1.\" $OpenBSD: popen.3,v 1.10 2001/07/17 17:08:29 aaron 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. All advertising materials mentioning features or use of this software 15.\" must display the following acknowledgement: 16.\" This product includes software developed by the University of 17.\" California, Berkeley and its contributors. 18.\" 4. Neither the name of the University nor the names of its contributors 19.\" may be used to endorse or promote products derived from this software 20.\" without specific prior written permission. 21.\" 22.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 23.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 24.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 25.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 26.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 27.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 28.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 29.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 30.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 31.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 32.\" SUCH DAMAGE. 33.\" 34.Dd June 4, 1993 35.Dt POPEN 3 36.Os 37.Sh NAME 38.Nm popen , 39.Nm pclose 40.Nd process 41.Tn I/O 42.Sh SYNOPSIS 43.Fd #include <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 a pipe, forking, and invoking the shell. 54Since a pipe is by definition unidirectional, the 55.Fa type 56argument may specify only reading or writing, not both; 57the resulting stream is correspondingly read-only or write-only. 58.Pp 59The 60.Fa command 61argument is a pointer to a null-terminated 62string containing a shell command line. 63This command is passed to 64.Pa /bin/sh 65using the 66.Fl c 67flag; interpretation, if any, is performed by the shell. 68The 69.Fa type 70argument is a pointer to a null-terminated 71string which must be either 72.Qq r 73for reading 74or 75.Qq w 76for writing. 77.Pp 78The return value from 79.Fn popen 80is a normal standard 81.Tn I/O 82stream in all respects 83except that it must be closed with 84.Fn pclose 85rather than 86.Fn fclose . 87Writing to such a stream 88writes to the standard input of the command; 89the command's standard output is the same as that of the process that called 90.Fn popen , 91unless this is altered by the command itself. 92Conversely, reading from a 93.Dq popened 94stream reads the command's standard output, and 95the command's standard input is the same as that of the process that called 96.Fn popen . 97.Pp 98Note that output 99.Fn popen 100streams are fully buffered by default. 101.Pp 102The 103.Fn pclose 104function waits for the associated process to terminate and returns the 105exit status of the command as returned by 106.Xr wait4 2 . 107.Sh RETURN VALUES 108The 109.Fn popen 110function returns 111.Dv NULL 112if the 113.Xr fork 2 114or 115.Xr pipe 2 116calls fail, 117or if it cannot allocate memory. 118.Pp 119The 120.Fn pclose 121function returns \-1 if 122.Fa stream 123is not associated with a 124.Dq popened 125command, if 126.Fa stream 127already 128.Dq pclosed , 129or if 130.Xr wait4 131returns an error. 132.Sh ERRORS 133The 134.Fn popen 135function does not reliably set 136.Va errno . 137.Sh SEE ALSO 138.Xr sh 1 , 139.Xr fork 2 , 140.Xr pipe 2 , 141.Xr wait4 2 , 142.Xr fclose 3 , 143.Xr fflush 3 , 144.Xr fopen 3 , 145.Xr stdio 3 , 146.Xr system 3 147.Sh BUGS 148Since the standard input of a command opened for reading 149shares its seek offset with the process that called 150.Fn popen , 151if the original process has done a buffered read, 152the command's input position may not be as expected. 153Similarly, the output from a command opened for writing 154may become intermingled with that of the original process. 155The latter can be avoided by calling 156.Xr fflush 3 157before 158.Fn popen . 159.Pp 160Failure to execute the shell is indistinguishable from the shell's 161failure to execute command, or an immediate exit of the command. 162The only hint is an exit status of 127. 163.Pp 164The 165.Fn popen 166argument always calls 167.Xr sh ; 168it never calls 169.Xr csh . 170.Sh HISTORY 171A 172.Fn popen 173and a 174.Fn pclose 175function appeared in 176.At v7 . 177