[apple/libc.git] / gen / popen.3

.\" Copyright (c) 1991, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\"    must display the following acknowledgement:
.\"	This product includes software developed by the University of
.\"	California, Berkeley and its contributors.
.\" 4. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\"     @(#)popen.3	8.2 (Berkeley) 5/3/95
.\" $FreeBSD: src/lib/libc/gen/popen.3,v 1.14 2001/10/01 16:08:51 ru Exp $
.\"
.Dd May 3, 1995
.Dt POPEN 3
.Os
.Sh NAME
.Nm popen ,
.Nm pclose
.Nd process
.Tn I/O
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdio.h
.Ft FILE *
.Fn popen "const char *command" "const char *type"
.Ft int
.Fn pclose "FILE *stream"
.Sh DESCRIPTION
The
.Fn popen
function
.Dq opens
a process by creating a bidirectional pipe
forking,
and invoking the shell.
Any streams opened by previous
.Fn popen
calls in the parent process are closed in the new child process.
Historically,
.Fn popen
was implemented with a unidirectional pipe;
hence many implementations of
.Fn popen
only allow the
.Fa type
argument to specify reading or writing, not both.
Since
.Fn popen
is now implemented using a bidirectional pipe, the
.Fa type
argument may request a bidirectional data flow.
The
.Fa type
argument is a pointer to a null-terminated string
which must be
.Ql r
for reading,
.Ql w
for writing, or
.Ql r+
for reading and writing.
.Pp
The
.Fa command
argument is a pointer to a null-terminated string
containing a shell command line.
This command is passed to
.Pa /bin/sh
using the
.Fl c
flag; interpretation, if any, is performed by the shell.
.Pp
The return value from
.Fn popen
is a normal standard
.Tn I/O
stream in all respects
save that it must be closed with
.Fn pclose
rather than
.Fn fclose .
Writing to such a stream
writes to the standard input of the command;
the command's standard output is the same as that of the process that called
.Fn popen ,
unless this is altered by the command itself.
Conversely, reading from a
.Dq popened
stream reads the command's standard output, and
the command's standard input is the same as that of the process that called
.Fn popen .
.Pp
Note that output
.Fn popen
streams are fully buffered by default.
.Pp
The
.Fn pclose
function waits for the associated process to terminate
and returns the exit status of the command
as returned by
.Fn wait4 .
.Sh RETURN VALUES
The
.Fn popen
function returns
.Dv NULL
if the
.Xr fork 2
or
.Xr pipe 2
calls fail,
or if it cannot allocate memory.
.Pp
The
.Fn pclose
function
returns \-1 if
.Fa stream
is not associated with a
.Dq popened
command, if
.Fa stream
already
.Dq pclosed ,
or if
.Xr wait4
returns an error.
.Sh ERRORS
The
.Fn popen
function does not reliably set
.Va errno .
.Sh SEE ALSO
.Xr sh 1 ,
.Xr fork 2 ,
.Xr pipe 2 ,
.Xr wait4 2 ,
.Xr fclose 3 ,
.Xr fflush 3 ,
.Xr fopen 3 ,
.Xr stdio 3 ,
.Xr system 3
.Sh BUGS
Since the standard input of a command opened for reading
shares its seek offset with the process that called
.Fn popen ,
if the original process has done a buffered read,
the command's input position may not be as expected.
Similarly, the output from a command opened for writing
may become intermingled with that of the original process.
The latter can be avoided by calling
.Xr fflush 3
before
.Fn popen .
.Pp
Failure to execute the shell
is indistinguishable from the shell's failure to execute command,
or an immediate exit of the command.
The only hint is an exit status of 127.
.Pp
The
.Fn popen
argument
always calls
.Xr sh 1 ,
never calls
.Xr csh 1 .
.Sh HISTORY
A
.Fn popen
and a
.Fn pclose
function appeared in
.At v7 .
.Pp
Bidirectional functionality was added in
.Fx 2.2.6 .
Commit	Line	Data
5b2abdfb A	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 1.14 2001/10/01 16:08:51 ru 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	.Fn wait4 .
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
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	argument
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 .