.\"	$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 December 1, 2010
.Dt DUP 2
.Os BSD 4
.Sh NAME
.Nm dup ,
.Nm dup2
.Nd duplicate an existing file descriptor
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Ft int
.Fo dup
.Fa "int fildes"
.Fc
.Ft int
.Fo dup2
.Fa "int fildes"
.Fa "int fildes2"
.Fc
.Sh DESCRIPTION
.Fn dup
duplicates an existing object descriptor
and returns its value to the calling process
.Fa ( fildes2
=
.Fn dup fildes ) .
The argument
.Fa fildes
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 2 .
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 fildes
and
.Fa fildes2
in any way.
Thus if
.Fa fildes2
and
.Fa fildes
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 fildes2
is specified.
If
.Fa fildes
and
.Fa fildes2
are equal, then
.Fn dup2 
just returns
.Fa fildes2 ;
no other changes are made to the existing descriptor.
Otherwise, if descriptor
.Fa fildes2
is already in use, it is first deallocated as if a
.Xr close 2
call had been done first.
.Sh RETURN VALUES
Upon successful completion, the new file descriptor is returned.
Otherwise, a value of -1 is returned and the global integer variable
.Va errno
is set to indicate the error.
.Sh ERRORS
The
.Fn dup
and
.Fn dup2
system calls will fail if:
.Bl -tag -width Er
.\" ==========
.It Bq Er EBADF
.Fa fildes
is not an active, valid file descriptor.
.\" ==========
.It Bq Er EINTR
Execution is interrupted by a signal.
.\" ==========
.It Bq Er EMFILE
Too many file descriptors are active.
.El
.Pp
The
.Fn dup2
system call will fail if:
.Bl -tag -width Er
.\" ==========
.It Bq Er EBADF
.Fa fildes2
is negative or greater than the maximum allowable number (see getdtablesize(2)).
.El
.Sh SEE ALSO
.Xr accept 2 ,
.Xr close 2 ,
.Xr fcntl 2 ,
.Xr getdtablesize 2 ,
.Xr open 2 ,
.Xr pipe 2 ,
.Xr socket 2 ,
.Xr socketpair 2
.Sh STANDARDS
.Fn dup
and
.Fn dup2
are expected to conform to 
.St -p1003.1-88 .