Just have a try with the examples provided by Operating System Concepts, P103, with some modification.

Run sm_alloc first, and you’ll receive a shared memory segment ID.

Then run sm_write and provide it with the ID you received just now, it will write a string Hi, shared memory! into that memory.

Then run sm_read and provide the ID, it will print the string in that memory.

Finally run sm_free and provide the ID, it will free that memory.

//sm_alloc.c
#include <stdio.h>
#include <sys/shm.h>
#include <sys/stat.h>

int main()
{
    int sid = 0;
    int size = 4096;

    sid = shmget(IPC_PRIVATE,size,S_IRUSR | S_IWUSR);
    printf("%d\n",sid);

    return 0;
}

//sm_write.c
#include <stdio.h>
#include <sys/shm.h>
#include <sys/stat.h>

int main()
{
    int sid = 0;
    scanf("%d",&sid);
    char *sm = (char *)shmat(sid,NULL,0);
    sprintf(sm,"Hi, shared memory!");
    shmdt(sm);

    return 0;
}

//sm_read.c
#include <stdio.h>
#include <sys/shm.h>
#include <sys/stat.h>

int main()
{
    int sid = 0;
    scanf("%d",&sid);
    char *sm = (char *)shmat(sid,NULL,0);
    printf("%s\n",sm);
    shmdt(sm);

    return 0;
}

//sm_free.c
#include <stdio.h>
#include <sys/shm.h>
#include <sys/stat.h>

int main()
{
    int sid = 0;
    scanf("%d",&sid);
    shmctl(sid,IPC_RMID,NULL);

    return 0;
}

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