1.

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>

/* define key */
int key=2709;

/* message */
char *messu[] = { 
	"Hello",
	"Countdown",
	"5", "4","3","1","sorry, 2","1","GO\007",
	"------------------------------"
	};

/* number of fields in message */
int n=10;

char *shmat();

main()
{
  int id,i;
  char *seg;

  /* transform key to identifier of the shared segment */
  /* key = 2709, size of the segment is 1024 bytes, create with */
  /* permissions 0777 */
  /* IPC = InterProcess Communication */
  /* see man shmget, man shmat */
  id = shmget(key,1024,IPC_CREAT | 0777);
  if (id == -1) {perror("shmget"); exit(1);}

  /* place shared segment to memory and return it's address seg */
  /* (char *) 0 means that OS will decide the location for a shared memory */
  seg = shmat(id,(char *)0,0);

  /* write into shared memory, sleep 1 second */
  for(i=0;;) {
 	sprintf(seg,"%s",messu[i]);
	i++; i = i % n;
	sleep(1);
  }
}


2.	
  
  
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>

char *shmat();

/* key is 2709*/
int key=2709;

main()
{
  int id,i;
  char *seg;

  /* transform key to id of the shared segment */
  /* key = 2709, size of the segment is 1024 bytes, create with */
  /* readonly-permission */
  id = shmget(key,1024, SHM_RDONLY);
  if (id == -1) {perror("shmget"); exit(1);}

  /* Locate seg to shared memory */
  /* (char *)0 means that system will find the corresponding address */

  seg = shmat(id,(char *)0,0);

  /* Read from shared memory, sleep 1 second */
  while(1){
    printf("%s\n",seg);
    sleep(1);
  }
}
	
3.

Program 1 writes a message in shared memory segment in every 1 second and
program 2 reads and display the contents of the shared memory in every
1 second.