Brief

  • As the name suggests, shared memory is a memory that may be shared by multiple programs with an intent to provide communication among them or avoid redundant copies.

Points To Catch

  • shmget() Creates a shared memory segment, The key argument could be semaphore ID
  • shmat(): Shared segment can be attached to a process address space using this API
  • It can be detached using shmdt(), A shared segment can be attached multiple times by the same process
  • The original owner of a shared memory segment can assign ownership to another user with `shmctl()
  • List out shared memory areas by ipcs -m
  • Get more info on the particular shared memory area ipcs -m -i [shmid]
  • Remove shared memory ipcrm shm [shmid]

Server.c

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

#define SHMSZ     27    //Size of shared memory

int main()
{
        char c;
        int shmid;
        int shmflg = IPC_CREAT | 0666;  // flags like create, access permission
        key_t key = 5678;               // Name of Shared Memory Segment
        char *shm, *s;

        if ((shmid = shmget(key, SHMSZ, shmflg)) < 0) {         //Create the segment
                perror("shmget");
                return 1;
        }

        if ((shm = shmat(shmid, NULL, 0)) == (void *) -1) {     //Attach segment to data space
                perror("shmat");
                return 1;
        }

        s = shm;                // Fill memory to read by other process

        for (c = 'a'; c <= 'z'; c++)
                *s++ = c;
        *s = NULL;

        while (*shm != '*')     // Wait for other process acknowledgement
                sleep(1);

        return 0;
}

Client.c

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

#define SHMSZ     27

int main()
{
        int shmid;
        int shmflg = 0666;  // flags like create, access permission
        key_t key = 5678;
        char *shm, *s;

        if ((shmid = shmget(key, SHMSZ, shmflg)) < 0) {         // Locate the segment
                perror("shmget");
                return 1;
        }

        if ((shm = shmat(shmid, NULL, 0)) == (void *) -1) {     // Attach segment to data space
                perror("shmat");
                return 1;
        }

        for (s = shm; *s != NULL; s++)  // Read what other process writes here
                putchar(*s);
        putchar('\n');

        *shm = '*';                     // Acknowledge other proces

        return 0;
}