#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/syscall.h>
#include <sys/times.h>
#include <unistd.h>
#include <sys/param.h>
#include <fcntl.h>
#include <string.h>

extern int errno;

void err_sys(const char *msg) {
    perror(msg);
    exit(errno);
}

int
main (int argc, char **argv)
{
    int fd = open("A.txt", 
                  O_WRONLY | O_CREAT | O_TRUNC, 
                  S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH);
    if (fd == -1) {
        err_sys("open");
    }
    int fd2 = dup(fd);
    if (fd2 == -1) {
        err_sys("dup");
    }

    const char *msg;
    size_t n;
    
    msg = "first message";
    n = strlen(msg);
    if (write(fd, msg, n) != n) {
        err_sys("write fd");
    }
    msg = "second message";
    n = strlen(msg);
    if (write(fd2, msg, n) != n) {
        err_sys("write fd2");
    }
}