popen, pclose — process I/O
#include <stdio.h>
FILE
*popen( |
const char * | command, |
| const char * | type); |
int
pclose( |
FILE * | stream); |
The popen() function opens a
process by creating a pipe, forking, and invoking the shell.
Since a pipe is by definition unidirectional, the type argument may specify only
reading or writing, not both; the resulting stream is
correspondingly read-only or write-only.
The command
argument is a pointer to a null-terminated string containing
a shell command line. This command is passed to /bin/sh using the −c flag; interpretation, if any, is
performed by the shell. The type argument is a pointer to a
null-terminated string which must be either "r" for reading
or "w" for writing.
The return value from popen() is a normal standard I/O stream in
all respects save that it must be closed with pclose() rather than 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 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 popen().
Note that output popen()
streams are fully buffered by default.
The pclose() function waits
for the associated process to terminate and returns the exit
status of the command as returned by wait4(2).
The popen() function returns
NULL if the fork(2) or pipe(2) calls fail, or if
it cannot allocate memory.
The pclose() function
returns −1 if wait4(2) returns an error,
or some other error is detected.
The popen() function does
not set errno if memory
allocation fails. If the underlying fork(2) or pipe(2) fails, errno is set appropriately. If the
type argument is
invalid, and this condition is detected, errno is set to EINVAL.
If pclose() cannot obtain
the child status, errno is set
to ECHILD.
Since the standard input of a command opened for reading
shares its seek offset with the process that called
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
fflush(3) before
popen().
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.
This page is part of release 2.79 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man-pages/.
|
Copyright 1991 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. All advertising materials mentioning features or use of this software must display the following acknowledgement: This product includes software developed by the University of California, Berkeley and its contributors. 4. 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. (#)popen.3 6.4 (Berkeley) 4/30/91 Converted for Linux, Mon Nov 29 14:45:38 1993, faithcs.unc.edu Modified Sat May 18 20:37:44 1996 by Martin Schulze (joeylinux.de) Modified 7 May 1998 by Joseph S. Myers (jsm28cam.ac.uk) |