POSIX shared memory demo

shm_open_get.c

// Eg: shm_open_get // => "Hello"

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

#define SHM_NAME "shm_open_demo"
#define SHM_SIZE 1024

int main(int argc, const char *argv[])
{
    int fd;
    char *addr;
    struct stat st;

    if (argc >= 2) {
        fprintf(stderr, "usage: shm_open_get\n");
        exit(1);
    }

    fd = shm_open(SHM_NAME, O_RDONLY | O_CREAT, S_IRUSR | S_IWUSR);
    if (fd == -1) {
        perror("shm_open");
        exit(1);
    }

    if (fstat(fd, &st) != -1 && st.st_size == 0) {
        if (ftruncate(fd, SHM_SIZE) == -1) {
            perror("ftruncate");
            exit(1);
        }
    }

    addr = mmap(NULL, SHM_SIZE, PROT_READ, MAP_SHARED, fd, 0);

    printf("segment contains: \"%s\"\n", addr);

    if (munmap(addr, SHM_SIZE) == -1) {
        perror("munmap");
        exit(1);
    }

    if (shm_unlink(SHM_NAME) == -1) {
        perror("shm_unlink");
        exit(1);
    }

    return 0;
}

shm_open_set.c

// Eg: shm_open_set "Hello"

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

#define SHM_NAME "shm_open_demo"
#define SHM_SIZE 1024

int main(int argc, const char *argv[])
{
    int fd;
    char *addr;
    struct stat st;

    if (argc < 2) {
        fprintf(stderr, "usage: shm_open_set [data to write]\n");
        exit(1);
    }

    fd = shm_open(SHM_NAME, O_RDWR | O_CREAT, S_IRUSR | S_IWUSR);
    if (fd == -1) {
        perror("shm_open");
        exit(1);
    }

    if (fstat(fd, &st) != -1 && st.st_size == 0) {
        if (ftruncate(fd, SHM_SIZE) == -1) {
            perror("ftruncate");
            exit(1);
        }
    }

    addr = mmap(NULL, SHM_SIZE, PROT_WRITE, MAP_SHARED, fd, 0);

    printf("Writing to segment: \"%s\"\n", argv[1]);
    strncpy(addr, argv[1], SHM_SIZE - 1);

    if (munmap(addr, SHM_SIZE) == -1) {
        perror("munmap");
        exit(1);
    }

    return 0;
}