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.\" $OpenBSD: popen.3,v 1.18 2014/01/19 10:39:00 schwarze Exp $
.\"
.\" Copyright (c) 1991, 1993
.\" The Regents of the University of California. All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\" 3. Neither the name of the University nor the names of its contributors
.\" may be used to endorse or promote products derived from this software
.\" without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.Dd $Mdocdate: January 19 2014 $
.Dt POPEN 3
.Os
.Sh NAME
.Nm popen ,
.Nm pclose
.Nd process I/O
.Sh SYNOPSIS
.In stdio.h
.Ft FILE *
.Fn popen "const char *command" "const char *type"
.Ft int
.Fn pclose "FILE *stream"
.Sh DESCRIPTION
The
.Fn popen
function
.Dq opens
a process by creating a pipe, forking, and invoking the shell.
Since a pipe is by definition unidirectional, the
.Fa type
argument may specify only reading or writing, not both;
the resulting stream is correspondingly read-only or write-only.
.Pp
The
.Fa command
argument is a pointer to a NUL-terminated
string containing a shell command line.
This command is passed to
.Pa /bin/sh
using the
.Fl c
flag; interpretation, if any, is performed by the shell.
The
.Fa type
argument is a pointer to a NUL-terminated
string which must be either
.Qq r
for reading
or
.Qq w
for writing.
.Pp
The return value from
.Fn popen
is a normal standard I/O stream in all respects
except that it must be closed with
.Fn pclose
rather than
.Xr 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
.Fn popen ,
unless this is altered by the command itself.
Conversely, reading from a
.Dq popened
stream reads the command's standard output, and
the command's standard input is the same as that of the process that called
.Fn popen .
.Pp
Note that
.Fn popen
output streams are fully buffered by default.
In addition, fork handlers established using
.Xr pthread_atfork 3
are not called when a multithreaded program calls
.Fn popen .
.Pp
The
.Fn pclose
function waits for the associated process to terminate and returns the
exit status of the command as returned by
.Xr wait4 2 .
.Sh RETURN VALUES
The
.Fn popen
function returns
.Dv NULL
if the
.Xr fork 2
or
.Xr pipe 2
calls fail,
or if it cannot allocate memory.
.Pp
The
.Fn pclose
function returns \-1 if
.Fa stream
is not associated with a
.Dq popened
command, if
.Fa stream
already
.Dq pclosed ,
or if
.Xr wait4 2
returns an error.
.Sh ERRORS
The
.Fn popen
function does not reliably set
.Va errno .
.Sh SEE ALSO
.Xr sh 1 ,
.Xr fork 2 ,
.Xr pipe 2 ,
.Xr wait4 2 ,
.Xr fclose 3 ,
.Xr fflush 3 ,
.Xr fopen 3 ,
.Xr stdio 3 ,
.Xr system 3
.Sh HISTORY
A
.Fn popen
and a
.Fn pclose
function appeared in
.At v7 .
.Sh BUGS
Since the standard input of a command opened for reading
shares its seek offset with the process that called
.Fn 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
.Xr fflush 3
before
.Fn popen .
.Pp
Failure to execute the shell is indistinguishable from the shell's
failure to execute
.Fa command ,
or an immediate exit of the command.
The only hint is an exit status of 127.
.Pp
The
.Fn popen
argument always calls
.Xr sh 1 .
|
