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.\"     @(#)popen.3	6.4 (Berkeley) 4/30/91
.\"
.\" Converted for Linux, Mon Nov 29 14:45:38 1993, faith@cs.unc.edu
.\" Modified Sat May 18 20:37:44 1996 by Martin Schulze (joey@linux.de)
.\" Modified 7 May 1998 by Joseph S. Myers (jsm28@cam.ac.uk)
.\"
.TH POPEN 3  "7 May 1998" "BSD MANPAGE" "Linux Programmer's Manual"
.SH NAME
popen, pclose \- process I/O
.SH SYNOPSIS
.B #include <stdio.h>
.sp
.BI "FILE *popen(const char *" command ", const char *" type );
.sp
.BI "int pclose(FILE *" stream );
.SH DESCRIPTION
The
.B popen()
function opens a process by creating a pipe, forking, and invoking the
shell.  Since a pipe is by definition unidirectional, the
.I type
argument may specify only reading or writing, not both; the resulting
stream is correspondingly read-only or write-only.
.PP
The
.I command
argument is a pointer to a null-terminated string containing a shell
command line.  This command is passed to
.I /bin/sh
using the
.B \-c
flag; interpretation, if any, is performed by the shell.  The
.I mode
argument is a pointer to a null-terminated string which must be either `r'
for reading or `w' for writing.
.PP
The return value from
.B popen()
is a normal standard I/O stream in all respects save that it must be closed
with
.B pclose()
rather than
.BR fclose() .
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
.BR popen() ,
unless this is altered by the command itself.  Conversely, reading from a
``popened'' stream reads the command's standard output, and the command's
standard input is the same as that of the process that called
.BR popen .
.PP
Note that output
.B popen
streams are fully buffered by default.
.PP
The
.B pclose
function waits for the associated process to terminate and returns the exit
status of the command as returned by
.BR wait4 .
.SH "RETURN VALUE"
The
.B popen
function returns
.B NULL
if the
.BR fork (2)
or
.BR pipe (2)
calls fail, or if it cannot allocate memory.
.PP
The
.B pclose
function returns \-1 if
.\" These conditions actually give undefined results, so I commented
.\" them out.
.\" .I stream
.\" is not associated with a ``popened'' command, if
.\".I stream
.\" already ``pclosed'', or if
.B wait4
returns an error, or some other error is detected.
.SH ERRORS
The
.B popen
function does not set
.I errno
if memory allocation fails.  If the underlying
.BR fork() " or " pipe()
fails,
.I errno
is set appropriately.  If the
.I mode
argument is invalid, and this condition is detected,
.I errno
is set to
.BR EINVAL .
.PP
If
.B pclose()
cannot obtain the child status,
.I errno
is set to
.BR ECHILD .
.SH "CONFORMING TO"
POSIX.2
.SH BUGS
Since the standard input of a command opened for reading shares its seek
offset with the process that called
.BR 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
.BR fflush (3)
before
.BR popen .
.PP
Failure to execute the shell is indistinguishable from the shell's failure
to execute command, or an immediate exit of the command.  The only hint is
an exit status of 127.
.SH HISTORY
A
.B popen()
and a
.B pclose()
function appeared in Version 7 AT&T UNIX.
.SH "SEE ALSO"
.BR fork (2),
.BR sh (1),
.BR pipe (2),
.BR wait4 (2),
.BR fflush "(3),
.BR fclose (3),
.BR fopen (3),
.BR stdio (3),
.BR system (3).