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

#ifndef INITIAL_LIMIT
# define INITIAL_LIMIT 1025
#endif
char buf[2*INITIAL_LIMIT];

write_to_file(int id) {

    char link[64];
    char * links = "/proc/self/fd/";
    sprintf(link, "%s", links);
    sprintf(link+strlen(links), "%d", id);
    //printf("link=%s\n", link);
    readlink(link, buf, INITIAL_LIMIT);
    //printf("buf=%s\n", buf);
    close(id);
    id = open(buf, O_WRONLY);
    write(id, "Hello world!\n", 13);
    close(id);
}

int main(int argc, char * argv[]) {

    int id = open("tmp.txt", O_RDONLY);

    write_to_file(id);

    return (0);
}