DUP
Section: System Calls (2)
Updated: May 13, 1986
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NAME
dup, dup2 - duplicate a descriptor
SYNOPSIS
newd = dup(oldd)
int newd, oldd;
dup2(oldd, newd)
int oldd, newd;
DESCRIPTION
Dup
duplicates an existing object descriptor.
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(2).
The new descriptor returned by the call,
newd,
is the lowest numbered descriptor that is
not currently in use by the process.
The object referenced by the descriptor does not distinguish
between references using 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 the second form of the call, the value of
newd
desired is specified. If this descriptor is already
in use, the descriptor is first deallocated as if a
close(2)
call had been done first.
RETURN VALUE
The value -1 is returned if an error occurs in either call.
The external variable
errno
indicates the cause of the error.
ERRORS
Dup
and
dup2
fail if:
- [EBADF]
-
Oldd or
newd is not a valid active descriptor
- [EMFILE]
-
Too many descriptors are active.
SEE ALSO
accept(2),
open(2),
close(2),
fcntl(2),
pipe(2),
socket(2),
socketpair(2),
getdtablesize(2)
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- SEE ALSO
-
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