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.\"	$NetBSD: dup.2,v 1.4 1995/02/27 12:32:21 cgd Exp $
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.\"     @(#)dup.2	8.1 (Berkeley) 6/4/93
.\"
.Dd $Mdocdate: June 25 2018 $
.Dt DUP 2
.Os
.Sh NAME
.Nm dup ,
.Nm dup2 ,
.Nm dup3
.Nd duplicate an existing file descriptor
.Sh SYNOPSIS
.In unistd.h
.Ft int
.Fn dup "int oldd"
.Ft int
.Fn dup2 "int oldd" "int newd"
.In fcntl.h
.In unistd.h
.Ft int
.Fn dup3 "int oldd" "int newd" "int flags"
.Sh DESCRIPTION
.Fn dup
duplicates an existing object descriptor and returns its value to
the calling process
.Fa ( newd
=
.Fn dup oldd ) .
The argument
.Fa 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
.Xr getdtablesize 3 .
The new descriptor returned by the call is the lowest numbered descriptor
currently not in use by the process.
.Pp
The object referenced by the descriptor does not distinguish between
.Fa oldd
and
.Fa newd
in any way.
Thus if
.Fa newd
and
.Fa oldd
are duplicate references to an open
file,
.Xr read 2 ,
.Xr write 2
and
.Xr 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
.Xr open 2
call.
The close-on-exec flag on the new file descriptor is unset.
.Pp
In
.Fn dup2 ,
the value of the new descriptor
.Fa newd
is specified.
If this descriptor is already in use, it is first deallocated as if a
.Xr close 2
call had been done first.
When
.Fa newd
equals
.Fa oldd ,
.Fn dup2
just returns without affecting the close-on-exec flag.
.Pp
In
.Fn dup3 ,
both the value of the new descriptor and the close-on-exec flag on
the new file descriptor are specified:
.Fa newd
specifies the value and the
.Dv O_CLOEXEC
bit in
.Fa flags
specifies the close-on-exec flag.
Unlike
.Fn dup2 ,
if
.Fa oldd
and
.Fa newd
are equal then
.Fn dup3
fails.
Otherwise, if
.Fa flags
is zero then
.Fn dup3
is identical to a call to
.Fn dup2 .
.Sh RETURN VALUES
Upon successful completion, the value of the new descriptor is returned.
The value \-1 is returned if an error occurs in either call.
The external variable
.Va errno
indicates the cause of the error.
.Sh ERRORS
.Fn dup
will fail if:
.Bl -tag -width Er
.It Bq Er EBADF
.Fa oldd
is not a valid active descriptor.
.It Bq Er EMFILE
Too many descriptors are active.
.El
.Pp
.Fn dup2
and
.Fn dup3
will fail if:
.Bl -tag -width Er
.It Bq Er EBADF
.Fa oldd
is not a valid active descriptor or
.Fa newd
is negative or greater than or equal to the process's
.Dv RLIMIT_NOFILE
limit.
.It Bq Er EBUSY
A race condition with
.Xr accept 2
or
.Xr open 2
has been detected.
.It Bq Er EINTR
An interrupt was received.
.It Bq Er EIO
An I/O error occurred while writing to the file system.
.El
.Pp
In addition,
.Fn dup3
will return the following error:
.Bl -tag -width Er
.It Bq Er EINVAL
.Fa oldd
is equal to
.Fa newd
or
.Fa flags
is invalid.
.El
.Sh SEE ALSO
.Xr accept 2 ,
.Xr close 2 ,
.Xr fcntl 2 ,
.Xr getrlimit 2 ,
.Xr open 2 ,
.Xr pipe 2 ,
.Xr socket 2 ,
.Xr socketpair 2 ,
.Xr getdtablesize 3
.Sh STANDARDS
.Fn dup
and
.Fn dup2
conform to
.St -p1003.1-2008 .
The
.Fn dup3
function is expected to conform to a future revision of that standard.
.Sh HISTORY
The
.Fn dup
system call first appeared in
.At v3 ,
.Fn dup2
in
.At v7 ,
and
.Fn dup3
in
.Ox 5.7 .