1.\" $NetBSD: popen.3,v 1.16 2007/08/02 23:45:10 wiz Exp $ 2.\" 3.\" Copyright (c) 1991, 1993 4.\" The Regents of the University of California. All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 3. Neither the name of the University nor the names of its contributors 15.\" may be used to endorse or promote products derived from this software 16.\" without specific prior written permission. 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 19.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 20.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 21.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 22.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 23.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 24.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 25.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 26.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 27.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 28.\" SUCH DAMAGE. 29.\" 30.\" @(#)popen.3 8.2 (Berkeley) 5/3/95 31.\" 32.Dd August 2, 2007 33.Dt POPEN 3 34.Os 35.Sh NAME 36.Nm popen , 37.Nm pclose 38.Nd process 39.Tn I/O 40.Sh LIBRARY 41.Lb libc 42.Sh SYNOPSIS 43.In stdio.h 44.Ft FILE * 45.Fn popen "const char *command" "const char *type" 46.Ft int 47.Fn pclose "FILE *stream" 48.Sh DESCRIPTION 49The 50.Fn popen 51function 52.Dq opens 53a process by creating an IPC connection, 54forking, 55and invoking the shell. 56Historically, 57.Nm popen 58was implemented with a unidirectional pipe; 59hence many implementations of 60.Nm popen 61only allow the 62.Fa type 63argument to specify reading or writing, not both. 64Since 65.Nm popen 66is now implemented using sockets, the 67.Fa type 68may request a bidirectional data flow. 69The 70.Fa type 71argument is a pointer to a null-terminated string 72which must be 73.Ql r 74for reading, 75.Ql w 76for writing, or 77.Ql r+ 78for reading and writing. 79.Pp 80The 81.Fa command 82argument is a pointer to a null-terminated string 83containing a shell command line. 84This command is passed to 85.Pa /bin/sh 86using the 87.Fl c 88flag; interpretation, if any, is performed by the shell. 89.Pp 90The return value from 91.Fn popen 92is a normal standard 93.Tn I/O 94stream in all respects 95save that it must be closed with 96.Fn pclose 97rather than 98.Fn fclose . 99Writing to such a stream 100writes to the standard input of the command; 101the command's standard output is the same as that of the process that called 102.Fn popen , 103unless this is altered by the command itself. 104Conversely, reading from a 105.Dq popened 106stream reads the command's standard output, and 107the command's standard input is the same as that of the process that called 108.Fn popen . 109.Pp 110Note that output 111.Fn popen 112streams are fully buffered by default. 113.Pp 114The 115.Fn pclose 116function waits for the associated process to terminate 117and returns the exit status of the command 118as returned by 119.Fn wait4 . 120.Sh RETURN VALUES 121The 122.Fn popen 123function returns 124.Dv NULL 125if the 126.Xr fork 2 , 127.Xr pipe 2 , 128or 129.Xr socketpair 2 130calls fail, 131or if it cannot allocate memory. 132.Pp 133The 134.Fn pclose 135function 136returns \-1 if 137.Fa stream 138is not associated with a 139.Dq popened 140command, if 141.Fa stream 142has already been 143.Dq pclosed , 144or if 145.Xr wait4 2 146returns an error. 147.Sh ERRORS 148The 149.Fn popen 150function does not reliably set 151.Va errno . 152.Sh SEE ALSO 153.Xr sh 1 , 154.Xr fork 2 , 155.Xr pipe 2 , 156.Xr socketpair 2 , 157.Xr wait4 2 , 158.Xr fclose 3 , 159.Xr fflush 3 , 160.Xr fopen 3 , 161.Xr shquote 3 , 162.Xr stdio 3 , 163.Xr system 3 164.Sh STANDARDS 165The 166.Fn popen 167and 168.Fn pclose 169functions conform to 170.St -p1003.2-92 . 171.Sh HISTORY 172A 173.Fn popen 174and a 175.Fn pclose 176function appeared in 177.At v7 . 178.Sh BUGS 179Since the standard input of a command opened for reading 180shares its seek offset with the process that called 181.Fn popen , 182if the original process has done a buffered read, 183the command's input position may not be as expected. 184Similarly, the output from a command opened for writing 185may become intermingled with that of the original process. 186The latter can be avoided by calling 187.Xr fflush 3 188before 189.Fn popen . 190.Pp 191Failure to execute the shell 192is indistinguishable from the shell's failure to execute command, 193or an immediate exit of the command. 194The only hint is an exit status of 127. 195.Pp 196The 197.Fn popen 198argument 199always calls 200.Xr sh 1 , 201never calls 202.Xr csh 1 . 203