int
dup(
int oldd
)
int
dup2(
int oldd
, int newd
)
)
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.
)
and
dup2(
)
fail if:
EBADF
]
oldd
or
newd
is not a valid active descriptor
EMFILE
]
)
and
dup2(
)
functions conform to
ISO/IEC 9945-1:1990 (``POSIX.1'') .