1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
|
'\" t
.\" Copyright 1991 The Regents of the University of California.
.\" All rights reserved.
.\"
.\" SPDX-License-Identifier: BSD-4-Clause-UC
.\"
.\" @(#)popen.3 6.4 (Berkeley) 4/30/91
.\"
.\" Converted for Linux, Mon Nov 29 14:45:38 1993, faith@cs.unc.edu
.\" Modified Sat May 18 20:37:44 1996 by Martin Schulze (joey@linux.de)
.\" Modified 7 May 1998 by Joseph S. Myers (jsm28@cam.ac.uk)
.\"
.TH popen 3 2023-10-31 "Linux man-pages 6.7"
.SH NAME
popen, pclose \- pipe stream to or from a process
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <stdio.h>
.P
.BI "FILE *popen(const char *" command ", const char *" type );
.BI "int pclose(FILE *" stream );
.fi
.P
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.P
.BR popen (),
.BR pclose ():
.nf
_POSIX_C_SOURCE >= 2
|| /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
.fi
.SH DESCRIPTION
The
.BR popen ()
function opens a process by creating a pipe, forking, and invoking the
shell.
Since a pipe is by definition unidirectional, the
.I type
argument may specify only reading or writing, not both; the resulting
stream is correspondingly read-only or write-only.
.P
The
.I command
argument is a pointer to a null-terminated string containing a shell
command line.
This command is passed to
.I /bin/sh
using the
.B \-c
flag; interpretation, if any, is performed by the shell.
.P
The
.I type
argument is a pointer to a null-terminated string which must contain
either the letter \[aq]r\[aq] for reading or the letter \[aq]w\[aq] for writing.
Since glibc 2.9,
this argument can additionally include the letter \[aq]e\[aq],
which causes the close-on-exec flag
.RB ( FD_CLOEXEC )
to be set on the underlying file descriptor;
see the description of the
.B O_CLOEXEC
flag in
.BR open (2)
for reasons why this may be useful.
.P
The return value from
.BR popen ()
is a normal standard I/O stream in all respects save that it must be closed
with
.BR pclose ()
rather than
.BR fclose (3).
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
.BR popen (),
unless this is altered by the command itself.
Conversely, reading from
the stream reads the command's standard output, and the command's
standard input is the same as that of the process that called
.BR popen ().
.P
Note that output
.BR popen ()
streams are block buffered by default.
.P
The
.BR pclose ()
function waits for the associated process to terminate and returns the exit
status of the command as returned by
.BR wait4 (2).
.SH RETURN VALUE
.BR popen ():
on success, returns a pointer to an open stream that
can be used to read or write to the pipe;
if the
.BR fork (2)
or
.BR pipe (2)
calls fail, or if the function cannot allocate memory,
NULL is returned.
.P
.BR pclose ():
on success, returns the exit status of the command; if
.\" These conditions actually give undefined results, so I commented
.\" them out.
.\" .I stream
.\" is not associated with a "popen()ed" command, if
.\".I stream
.\" already "pclose()d", or if
.BR wait4 (2)
returns an error, or some other error is detected,
\-1 is returned.
.P
On failure, both functions set
.I errno
to indicate the error.
.SH ERRORS
The
.BR popen ()
function does not set
.I errno
if memory allocation fails.
If the underlying
.BR fork (2)
or
.BR pipe (2)
fails,
.I errno
is set to indicate the error.
If the
.I type
argument is invalid, and this condition is detected,
.I errno
is set to
.BR EINVAL .
.P
If
.BR pclose ()
cannot obtain the child status,
.I errno
is set to
.BR ECHILD .
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbx lb lb
l l l.
Interface Attribute Value
T{
.na
.nh
.BR popen (),
.BR pclose ()
T} Thread safety MT-Safe
.TE
.SH VERSIONS
The \[aq]e\[aq] value for
.I type
is a Linux extension.
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
POSIX.1-2001.
.SH CAVEATS
Carefully read Caveats in
.BR system (3).
.SH BUGS
Since the standard input of a command opened for reading shares its seek
offset with the process that called
.BR 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
.BR fflush (3)
before
.BR popen ().
.P
Failure to execute the shell is indistinguishable from the shell's failure
to execute the command, or an immediate exit of the command.
The only hint is an exit status of 127.
.\" .SH HISTORY
.\" A
.\" .BR popen ()
.\" and a
.\" .BR pclose ()
.\" function appeared in Version 7 AT&T UNIX.
.SH SEE ALSO
.BR sh (1),
.BR fork (2),
.BR pipe (2),
.BR wait4 (2),
.BR fclose (3),
.BR fflush (3),
.BR fopen (3),
.BR stdio (3),
.BR system (3)
|
