Redirecting STDIN/OUT/ERR - c

Question

I'm trying to create a linux daemon in c and found some sample code on this page.
I understand all the code except where it tries to redirect STDIN, STDOUT and STDERR (to /dev/null/). I also found a number of questions on here related to why these should be redirected (which I understand).
Specifically the section of code my question relates to is:
/* Route I/O connections */
/* Open STDIN */
i = open("/dev/null", O_RDWR);
/* STDOUT */
dup(i);
/* STDERR */
dup(i);
Reading the man page for dup() it implies that dup() simply duplicates a file descriptor.
So I don't understand how this does the redirect ? Is the compiler taking hints from the comments in the line above ?, or is it missing some code ?, is it plain wrong ?, or am I missing something ?

Answer 1

It's import to understand the previous bit of the example code you link to:
/* close all descriptors */
for (i = getdtablesize(); i >= 0; --i)
{
close(i);
}
This closes all open file descriptors including STDIN, STDOUT and STDERR.
As the manpage for open() states
The file descriptor returned by a successful call will be the lowest-numbered file descriptor not currently open for the process
So the subsequent call to open() in the example code will redirect file descriptor 0 which is STDIN, to /dev/null.
The subsequent calls to dup() will duplicate the file descriptor using the next lowest numbers. STDOUT is 1, and STDERR is 2.
The manpage for dup() states:
The dup() system call creates a copy of the file descriptor oldfd, using the lowest-numbered unused descriptor for the new descriptor

Answer 2

From the man page of dup:
The dup(oldfd) system call creates a copy of the file descriptor oldfd,
using the lowest-numbered unused descriptor for the new descriptor.
If you see the referenced code, he is first closing all the open file descriptors:
for (i = getdtablesize(); i >= 0; --i)
{
close(i);
}
After that when you call dup(i), it will copy the file descriptor i to the lowest available descriptor, which will be 0 (stdin). Doing that again will copy it to descriptor 1 (stdout) and similarly for descriptor 2 (stderr). In this way, the stdin, stdout, and stderr of the daemon process are pointing to /dev/null.
Every process gets three open file descriptors which are the stdin, stdout, and stderr (these descriptors usually have the values 0, 1, and 2 respectively). When you call printf(), for example, it writes to the file pointed to by the stdout descriptor. By pointing this descriptor to another file (such as /dev/null), any output from this process will get redirected to that file. Same logic applies for stdin and stderr.
On the shell, when you run something like ls > ls.out, the shell does the same. It fork()s a new process, opens ls.out for writing, and calls dup (or dup2) to copy the file descriptor of ls.out to this process' stdout.