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

#define FILE_PATH "mapped.txt"
#define FILE_SIZE 4096

int main() {
    int fd = open(FILE_PATH, O_RDWR | O_CREAT, 0644);
    if (fd == -1) {
        perror("open");
        return 1;
    }

    /* We want some space to write */
    if (ftruncate(fd, FILE_SIZE) == -1) {
        perror("ftruncate");
        close(fd);
        return 1;
    }

    /* NULL means we do not care where the mapping goes, anywhere in canonical memory will do
     * PROT_READ/WRITE are just protection flags that allows our process rw rights
     * MAP_SHARED means our (real time, or close to) changes are visible to other processes mmping the same file */
    void *file_memory = mmap(NULL, FILE_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
    if (file_memory == MAP_FAILED) {
        perror("mmap");
        close(fd);
        return 1;
    }

    const char *message = "This is written to file!";
    strncpy(file_memory, message, strlen(message) + 1);

    /* Re-truncate the file to known length, otherwise the entire buffer
     * (4k, with garbage from whatever the kernel gave us) will be in the file*/
    if (ftruncate(fd, strnlen(message, FILE_SIZE + 1)) == -1) {
        perror("ftruncate");
        close(fd);
        return 1;
    }

    printf("Mapped region content: %s\n", (char *)file_memory);

    munmap(file_memory, FILE_SIZE);

    close(fd);

    /* Optional finishing step, but i assume you want to inspect the file */
    // remove(file_path);

    return 0;
}