If you want to see how shared memory is implemented then go look at the Linux kernel source code. That is an excercise I leave up to you. However, I will warn you that it is not pretty, and probably not going to make sense unless you have a high level understanding of C programming and the implementation of the Linux kernel.
As far as using shared memory goes here is an incredibly simplistic client/server example using shared memory. Mind you in production code you want to use semaphores or a record locking mechanism for syncronization and concurrency which raise the code complexity by several magnitudes.
Server.c
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#include <sys/types.h>
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#include <sys/ipc.h>
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#include <sys/shm.h>
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#include <stdio.h>
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#define SHMSZ 27
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main()
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{
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char c;
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int shmid;
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key_t key;
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char *shm, *s;
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/*
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* We'll name our shared memory segment
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* "5678".
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*/
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key = 5678;
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/*
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* Create the segment.
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*/
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if ((shmid = shmget(key, SHMSZ, IPC_CREAT | 0666)) < 0) {
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perror("shmget");
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exit(1);
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}
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/*
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* Now we attach the segment to our data space.
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*/
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if ((shm = shmat(shmid, NULL, 0)) == (char *) -1) {
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perror("shmat");
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exit(1);
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}
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/*
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* Now put some things into the memory for the
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* other process to read.
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*/
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s = shm;
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for (c = 'a'; c <= 'z'; c++)
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*s++ = c;
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*s = NULL;
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/*
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* Finally, we wait until the other process
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* changes the first character of our memory
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* to '*', indicating that it has read what
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* we put there.
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*/
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while (*shm != '*')
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sleep(1);
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exit(0);
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}
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Client.c
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/*
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* shm-client - client program to demonstrate shared memory.
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*/
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#include <sys/types.h>
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#include <sys/ipc.h>
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#include <sys/shm.h>
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#include <stdio.h>
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#define SHMSZ 27
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main()
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{
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int shmid;
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key_t key;
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char *shm, *s;
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/*
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* We need to get the segment named
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* "5678", created by the server.
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*/
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key = 5678;
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/*
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* Locate the segment.
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*/
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if ((shmid = shmget(key, SHMSZ, 0666)) < 0) {
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perror("shmget");
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exit(1);
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}
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/*
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* Now we attach the segment to our data space.
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*/
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if ((shm = shmat(shmid, NULL, 0)) == (char *) -1) {
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perror("shmat");
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exit(1);
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}
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/*
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* Now read what the server put in the memory.
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*/
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for (s = shm; *s != NULL; s++)
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putchar(*s);
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putchar('\n');
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/*
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* Finally, change the first character of the
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* segment to '*', indicating we have read
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* the segment.
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*/
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*shm = '*';
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exit(0);
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}
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