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	1	.\" Copyright (c) 1991, 1993
	2	.\" The Regents of the University of California. All rights reserved.
	3	.\"
	4	.\" Redistribution and use in source and binary forms, with or without
	5	.\" modification, are permitted provided that the following conditions
	6	.\" are met:
	7	.\" 1. Redistributions of source code must retain the above copyright
	8	.\" notice, this list of conditions and the following disclaimer.
	9	.\" 2. Redistributions in binary form must reproduce the above copyright
	10	.\" notice, this list of conditions and the following disclaimer in the
	11	.\" documentation and/or other materials provided with the distribution.
	12	.\" 3. All advertising materials mentioning features or use of this software
	13	.\" must display the following acknowledgement:
	14	.\" This product includes software developed by the University of
	15	.\" California, Berkeley and its contributors.
	16	.\" 4. Neither the name of the University nor the names of its contributors
	17	.\" may be used to endorse or promote products derived from this software
	18	.\" without specific prior written permission.
	19	.\"
	20	.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
	21	.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
	22	.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
	23	.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
	24	.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
	25	.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
	26	.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
	27	.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
	28	.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
	29	.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
	30	.\" SUCH DAMAGE.
	31	.\"
	32	.\" @(#)popen.3 8.2 (Berkeley) 5/3/95
	33	.\" $FreeBSD: src/lib/libc/gen/popen.3,v 1.16 2003/06/08 10:01:51 charnier Exp $
	34	.\"
	35	.Dd May 3, 1995
	36	.Dt POPEN 3
	37	.Os
	38	.Sh NAME
	39	.Nm popen ,
	40	.Nm pclose
	41	.Nd process
	42	.Tn I/O
	43	.Sh LIBRARY
	44	.Lb libc
	45	.Sh SYNOPSIS
	46	.In stdio.h
	47	.Ft FILE *
	48	.Fn popen "const char command" "const char type"
	49	.Ft int
	50	.Fn pclose "FILE *stream"
	51	.Sh DESCRIPTION
	52	The
	53	.Fn popen
	54	function
	55	.Dq opens
	56	a process by creating a bidirectional pipe
	57	forking,
	58	and invoking the shell.
	59	Any streams opened by previous
	60	.Fn popen
	61	calls in the parent process are closed in the new child process.
	62	Historically,
	63	.Fn popen
	64	was implemented with a unidirectional pipe;
	65	hence many implementations of
	66	.Fn popen
	67	only allow the
	68	.Fa type
	69	argument to specify reading or writing, not both.
	70	Since
	71	.Fn popen
	72	is now implemented using a bidirectional pipe, the
	73	.Fa type
	74	argument may request a bidirectional data flow.
	75	The
	76	.Fa type
	77	argument is a pointer to a null-terminated string
	78	which must be
	79	.Ql r
	80	for reading,
	81	.Ql w
	82	for writing, or
	83	.Ql r+
	84	for reading and writing.
	85	.Pp
	86	The
	87	.Fa command
	88	argument is a pointer to a null-terminated string
	89	containing a shell command line.
	90	This command is passed to
	91	.Pa /bin/sh
	92	using the
	93	.Fl c
	94	flag; interpretation, if any, is performed by the shell.
	95	.Pp
	96	The return value from
	97	.Fn popen
	98	is a normal standard
	99	.Tn I/O
	100	stream in all respects
	101	save that it must be closed with
	102	.Fn pclose
	103	rather than
	104	.Fn fclose .
	105	Writing to such a stream
	106	writes to the standard input of the command;
	107	the command's standard output is the same as that of the process that called
	108	.Fn popen ,
	109	unless this is altered by the command itself.
	110	Conversely, reading from a
	111	.Dq popened
	112	stream reads the command's standard output, and
	113	the command's standard input is the same as that of the process that called
	114	.Fn popen .
	115	.Pp
	116	Note that output
	117	.Fn popen
	118	streams are fully buffered by default.
	119	.Pp
	120	The
	121	.Fn pclose
	122	function waits for the associated process to terminate
	123	and returns the exit status of the command
	124	as returned by
	125	.Xr wait4 2 .
	126	.Sh RETURN VALUES
	127	The
	128	.Fn popen
	129	function returns
	130	.Dv NULL
	131	if the
	132	.Xr fork 2
	133	or
	134	.Xr pipe 2
	135	calls fail,
	136	or if it cannot allocate memory.
	137	.Pp
	138	The
	139	.Fn pclose
	140	function
	141	returns \-1 if
	142	.Fa stream
	143	is not associated with a
	144	.Dq popened
	145	command, if
	146	.Fa stream
	147	already
	148	.Dq pclosed ,
	149	or if
	150	.Xr wait4 2
	151	returns an error.
	152	.Sh ERRORS
	153	The
	154	.Fn popen
	155	function does not reliably set
	156	.Va errno .
	157	.Sh SEE ALSO
	158	.Xr sh 1 ,
	159	.Xr fork 2 ,
	160	.Xr pipe 2 ,
	161	.Xr wait4 2 ,
	162	.Xr fclose 3 ,
	163	.Xr fflush 3 ,
	164	.Xr fopen 3 ,
	165	.Xr stdio 3 ,
	166	.Xr system 3
	167	.Sh BUGS
	168	Since the standard input of a command opened for reading
	169	shares its seek offset with the process that called
	170	.Fn popen ,
	171	if the original process has done a buffered read,
	172	the command's input position may not be as expected.
	173	Similarly, the output from a command opened for writing
	174	may become intermingled with that of the original process.
	175	The latter can be avoided by calling
	176	.Xr fflush 3
	177	before
	178	.Fn popen .
	179	.Pp
	180	Failure to execute the shell
	181	is indistinguishable from the shell's failure to execute command,
	182	or an immediate exit of the command.
	183	The only hint is an exit status of 127.
	184	.Pp
	185	The
	186	.Fn popen
	187	function
	188	always calls
	189	.Xr sh 1 ,
	190	never calls
	191	.Xr csh 1 .
	192	.Sh HISTORY
	193	A
	194	.Fn popen
	195	and a
	196	.Fn pclose
	197	function appeared in
	198	.At v7 .
	199	.Pp
	200	Bidirectional functionality was added in
	201	.Fx 2.2.6 .