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

#define MAP_SIZE 10

int main(int argc, char *argv[])
{
    if (argc < 2) {
        printf("*** Usage: filepath [new contents]\n");
        return -1;
    }
    
    int fd = open(argv[1], O_RDWR);
    if (fd == -1) {
        printf("*** open() failed.\n");
        return -2;
    }
    
    char *addr = mmap(NULL, MAP_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
    if (addr == MAP_FAILED) {
        printf("*** mmap() failed.\n");
        return -3;
    }
    
    if (close(fd) == -1) {  // file descriptor no longer needed
        printf("*** close(fd) failed.\n");
        return -4;
    }
    
    printf("Current contents = \"%.*s\"\n", MAP_SIZE, addr);
    
    if (argc > 2) {
        memset(addr, 0, MAP_SIZE);  // zero out region
        
        strncpy(addr, argv[2], MAP_SIZE-1);
        if (msync(addr, MAP_SIZE, MS_SYNC) == -1) {  // flush to disk
            printf("*** msync() failed.\n");
            return -5;
        }
        
        printf("Copied \"%s\" to mapped file.\n", argv[2]);
    }
    
    return 0;
}