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

int main(int argc, char *argv[]) {
    if (argc != 2) {
        fprintf(stderr, "Usage: %s <file>\n", argv[0]);
        return 1;
    }

    int fd = open(argv[1], O_RDONLY);
    if (fd == -1) {
        perror("open");
        return 1;
    }

    struct stat st;
    if (fstat(fd, &st) == -1) {
        perror("fstat");
        close(fd);
        return 1;
    }

    size_t len = st.st_size;
    char *addr = mmap(NULL, len, PROT_READ, MAP_PRIVATE, fd, 0);
    if (addr == MAP_FAILED) {
        perror("mmap");
        close(fd);
        return 1;
    }

    close(fd);

    char *new_addr = malloc(len);
    if (new_addr == NULL) {
        perror("malloc");
        munmap(addr, len);
        return 1;
    }

    memcpy(new_addr, addr, len);
    memset(new_addr, 'A', len);

    int fd2 = open(argv[1], O_WRONLY);
    if (fd2 == -1) {
        perror("open write");
        free(new_addr);
        munmap(addr, len);
        return 1;
    }

    if (write(fd2, new_addr, len) != (ssize_t)len) {
        perror("write");
        close(fd2);
        free(new_addr);
        munmap(addr, len);
        return 1;
    }

    close(fd2);
    free(new_addr);
    munmap(addr, len);

    return 0;
}