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32 .\" @(#)popen.3 8.2 (Berkeley) 5/3/95
33 .\" $FreeBSD: src/lib/libc/gen/popen.3,v 1.10.2.4 2003/03/15 15:11:05 trhodes Exp $
34 .\" $DragonFly: src/lib/libc/gen/popen.3,v 1.4 2006/04/08 08:17:06 swildner Exp $
35 .\"
36 .Dd May 3, 1995
37 .Dt POPEN 3
38 .Os
39 .Sh NAME
40 .Nm popen ,
41 .Nm pclose
42 .Nd process
43 .Tn I/O
44 .Sh LIBRARY
45 .Lb libc
46 .Sh SYNOPSIS
47 .In stdio.h
48 .Ft FILE *
49 .Fn popen "const char *command" "const char *type"
50 .Ft int
51 .Fn pclose "FILE *stream"
52 .Sh DESCRIPTION
53 The
54 .Fn popen
55 function
56 .Dq opens
57 a process by creating a bidirectional pipe
58 forking,
59 and invoking the shell.
60 Any streams opened by previous
61 .Fn popen
62 calls in the parent process are closed in the new child process.
63 Historically,
64 .Fn popen
65 was implemented with a unidirectional pipe;
66 hence many implementations of
67 .Fn popen
68 only allow the
69 .Fa type
70 argument to specify reading or writing, not both.
71 Since
72 .Fn popen
73 is now implemented using a bidirectional pipe, the
74 .Fa type
75 argument may request a bidirectional data flow.
76 The
77 .Fa type
78 argument is a pointer to a null-terminated string
79 which must be
80 .Ql r
81 for reading,
82 .Ql w
83 for writing, or
84 .Ql r+
85 for reading and writing.
86 .Pp
87 The
88 .Fa command
89 argument is a pointer to a null-terminated string
90 containing a shell command line.
91 This command is passed to
92 .Pa /bin/sh
93 using the
94 .Fl c
95 flag; interpretation, if any, is performed by the shell.
96 .Pp
97 The return value from
98 .Fn popen
99 is a normal standard
100 .Tn I/O
101 stream in all respects
102 save that it must be closed with
103 .Fn pclose
104 rather than
105 .Fn fclose .
106 Writing to such a stream
107 writes to the standard input of the command;
108 the command's standard output is the same as that of the process that called
109 .Fn popen ,
110 unless this is altered by the command itself.
111 Conversely, reading from a
112 .Dq popened
113 stream reads the command's standard output, and
114 the command's standard input is the same as that of the process that called
115 .Fn popen .
116 .Pp
117 Note that output
118 .Fn popen
119 streams are fully buffered by default.
120 .Pp
121 The
122 .Fn pclose
123 function waits for the associated process to terminate
124 and returns the exit status of the command
125 as returned by
126 .Fn wait4 .
127 .Sh RETURN VALUES
128 The
129 .Fn popen
130 function returns
131 .Dv NULL
132 if the
133 .Xr fork 2
134 or
135 .Xr pipe 2
136 calls fail,
137 or if it cannot allocate memory.
138 .Pp
139 The
140 .Fn pclose
141 function
142 returns \-1 if
143 .Fa stream
144 is not associated with a
145 .Dq popened
146 command, if
147 .Fa stream
148 already
149 .Dq pclosed ,
150 or if
151 .Xr wait4 2
152 returns an error.
153 .Sh ERRORS
154 The
155 .Fn popen
156 function does not reliably set
157 .Va errno .
158 .Sh SEE ALSO
159 .Xr sh 1 ,
160 .Xr fork 2 ,
161 .Xr pipe 2 ,
162 .Xr wait4 2 ,
163 .Xr fclose 3 ,
164 .Xr fflush 3 ,
165 .Xr fopen 3 ,
166 .Xr stdio 3 ,
167 .Xr system 3
168 .Sh HISTORY
169 A
170 .Fn popen
171 and a
172 .Fn pclose
173 function appeared in
174 .At v7 .
175 .Pp
176 Bidirectional functionality was added in
177 .Fx 2.2.6 .
178 .Sh BUGS
179 Since the standard input of a command opened for reading
180 shares its seek offset with the process that called
181 .Fn popen ,
182 if the original process has done a buffered read,
183 the command's input position may not be as expected.
184 Similarly, the output from a command opened for writing
185 may become intermingled with that of the original process.
186 The latter can be avoided by calling
187 .Xr fflush 3
188 before
189 .Fn popen .
190 .Pp
191 Failure to execute the shell
192 is indistinguishable from the shell's failure to execute command,
193 or an immediate exit of the command.
194 The only hint is an exit status of 127.
195 .Pp
196 The
197 .Fn popen
198 function
199 always calls
200 .Xr sh 1 ,
201 never calls
202 .Xr csh 1 .