backup file descriptor of stdout, to restore printing on to terminal later using only dup2, without using dup system call in C

Question

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


int main()
{
    int stdout_fd   = dup2(1, 1);
    printf("stdout_fd:%d
", stdout_fd);
    
    int fd_file     = open("file.txt", O_WRONLY|O_TRUNC);
    fd_file         = dup2(fd_file, 1);
    printf("file descriptor of file.txt : %d
", fd_file);
    
    fd_file         = dup2(fd_file, stdout_fd);
    printf("file descriptor of file.txt : %d
", fd_file);
    return 0;
}
O/P:
stdout
______
stdout_fd:1                                                                                                                                                 

file.txt
________
file descriptor of file.txt : 1
file descriptor of file.txt : 1

Here I am trying to print first time to required "file.txt" instead of stdout then for 2nd printf, I am trying to print on stdout i.e, restore file descriptor 0 from from file.txt to stdout again.

I want to implement it using only dup2, without using dup. I dont want to use something like below

int stdout_fd = dup(fd_file);
dup2(fd_file, 1);
printf("I am printing on file.txt");
dup2(fd_file, stdout_fd);
printf("I am printing on file.txt");

Answer 1

等待大神答复