dup() 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 value must be less than the size of the table, which is returned by
getdtablesize(3). 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(), 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 had been done first. If
newd and
oldd are the same, the call has no effect.