Example on semaphore & shared memory

/* sem.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);
}
}

COMPILE:   $ cc sem.c -o simplesema
                     $ ./simplesema