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

#define SHM_SIZE 1024  /* a 1K shared memory segment */

int main(int argc, char *argv[]) {
    key_t key;
    int shmid;
    char *data;
    int mode;

    /* make the key: */
    if ((key = ftok("29-shm.c", 'R')) == -1) {
        perror("ftok");
        exit(1);
    }
    /* connect to (and possibly create) the segment: */
    if ((shmid = shmget(key, SHM_SIZE, 0644 | IPC_CREAT)) == -1) {
        perror("shmget");
        exit(1);
    }
    /* attach to the segment to get a pointer to it: */
    data = shmat(shmid, (void *)0, 0);
    if (data == (char *)(-1)) {
        perror("shmat");
        exit(1);
    }

    /* read or modify the segment, based on the command line: */
    if (argc == 2) {
        printf("writing to segment: \"%s\"\n", argv[1]);
        strncpy(data, argv[1], SHM_SIZE);
    } else
        printf("segment contains: \"%s\"\n", data);

    /* detach from the segment: */
    if (shmdt(data) == -1) {
        perror("shmdt");
        exit(1);
    }

    return 0;
}