dup(), dup2() - duplicate an existing file descriptor
#include <unistd.h>
int dup (int oldd)
int dup2 (int oldd, int newd)
The dup(2) function duplicates an existing object descriptor and returns its value to the calling process (newd The argument oldd is a small non-negative integer which typically refers to a file or a socket. The new descriptor returned by the call is the lowest numbered descriptor currently not in use by the process.
The object referenced by the descriptor does not distinguish between oldd and newd in any way. Thus if newd and oldd are duplicate references to an open file, read(2), write(2) and lseek(2) calls all move a single pointer into the file, and append mode, non-blocking I/O and asynchronous I/O options are shared between the references. If a separate pointer into the file is desired, a different object reference to the file must be obtained by issuing an additional open(2) call. The close-on-exec flag on the new file descriptor is unset.
In dup2(2), the value of the new descriptor newd is specified. If this descriptor is already in use, the descriptor is first deallocated as if a close(2) call were done first.
Both functions return -1 if an error occurs, and both set the external variable errno to indicate the error.
The dup(2) and dup2(2) functions fail if:
open(2)
close(2)
fcntl(2)
pipe(2)
socket(2)