#define _POSIX_C_SOURCE 200809L

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

#define ERR(source) (perror(source), fprintf(stderr, "%s:%d\n", __FILE__, __LINE__), exit(EXIT_FAILURE))

void create_file()
{
    int fd = open("a.txt", O_WRONLY | O_CREAT | O_TRUNC, 0644);
    if (fd == -1)
        ERR("open");
    char *data = "abcdefghijklmnopqrstuvwxyz\n";
    if (write(fd, data, strlen(data)) < 0)
        ERR("write");
    close(fd);
}

int main(void)
{
    create_file();

    int fd = open("a.txt", O_RDWR);
    if (fd == -1)
        ERR("open");

    struct stat sb;
    if (fstat(fd, &sb) == -1)
        ERR("fstat");
    size_t length = sb.st_size;
    char *buff = mmap(NULL, length, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
    if (buff == MAP_FAILED)
        ERR("mmap");

    printf("a.txt[3] = %c (ASCII: %d)\n", buff[3], buff[3]);
    buff[3] = 'Z';
    msync(buff, length, MS_SYNC);

    munmap(buff, length);
    close(fd);
    return 0;
}