The
dup()
system call
duplicates an existing object descriptor and returns its value to
the calling process
(,
newd
=
dup(oldd)).
The argument
oldd
is a small non-negative integer index in
the per-process descriptor table.
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)
system call.
The close-on-exec flag on the new file descriptor is unset.
In
dup2(),
the value of the new descriptor
newd
is specified.
If this descriptor is already in use and
oldd
≠
newd,
the descriptor is first deallocated as if the
close(2)
system call had been used.
If
oldd
is not a valid descriptor, then
newd
is not closed.
If
oldd
==
newd
and
oldd
is a valid descriptor, then
dup2()
is successful, and does nothing.
