#include <unistd.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>

#define SHARED_MEMORY_OBJECT_NAME "my_shared_memory"
#define SHARED_MEMORY_OBJECT_SIZE 50
#define SHM_CREATE 1
#define SHM_PRINT  3
#define SHM_CLOSE  4

void usage(const char * s) {
    printf("Usage: %s <create|write|read|unlink> ['text']\n", s);
}

int main (int argc, char ** argv) {
    int shm, len, cmd, mode = 0;
    char *addr;

    if ( argc < 2 ) {
        usage(argv[0]);
        return 1;
    }

    if ( (!strcmp(argv[1], "create") || !strcmp(argv[1], "write")) && (argc == 3) ) {
        len = strlen(argv[2]);
        len = (len<=SHARED_MEMORY_OBJECT_SIZE)?len:SHARED_MEMORY_OBJECT_SIZE;
        mode = O_CREAT;
        cmd = SHM_CREATE;
    } else if ( ! strcmp(argv[1], "print" ) ) {
        cmd = SHM_PRINT;
    } else if ( ! strcmp(argv[1], "unlink" ) ) {
        cmd = SHM_CLOSE;
    } else {
        usage(argv[0]);
        return 1;
    }

    if ( (shm = shm_open(SHARED_MEMORY_OBJECT_NAME, mode|O_RDWR, S_IRWXO|S_IRWXG|S_IRWXU)) == -1 ) {
        perror("shm_open");
        return 1;
    }

    if ( cmd == SHM_CREATE ) {
        if ( ftruncate(shm, SHARED_MEMORY_OBJECT_SIZE+1) == -1 ) {
            perror("ftruncate");
            return 1;
        }
    }

    if ( (addr = mmap(0, SHARED_MEMORY_OBJECT_SIZE+1, PROT_WRITE|PROT_READ, MAP_SHARED, shm, 0)) == (char*)-1 ) {
        perror("mmap");
        return 1;
    }

    switch ( cmd ) {
    case SHM_CREATE:
        memcpy(addr, argv[2], len);
        addr[len] = '\0';
        printf("Shared memory filled in. You may run '%s print' to see shared memory value.\n", argv[0]);
        break;
    case SHM_PRINT:
        printf("Got from shared memory: %s\n", addr);
        break;
    }

    munmap(addr, SHARED_MEMORY_OBJECT_SIZE);
    close(shm);

    if ( cmd == SHM_CLOSE ) {
        shm_unlink(SHARED_MEMORY_OBJECT_NAME);
    }

    return 0;
}