Shared Memory Access using semaphore

If you want to see how shared memory is implemented then go look at the Linux kernel source code.
Here there is an exercise for you to understand the logic. But this is usefule only is you have good understanding of system level C language.
This example if take from the website http://www.cs.cf.ac.uk (UNIX System Calls and Subroutines using C)

shm_server.c

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

#include <stdio.h>

#define SHMSZ     27

main()
{
   char c;
   int shmid;
   key_t key;

   char *shm, *s;

   /*
    * We'll name our shared memory segment
    * "5678".
    */
   key = 5678;

   /*

    * Create the segment.
    */
   if ((shmid = shmget(key, SHMSZ, IPC_CREAT | 0666)) < 0) {
       perror("shmget");
       exit(1);

   }

   /*
    * Now we attach the segment to our data space.
    */
   if ((shm = shmat(shmid, NULL, 0)) == (char *) -1) {
       perror("shmat");

       exit(1);
   }

   /*
    * Now put some things into the memory for the
    * other process to read.
    */
   s = shm;

   for (c = 'a'; c <= 'z'; c++)

       *s++ = c;
   *s = NULL;

   /*
    * Finally, we wait until the other process
    * changes the first character of our memory
    * to '*', indicating that it has read what
    * we put there.
    */

   while (*shm != '*')
       sleep(1);

   exit(0);
}

shm_client.c

/*
* shm-client - client program to demonstrate shared memory.
*/
#include <sys/types.h>

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

#define SHMSZ     27

main()
{
   int shmid;

   key_t key;
   char *shm, *s;

   /*
    * We need to get the segment named
    * "5678", created by the server.
    */
   key = 5678;


   /*
    * Locate the segment.
    */
   if ((shmid = shmget(key, SHMSZ, 0666)) < 0) {
       perror("shmget");

       exit(1);
   }

   /*
    * Now we attach the segment to our data space.
    */
   if ((shm = shmat(shmid, NULL, 0)) == (char *) -1) {

       perror("shmat");
       exit(1);
   }

   /*
    * Now read what the server put in the memory.
    */
   for (s = shm; *s != NULL; s++)

       putchar(*s);
   putchar('\n');

   /*
    * Finally, change the first character of the
    * segment to '*', indicating we have read
    * the segment.
    */
   *shm = '*';


   exit(0);
}