %s\n", memptr);
#ifdef SLOW
        sleep(4);
#endif
	printf("Parent process views msg = %s\n", memptr);
	sprintf(memptr, "Hello 2 from parent");
	printf("Parent process sets msg = %s\n", memptr);

#ifdef DEBUG
	printf("Waiting for child now...\n");
#endif
	wait(NULL);

	/* detach shared memory segment from process address space */
	shmdt(memptr);
#ifdef DEBUG
	printf("All done, and my child too!\n");
#endif
      }
  }