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

A file descriptor is a bit more than a number. It also carries various semi-hidden state with it (whether it's open or not, to which file description it refers, and also some flags). dup duplicates this information, so you can e.g. close the two descriptors independently. fd=fd2 does not.