Can someone explain what dup() in C does?

Question

I know that dup, dup2, dup3 \"create a copy of the file descriptor oldfd\"(from man pages). However I can\'t digest it.

As I know file descriptors are just

Answer 1

The single most important thing about dup() is it returns the smallest integer available for a new file descriptor. That's the basis of redirection:

int fd_redirect_to = open("file", O_CREAT);
close(1); /* stdout */
int fd_to_redirect = dup(fd_redirect_to); /* magically returns 1: stdout */
close(fd_redirect_to); /* we don't need this */

After this anything written to file descriptor 1 (stdout), magically goes into "file".