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:
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.
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.
If mode is any other value, the result is
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.
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 moder, and a
mode of anything else would be treated as modew.
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
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
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 Interactionof File Descriptors and
Standard I/O Streams ).
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