C language dup2 function

#include <sys/stat.h>

#include <dirent.h>
#include <unistd.h>
#include <sys/types.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <string.h>

int main(int argc, char *argv[])
{
    
    // dup2 function 
    // dup Function will directly return an unoccupied file descriptor （ The smallest file descriptor from idle ）
    //  however dup2 The function will accept a fd As the specified file descriptor 
    //  If the specified file descriptor is occupied , Then the file corresponding to the file descriptor will be closed 
    int fd = open("a.txt", O_RDWR | O_CREAT, 0664);

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

    int fd1 = open("b.txt", O_RDWR | O_CREAT, 0664);
   
    if (fd1 == -1) {
    
        perror("open");
        return -1;
    }

    
    printf("fd: %d, fd1: %d\n", fd, fd1);

    //  here fd1 Will point to fd Files pointed to 
    //  Then operate fd1 It's equivalent to operating a.txt
    int fd2 = dup2(fd, fd1);
    if( fd2 == -1) {
    
        perror("dup2");
        return -1;
    }

    char* str = "Hello, dup2";
    int ret = write(fd1, str, strlen(str)); 
    if (ret == -1) {
    
        perror("write");
        return -1;
    }

    printf("fd: %d, fd1: %d, fd2: %d\n", fd, fd1, fd2);

    close(fd);
    close(fd1);

    return 0;
}

Compared with dup function ,dup2 Function can redirect a file descriptor to the file pointed to by another file descriptor

In the code above ,fd1 Redirected to fd The point is a.txt file , So right. fd1 The operation of the file descriptor is equivalent to the operation a.txt

in addition dup2 The return value of a function has no real effect , The return value can be regarded as a judgment dup2 Whether the function call is successful depends on

In this code ,fd2 The value of and fd1 It's consistent