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[thirdparty/man-pages.git] / man3 / popen.3

.\" Copyright 1991 The Regents of the University of California.
.\" All rights reserved.
.\"
.\" SPDX-License-Identifier: BSD-4-Clause-UC
.\"
.\"     @(#)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 (date) "Linux man-pages (unreleased)"
.SH NAME
popen, pclose \- pipe stream to or from a process
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <stdio.h>
.PP
.BI "FILE *popen(const char *" command ", const char *" type );
.BI "int pclose(FILE *" stream );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR popen (),
.BR pclose ():
.nf
    _POSIX_C_SOURCE >= 2
        || /* Glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
.fi
.SH DESCRIPTION
The
.BR 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.
.PP
The
.I type
argument is a pointer to a null-terminated string which must contain
either the letter \(aqr\(aq for reading or the letter \(aqw\(aq for writing.
Since glibc 2.9,
this argument can additionally include the letter \(aqe\(aq,
which causes the close-on-exec flag
.RB ( FD_CLOEXEC )
to be set on the underlying file descriptor;
see the description of the
.B O_CLOEXEC
flag in
.BR open (2)
for reasons why this may be useful.
.PP
The return value from
.BR popen ()
is a normal standard I/O stream in all respects save that it must be closed
with
.BR pclose ()
rather than
.BR 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
.BR popen (),
unless this is altered by the command itself.
Conversely, reading from
the 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
.BR popen ()
streams are block buffered by default.
.PP
The
.BR pclose ()
function waits for the associated process to terminate and returns the exit
status of the command as returned by
.BR wait4 (2).
.SH RETURN VALUE
.BR popen ():
on success, returns a pointer to an open stream that
can be used to read or write to the pipe;
if the
.BR fork (2)
or
.BR pipe (2)
calls fail, or if the function cannot allocate memory,
NULL is returned.
.PP
.BR pclose ():
on success, returns the exit status of the command; if
.\" These conditions actually give undefined results, so I commented
.\" them out.
.\" .I stream
.\" is not associated with a "popen()ed" command, if
.\".I stream
.\" already "pclose()d", or if
.BR wait4 (2)
returns an error, or some other error is detected,
\-1 is returned.
.PP
On failure, both functions set
.I errno
to indicate the error.
.SH ERRORS
The
.BR popen ()
function does not set
.I errno
if memory allocation fails.
If the underlying
.BR fork (2)
or
.BR pipe (2)
fails,
.I errno
is set to indicate the error.
If the
.I type
argument is invalid, and this condition is detected,
.I errno
is set to
.BR EINVAL .
.PP
If
.BR pclose ()
cannot obtain the child status,
.I errno
is set to
.BR ECHILD .
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR popen (),
.BR pclose ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.sp 1
.SH STANDARDS
POSIX.1-2001, POSIX.1-2008.
.PP
The \(aqe\(aq value for
.I type
is a Linux extension.
.SH NOTES
.BR Note :
carefully read Caveats in
.BR system (3).
.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
.\" .BR popen ()
.\" and a
.\" .BR pclose ()
.\" function appeared in Version 7 AT&T UNIX.
.SH SEE ALSO
.BR sh (1),
.BR fork (2),
.BR pipe (2),
.BR wait4 (2),
.BR fclose (3),
.BR fflush (3),
.BR fopen (3),
.BR stdio (3),
.BR system (3)