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 numbers to keep track of file locations and their direction(input/output). Wouldn't it be easier to just

fd=fd2; 

Whenever we want to duplicate a file descriptor?

And something else..

dup() uses the lowest-numbered unused descriptor for the new descriptor.

Does that mean that it can also take as value stdin, stdout or stderr if we assume that we have close()-ed one of those?

Answer 1

Just wanted to respond to myself on the second question after experimenting a bit.

The answer is YES. A file descriptor that you make can take a value 0, 1, 2 if stdin, stdout or stderr are closed.

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 

Answer 2

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.