Section (3) popen
popen, pclose — pipe stream to or from a process
||const char *command,|
|const char *type
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.
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.
is a pointer to a null-terminated string which must contain
either the letter _zsingle_quotesz_r_zsingle_quotesz_ for reading or the letter _zsingle_quotesz_w_zsingle_quotesz_ for
writing. Since glibc 2.9, this argument can additionally
include the letter _zsingle_quotesz_e_zsingle_quotesz_, which causes the close-on-exec flag
FD_CLOEXEC) to be set on the
underlying file descriptor; see the description of the
O_CLOEXEC flag in open(2) for reasons why
this may be useful.
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_zsingle_quotesz_s standard output is the same as that of the process
this is altered by the command itself. Conversely, reading
from the stream reads the command_zsingle_quotesz_s standard output, and the
command_zsingle_quotesz_s standard input is the same as that of the process
Note that output
streams are block buffered by default.
pclose() function waits
for the associated process to terminate and returns the exit
status of the command as returned by wait4(2).
popen(): on success, returns
a pointer to an open stream that can be used to read or write
to the pipe; if the fork(2) or pipe(2) calls fail, or if
the function cannot allocate memory, NULL is returned.
pclose(): on success,
returns the exit status of the command; if wait4(2) returns an error,
or some other error is detected, −1 is returned.
Both functions set
an appropriate value in the case of an error.
popen() function does
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.
pclose() cannot obtain
the child status,
errno is set
For an explanation of the terms used in this section, see attributes(7).
The _zsingle_quotesz_e_zsingle_quotesz_ value for
type is a Linux extension.
carefully read Caveats in system(3).
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_zsingle_quotesz_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
Failure to execute the shell is indistinguishable from the shell_zsingle_quotesz_s failure to execute command, or an immediate exit of the command. The only hint is an exit status of 127.
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description of the project, information about reporting bugs,
and the latest version of this page, can be found at
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
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_zsingle_quotesz__zsingle_quotesz_ 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
(#)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)