POPEN
Section: POSIX Programmer's Manual (3P)
Updated: 2017
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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.
NAME
popen
--- initiate pipe streams to or from a process
SYNOPSIS
#include <stdio.h>
FILE *popen(const char *command, const char *mode);
DESCRIPTION
The
popen()
function shall execute the command specified by the string
command.
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.
The environment of the executed command shall be as if a child process
were created within the
popen()
call using the
fork()
function, and the child invoked the
sh
utility using the call:
-
execl(shell path, "sh", "-c", command, (char *)0);
where
shell path
is an unspecified pathname for the
sh
utility.
The
popen()
function shall ensure that any streams from previous
popen()
calls that remain open in the parent process are closed in the new
child process.
The
mode
argument to
popen()
is a string that specifies I/O mode:
- 1.
-
If
mode
is
r,
when the child process is started, its file descriptor STDOUT_FILENO
shall be the writable end of the pipe, and the file descriptor
fileno(stream) in the calling process, where
stream
is the stream pointer returned by
popen(),
shall be the readable end of the pipe.
- 2.
-
If
mode
is
w,
when the child process is started its file descriptor STDIN_FILENO
shall be the readable end of the pipe, and the file descriptor
fileno(stream) in the calling process, where
stream
is the stream pointer returned by
popen(),
shall be the writable end of the pipe.
- 3.
-
If
mode
is any other value, the result is unspecified.
After
popen(),
both the parent and the child process shall be capable of executing
independently before either terminates.
Pipe streams are byte-oriented.
RETURN VALUE
Upon successful completion,
popen()
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
errno
to indicate the error.
ERRORS
The
popen()
function shall fail if:
- EMFILE
-
{STREAM_MAX}
streams are currently open in the calling process.
The
popen()
function may fail if:
- EMFILE
-
{FOPEN_MAX}
streams are currently open in the calling process.
- EINVAL
-
The
mode
argument is invalid.
The
popen()
function may also set
errno
values as described by
fork()
or
pipe().
The following sections are informative.
EXAMPLES
Using popen() to Obtain a List of Files from the ls Utility
The following example demonstrates the use of
popen()
and
pclose()
to execute the command
ls*
in order to obtain a list of files in the current directory:
-
#include <stdio.h>
...
FILE *fp;
int status;
char path[PATH_MAX];
fp = popen("ls *", "r");
if (fp == NULL)
/* Handle error */;
while (fgets(path, PATH_MAX, fp) != NULL)
printf("%s", path);
status = pclose(fp);
if (status == -1) {
/* Error reported by pclose() */
...
} else {
/* Use macros described under wait() to inspect `status' in order
to determine success/failure of command executed by popen() */
...
}
APPLICATION USAGE
Since open files are shared, a mode
r
command can be used as an input filter and a mode
w
command as an output filter.
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
fflush().
A stream opened by
popen()
should be closed by
pclose().
The behavior of
popen()
is specified for values of
mode
of
r
and
w.
Other modes such as
rb
and
wb
might be supported by specific implementations, but these would not be
portable features. Note that historical implementations of
popen()
only check to see if the first character of
mode
is
r.
Thus, a
mode
of
robert the robot
would be treated as
mode
r,
and a
mode
of
anything else
would be treated as
mode
w.
If the application calls
waitpid()
or
waitid()
with a
pid
argument greater than 0, and it still has a stream that was called with
popen()
open, it must ensure that
pid
does not refer to the process started by
popen().
To determine whether or not the environment specified in the Shell and Utilities volume of POSIX.1-2017 is
present, use the function call:
-
sysconf(_SC_2_VERSION)
(See
sysconf()).
RATIONALE
The
popen()
function should not be used by programs that have set user (or group)
ID privileges. The
fork()
and
exec
family of functions (except
execlp()
and
execvp()),
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.
If the original and
popen()ed
processes both intend to read or write or read and write a common file,
and either will be using FILE-type C functions (fread(),
fwrite(),
and so on), the rules for sharing file handles must be observed (see
Section 2.5.1, Interaction of File Descriptors and Standard I/O Streams).
FUTURE DIRECTIONS
None.
SEE ALSO
Section 2.5, Standard I/O Streams,
fork(),
pclose(),
pipe(),
sysconf(),
system(),
wait(),
waitid()
The Base Definitions volume of POSIX.1-2017,
<stdio.h>
The Shell and Utilities volume of POSIX.1-2017,
sh
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 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 .
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
Index
- PROLOG
-
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- EXAMPLES
-
- Using popen() to Obtain a List of Files from the ls Utility
-
- APPLICATION USAGE
-
- RATIONALE
-
- FUTURE DIRECTIONS
-
- SEE ALSO
-
- COPYRIGHT
-
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