7.c

#include <stdio.h>

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>

#define FILENAME "./test"

int main(void)
{
    int fd;
    int ret;

    fd = open(FILENAME, O_RDWR);
    if (fd == -1)
    {
        perror("open");
        return -1;
    }

    printf("fd = %d\n", fd);

    close(1);
    ret = dup(fd);

    printf("ret = %d\n", ret);

    return 0;
}