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Linux 2.6 - man page for popen (linux section 3posix)

POPEN(P)			    POSIX Programmer's Manual				 POPEN(P)

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  inde-
       pendently 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  fil-
       ter mispositioned. Similar problems with an output filter may be prevented by careful buf-
       fer 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 priv-
       ileges. 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 Descrip-
       tors 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 orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003					 POPEN(P)


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