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POPEN(3)		 BSD Library Functions Manual		      POPEN(3)

     popen, pclose -- process I/O

     Standard C	Library	(libc, -lc)

     #include <stdio.h>

     FILE *
     popen(const char *command,	const char *type);

     pclose(FILE *stream);

     The popen() function "opens" a process by creating	a bidirectional	pipe
     forking, and invoking the shell.  Any streams opened by previous popen()
     calls in the parent process are closed in the new child process.  Histor-
     ically, popen() was implemented with a unidirectional pipe; hence many
     implementations of	popen()	only allow the type argument to	specify	read-
     ing or writing, not both.	Since popen() is now implemented using a bidi-
     rectional pipe, the type argument may request a bidirectional data	flow.
     The type argument is a pointer to a null-terminated string	which must be
     `r' for reading, `w' for writing, or `r+' for reading and writing.

     A letter `e' may be appended to that to request that the underlying file
     descriptor	be set close-on-exec.

     The command argument is a pointer to a null-terminated string containing
     a shell command line.  This command is passed to /bin/sh using the	-c
     flag; interpretation, if any, is performed	by the shell.

     The return	value from popen() is a	normal standard	I/O stream in all re-
     spects save that it must be closed	with pclose() rather than fclose().
     Writing to	such a stream writes to	the standard input of the command; the
     command's standard	output is the same as that of the process that called
     popen(), unless this is altered by	the command itself.  Conversely, read-
     ing from a	"popened" stream reads the command's standard output, and the
     command's standard	input is the same as that of the process that called

     Note that output popen() streams are fully	buffered by default.

     The pclose() function waits for the associated process to terminate and
     returns the exit status of	the command as returned	by wait4(2).

     The popen() function returns NULL if the fork(2) or pipe(2) calls fail,
     or	if it cannot allocate memory.

     The pclose() function returns -1 if stream	is not associated with a
     "popened" command,	if stream already "pclosed", or	if wait4(2) returns an

     The popen() function does not reliably set	errno.

     sh(1), fork(2), pipe(2), wait4(2),	fclose(3), fflush(3), fopen(3),
     stdio(3), system(3)

     A popen() and a pclose() function appeared	in Version 7 AT&T UNIX.

     Bidirectional functionality was added in FreeBSD 2.2.6.

     Since the standard	input of a command opened for reading shares its seek
     offset with the process that called popen(), if the original process has
     done a buffered read, the command's input position	may not	be as ex-
     pected.  Similarly, the output from a command opened for writing may be-
     come intermingled with that of the	original process.  The latter can be
     avoided by	calling	fflush(3) before popen().

     Failure to	execute	the shell is indistinguishable from the	shell's	fail-
     ure to execute command, or	an immediate exit of the command.  The only
     hint is an	exit status of 127.

     The popen() function always calls sh(1), never calls csh(1).

BSD				 May 20, 2013				   BSD


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