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

Example:

close(1);     //closing stdout
newfd=dup(1); //newfd takes value of least available fd number

Where this happens to file descriptors:

0 stdin     .--------------.     0 stdin     .--------------.     0 stdin
1 stdout   =|   close(1)   :=>   2 stderr   =| newfd=dup(1) :=>   1 newfd
2 stderr    '--------------'                 '--------------'     2 stderr

A question arose again: How can I dup() a file descriptor that I already closed?

I doubt that you conducted the above experiment with the shown result, because that would not be standard-conforming - cf. dup:

The dup() function shall fail if:

[EBADF]

The fildes argument is not a valid open file descriptor.

So, after the shown code sequence, newfd must be not 1, but rather -1, and errno EBADF.