POPEN(3P)						     POSIX Programmer's Manual							 POPEN(3P)

PROLOG

       This  manual page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (consult the correspond-
       ing 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 implemen-
       tations, 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.1-2008 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.1-2008, <stdio.h>

       The Shell and Utilities volume of POSIX.1-2008, sh

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition, Standard for Information Technol-
       ogy -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C)  2013  by	the  Institute	of
       Electrical and Electronics Engineers, Inc and The Open Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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 Stan-
       dard is the referee document. The original Standard can be obtained online at http://www.unix.org/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 .

IEEE
/The Open Group						       2013								 POPEN(3P)