man3/popen.3

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