.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "POPEN" 3P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" popen 
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.SH NAME
popen \- initiate pipe streams to or from a process
.SH SYNOPSIS
.LP
\fB#include <stdio.h>
.br
.sp
FILE *popen(const char *\fP\fIcommand\fP\fB, const char *\fP\fImode\fP\fB);
\fP
\fB
.br
\fP
.SH DESCRIPTION
.LP
The \fIpopen\fP() function shall execute the command specified by
the string \fIcommand\fP. It shall create a pipe between the
calling program and the executed command, and shall return a pointer
to a stream that can be used to either read from or write to
the pipe.
.LP
The environment of the executed command shall be as if a child process
were created within the \fIpopen\fP() call using the \fIfork\fP()
function, and the child invoked the \fIsh\fP
utility using the call:
.sp
.RS
.nf

\fBexecl(\fP\fIshell path\fP\fB, "sh", "-c",\fP \fIcommand\fP\fB, (char *)0);
\fP
.fi
.RE
.LP
where \fIshell path\fP is an unspecified pathname for the \fIsh\fP
utility.
.LP
The \fIpopen\fP() function shall ensure that any streams from previous
\fIpopen\fP() calls that remain open in the parent
process are closed in the new child process.
.LP
The \fImode\fP argument to \fIpopen\fP() is a string that specifies
I/O mode:
.IP " 1." 4
If \fImode\fP is \fIr\fP, when the child process is started, its file
descriptor STDOUT_FILENO shall be the writable end of
the pipe, and the file descriptor \fIfileno\fP(\fIstream\fP) in the
calling process, where \fIstream\fP is the stream pointer
returned by \fIpopen\fP(), shall be the readable end of the pipe.
.LP
.IP " 2." 4
If \fImode\fP is \fIw\fP, when the child process is started its file
descriptor STDIN_FILENO shall be the readable end of the
pipe, and the file descriptor \fIfileno\fP(\fIstream\fP) in the calling
process, where \fIstream\fP is the stream pointer
returned by \fIpopen\fP(), shall be the writable end of the pipe.
.LP
.IP " 3." 4
If \fImode\fP is any other value, the result is undefined.
.LP
.LP
After \fIpopen\fP(), both the parent and the child process shall be
capable of executing independently before either
terminates.
.LP
Pipe streams are byte-oriented.
.SH RETURN VALUE
.LP
Upon successful completion, \fIpopen\fP() shall return a pointer to
an open stream that can be used to read or write to the
pipe. Otherwise, it shall return a null pointer and may set \fIerrno\fP
to indicate the error.
.SH ERRORS
.LP
The \fIpopen\fP() function may fail if:
.TP 7
.B EMFILE
{FOPEN_MAX} or {STREAM_MAX} streams are currently open in the calling
process.
.TP 7
.B EINVAL
The \fImode\fP argument is invalid.
.sp
.LP
The \fIpopen\fP() function may also set \fIerrno\fP values as described
by \fIfork\fP() or \fIpipe\fP().
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
None.
.SH APPLICATION USAGE
.LP
Since open files are shared, a mode \fIr\fP command can be used as
an input filter and a mode \fIw\fP command as an output
filter.
.LP
Buffered reading before opening an input filter may leave the standard
input of that filter mispositioned. Similar problems with
an output filter may be prevented by careful buffer flushing; for
example, with \fIfflush\fP().
.LP
A stream opened by \fIpopen\fP() should be closed by \fIpclose\fP().
.LP
The behavior of \fIpopen\fP() is specified for values of \fImode\fP
of \fIr\fP and \fIw\fP. Other modes such as \fIrb\fP
and \fIwb\fP might be supported by specific implementations, but these
would not be portable features. Note that historical
implementations of \fIpopen\fP() only check to see if the first character
of \fImode\fP is \fIr\fP. Thus, a \fImode\fP of
\fIrobert the robot\fP would be treated as \fImode\fP \fIr\fP, and
a \fImode\fP of \fIanything else\fP would be treated as
\fImode\fP \fIw\fP.
.LP
If the application calls \fIwaitpid\fP() or \fIwaitid\fP() with a
\fIpid\fP argument greater than 0, and it still has a stream that
was called
with \fIpopen\fP() open, it must ensure that \fIpid\fP does not refer
to the process started by \fIpopen\fP().
.LP
To determine whether or not the environment specified in the Shell
and Utilities volume of IEEE\ Std\ 1003.1-2001 is
present, use the function call:
.sp
.RS
.nf

\fBsysconf(_SC_2_VERSION)
\fP
.fi
.RE
.LP
(See \fIsysconf\fP()).
.SH RATIONALE
.LP
The \fIpopen\fP() function should not be used by programs that have
set user (or group) ID privileges. The \fIfork\fP() and \fIexec\fP
family of functions (except \fIexeclp\fP() and \fIexecvp\fP()), should
be used
instead. This prevents any unforeseen manipulation of the environment
of the user that could cause execution of commands not
anticipated by the calling program.
.LP
If the original and \fIpopen\fP()ed processes both intend to read
or write or read and write a common file, and either will be
using FILE-type C functions ( \fIfread\fP(), \fIfwrite\fP(), and so
on), the rules for sharing file handles must be observed (see \fIInteraction
of File Descriptors and Standard I/O Streams\fP ).
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIpclose\fP(), \fIpipe\fP(), \fIsysconf\fP(),
\fIsystem\fP(), the Base Definitions volume of IEEE\ Std\ 1003.1-2001,
\fI<stdio.h>\fP, the Shell and Utilities volume of IEEE\ Std\ 1003.1-2001,
\fIsh\fP
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .