dup2 / dup - why would I need to duplicate a file descriptor?

Question

I'm trying to understand the use of dup2 and dup.

From the man page :

DESCRIPTION

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. They share locks, file position pointers and
flags; for example, if the file position is modified by using lseek on
one of the descriptors, the position is also changed for the other.

The two descriptors do not share the close-on-exec flag, however.

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

dup2 makes newfd be the copy of oldfd, closing newfd first if necessary.  

RETURN VALUE

dup and dup2 return the new descriptor, or -1 if an error occurred 
(in which case, errno is set appropriately).  

Why would I need that system call? what is the use of duplicating the file descriptor?

If I have the file descriptor, why would I want to make a copy of it?

I'd appreciate if you could explain and give me an example where dup2 / dup is needed.

Thanks

Answer 1

The dup system call duplicates an existing file descriptor, returning a new one that refers to the same underlying I/O object.

Dup allows shells to implement commands like this:

ls existing-file non-existing-file > tmp1  2>&1

The 2>&1 tells the shell to give the command a file descriptor 2 that is a duplicate of descriptor 1. (i.e stderr & stdout point to same fd).
Now the error message for calling ls on non-existing file and the correct output of ls on existing file show up in tmp1 file.

The following example code runs the program wc with standard input connected to the read end of a pipe.

int p[2];
char *argv[2];
argv[0] = "wc";
argv[1] = 0;
pipe(p);
if(fork() == 0) {
    close(STDIN); //CHILD CLOSING stdin
    dup(p[STDIN]); // copies the fd of read end of pipe into its fd i.e 0 (STDIN)
    close(p[STDIN]);
    close(p[STDOUT]);
    exec("/bin/wc", argv);
} else {
    write(p[STDOUT], "hello world\n", 12);
    close(p[STDIN]);
    close(p[STDOUT]);
}

The child dups the read end onto file descriptor 0, closes the file de scriptors in p, and execs wc. When wc reads from its standard input, it reads from the pipe.
This is how pipes are implemented using dup, well that one use of dup now you use pipe to build something else, that's the beauty of system calls,you build one thing after another using tools which are already there , these tool were inturn built using something else so on .. At the end system calls are the most basic tools you get in kernel

Cheers :)