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9bccf70c A	1	.\" $NetBSD: dup.2,v 1.4 1995/02/27 12:32:21 cgd Exp $
	2	.\"
	3	.\" Copyright (c) 1980, 1991, 1993
	4	.\" The Regents of the University of California. All rights reserved.
	5	.\"
	6	.\" Redistribution and use in source and binary forms, with or without
	7	.\" modification, are permitted provided that the following conditions
	8	.\" are met:
	9	.\" 1. Redistributions of source code must retain the above copyright
	10	.\" notice, this list of conditions and the following disclaimer.
	11	.\" 2. Redistributions in binary form must reproduce the above copyright
	12	.\" notice, this list of conditions and the following disclaimer in the
	13	.\" documentation and/or other materials provided with the distribution.
	14	.\" 3. All advertising materials mentioning features or use of this software
	15	.\" must display the following acknowledgement:
	16	.\" This product includes software developed by the University of
	17	.\" California, Berkeley and its contributors.
	18	.\" 4. Neither the name of the University nor the names of its contributors
	19	.\" may be used to endorse or promote products derived from this software
	20	.\" without specific prior written permission.
	21	.\"
	22	.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
	23	.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
	24	.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
	25	.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
	26	.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
	27	.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
	28	.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
	29	.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
	30	.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
	31	.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
	32	.\" SUCH DAMAGE.
	33	.\"
	34	.\" @(#)dup.2 8.1 (Berkeley) 6/4/93
	35	.\"
6d2010ae	36	.Dd December 1, 2010
9bccf70c A	37	.Dt DUP 2
	38	.Os BSD 4
	39	.Sh NAME
	40	.Nm dup ,
	41	.Nm dup2
	42	.Nd duplicate an existing file descriptor
	43	.Sh SYNOPSIS
	44	.Fd #include <unistd.h>
	45	.Ft int
2d21ac55 A	46	.Fo dup
	47	.Fa "int fildes"
	48	.Fc
9bccf70c	49	.Ft int
2d21ac55 A	50	.Fo dup2
	51	.Fa "int fildes"
	52	.Fa "int fildes2"
	53	.Fc
9bccf70c	54	.Sh DESCRIPTION
39037602	55	.Fn dup
2d21ac55 A	56	duplicates an existing object descriptor
	57	and returns its value to the calling process
	58	.Fa ( fildes2
9bccf70c	59	=
2d21ac55	60	.Fn dup fildes ) .
9bccf70c	61	The argument
2d21ac55 A	62	.Fa fildes
	63	is a small non-negative integer index
	64	in the per-process descriptor table.
	65	The value must be less than the size of the table,
	66	which is returned by
9bccf70c A	67	.Xr getdtablesize 2 .
	68	The new descriptor returned by the call
	69	is the lowest numbered descriptor
	70	currently not in use by the process.
	71	.Pp
	72	The object referenced by the descriptor does not distinguish
	73	between
2d21ac55	74	.Fa fildes
9bccf70c	75	and
2d21ac55	76	.Fa fildes2
9bccf70c A	77	in any way.
9bccf70c A	78	Thus if
2d21ac55	79	.Fa fildes2
9bccf70c	80	and
2d21ac55	81	.Fa fildes
9bccf70c A	82	are duplicate references to an open
	83	file,
	84	.Xr read 2 ,
	85	.Xr write 2
	86	and
	87	.Xr lseek 2
	88	calls all move a single pointer into the file,
	89	and append mode, non-blocking I/O and asynchronous I/O options
	90	are shared between the references.
2d21ac55 A	91	If a separate pointer into the file is desired,
	92	a different object reference to the file must be obtained
	93	by issuing an additional
9bccf70c A	94	.Xr open 2
	95	call.
	96	The close-on-exec flag on the new file descriptor is unset.
	97	.Pp
	98	In
	99	.Fn dup2 ,
	100	the value of the new descriptor
2d21ac55 A	101	.Fa fildes2
2d21ac55 A	102	is specified.
6d2010ae A	103	If
	104	.Fa fildes
	105	and
	106	.Fa fildes2
	107	are equal, then
	108	.Fn dup2
	109	just returns
	110	.Fa fildes2 ;
	111	no other changes are made to the existing descriptor.
	112	Otherwise, if descriptor
	113	.Fa fildes2
	114	is already in use, it is first deallocated as if a
9bccf70c A	115	.Xr close 2
	116	call had been done first.
	117	.Sh RETURN VALUES
b0d623f7	118	Upon successful completion, the new file descriptor is returned.
2d21ac55	119	Otherwise, a value of -1 is returned and the global integer variable
9bccf70c	120	.Va errno
2d21ac55	121	is set to indicate the error.
9bccf70c	122	.Sh ERRORS
2d21ac55 A	123	The
2d21ac55 A	124	.Fn dup
9bccf70c A	125	and
9bccf70c A	126	.Fn dup2
2d21ac55	127	system calls will fail if:
9bccf70c	128	.Bl -tag -width Er
2d21ac55	129	.\" ==========
9bccf70c	130	.It Bq Er EBADF
2d21ac55	131	.Fa fildes
2d21ac55 A	132	is not an active, valid file descriptor.
	133	.\" ==========
	134	.It Bq Er EINTR
	135	Execution is interrupted by a signal.
	136	.\" ==========
9bccf70c	137	.It Bq Er EMFILE
2d21ac55	138	Too many file descriptors are active.
9bccf70c	139	.El
b0d623f7 A	140	.Pp
	141	The
	142	.Fn dup2
	143	system call will fail if:
	144	.Bl -tag -width Er
	145	.\" ==========
	146	.It Bq Er EBADF
	147	.Fa fildes2
	148	is negative or greater than the maximum allowable number (see getdtablesize(2)).
	149	.El
9bccf70c A	150	.Sh SEE ALSO
9bccf70c A	151	.Xr accept 2 ,
9bccf70c A	152	.Xr close 2 ,
9bccf70c A	153	.Xr fcntl 2 ,
2d21ac55 A	154	.Xr getdtablesize 2 ,
2d21ac55 A	155	.Xr open 2 ,
9bccf70c A	156	.Xr pipe 2 ,
9bccf70c A	157	.Xr socket 2 ,
2d21ac55	158	.Xr socketpair 2
9bccf70c	159	.Sh STANDARDS
39037602	160	.Fn dup
9bccf70c A	161	and
	162	.Fn dup2
	163	are expected to conform to
	164	.St -p1003.1-88 .