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

Let's say you're writing a shell program and you want to redirect stdin and stdout in a program you want to run. It could look something like this:

fdin = open(infile, O_RDONLY);
fdout = open(outfile, O_WRONLY);
// Check for errors, send messages to stdout.
...
int pid = fork(0);
if(pid == 0) {
    close(0);
    dup(fdin);
    close(fdin);
    close(1);
    dup(fdout);
    close(fdout);
    execvp(program, argv);
}
// Parent process cleans up, maybe waits for child.
...

dup2() is a little more convenient way to do it the close() dup() can be replaced by:

dup2(fdin, 0);
dup2(fdout, 1);

The reason why you want to do this is that you want to report errors to stdout (or stderr) so you can't just close them and open a new file in the child process. Secondly, it would be a waste to do the fork if either open() call returned an error.