Semaphore /Shared memory simple example

/*

Shared memory is perhaps the most powerful of the SysV IPC methods, and it is the easiest to implement. As the name implies, a block of memory is shared between processes. Listing 7 shows a program that calls fork(2) to split itself into a parent process and a child process, communicating between the two using a shared memory segment.
A program illustrating the use of shared memory
The following program shows the contents of Filename : sema_simple.c

*/

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
int main(void) {
pid_t pid;
int *shared; /* pointer to the shm */
int shmid;
shmid = shmget(IPC_PRIVATE, sizeof(int), IPC_CREAT | 0666);
if (fork() == 0) { /* Child */
/* Attach to shared memory and print the pointer */
shared = shmat(shmid, (void *) 0, 0);
printf("Child pointer %u\n", shared);
*shared=1;
printf("Child value=%d\n", *shared);
sleep(2);
printf("Child value=%d\n", *shared);
} else { /* Parent */
/* Attach to shared memory and print the pointer */
shared = shmat(shmid, (void *) 0, 0);
printf("Parent pointer %u\n", shared);
printf("Parent value=%d\n", *shared);
sleep(1);
*shared=42;
printf("Parent value=%d\n", *shared);
sleep(5);
shmctl(shmid, IPC_RMID, 0);
}
}