C language DUP 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[])
{
    
    //  Copy file descriptor 
    //  The descriptor encoding is up to 1023
    // 0 1 2 Occupied by the system , These are standard inputs 、 standard output 、 The standard error 
    //  Other file descriptors must be from 3 Start 
    //  A file can be pointed to by multiple file descriptors 
    // dup The function returns a file descriptor , It will select the smallest of the idle file descriptors middle note as the new file descriptor returned  
    int fd = open("a.txt", O_RDWR | O_CREAT, 0664);

    int fd1 = dup(fd);
   
    if (fd1 == -1) {
    
        perror("open");
        return -1;
    }

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

    close(fd);

    char* str = "hello world";
    int ret = write(fd1, str, strlen(str));

    if (ret == -1) {
    
        perror("write");
        return -1;
    }
	return 0;
}

The above code will use dup Function will a.txt File descriptor for fd Copied to the fd1, After copying , Both file descriptors point to a.txt, operation fd and fd1 Any file descriptor is equivalent to an operation a.txt

a.txt Is written in hello world