Question

Let’s suppose that in my OS exists N file descriptors. How many file descriptors will OS have after executing the code below:

    int fd = dup(oldfd);

How about :

   int fd = dup2(oldfd,newfd);

Thanks!

Answer 1

Its given in man pages. You’ll have N+1 file descriptors after calling either one of them.

From manpages
    ...
    dup and dup2 create a copy of the file descriptor oldfd.

    After successful return of dup or dup2, the old and new descriptors 
    may be used interchangeably. 

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

So, an unused descriptor is used as a new one. That should answer your question.