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 undefined.
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 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.
EXAMPLES
None.
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 IEEE Std 1003.1-2001 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 Interaction of File Descriptors and
Standard I/O Streams ).
FUTURE DIRECTIONS
None.
SEE ALSO
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
COPYRIGHT
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 .