Originální popis anglicky:
popen - initiate pipe streams to or from a process
Návod, kniha: POSIX Programmer's Manual
#include <stdio.h>
FILE *popen(const char *
command, const char
* mode);
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
undefined.
After
popen(), both the parent and the child process shall be capable of
executing independently before either terminates.
Pipe streams are byte-oriented.
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.
The
popen() function may fail if:
- EMFILE
- {FOPEN_MAX} or {STREAM_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.
None.
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 IEEE Std 1003.1-2001 is present, use the function
call:
(See
sysconf() ).
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
Interaction of File Descriptors and
Standard I/O Streams ).
None.
pclose() ,
pipe() ,
sysconf() ,
system() , the Base
Definitions volume of IEEE Std 1003.1-2001,
<stdio.h>, the Shell and Utilities volume of
IEEE Std 1003.1-2001,
sh
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 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
.
