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POPEN(3)		   Linux Programmer's Manual		      POPEN(3)

NAME
       popen, pclose - process I/O

SYNOPSIS
       #include	<stdio.h>

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

       int pclose(FILE *stream);

DESCRIPTION
       The  popen()  function opens a process by creating a pipe, forking, and
       invoking	the shell.  Since a pipe is by definition unidirectional,  the
       type  argument  may  specify  only  reading  or	writing, not both; the
       resulting stream	is correspondingly read-only or	write-only.

       The command argument is a pointer to a null-terminated string  contain-
       ing  a shell command line.  This	command	is passed to /bin/sh using the
       -c flag;	interpretation,	if any,	is performed by	the shell.   The  mode
       argument	 is a pointer to a null-terminated string which	must be	either
       `r' for reading or `w' for writing.

       The return value	from popen() is	a normal standard I/O  stream  in  all
       respects	 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,  reading from a ``popened'' stream	reads the com-
       mand's standard output, and the command's standard input	is the same as
       that of the process that	called popen.

       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.

RETURN VALUE
       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 wait4 returns an error, or some other
       error is	detected.

ERRORS
       The popen function does not set errno if	memory allocation  fails.   If
       the  underlying fork() or pipe()	fails, errno is	set appropriately.  If
       the mode	argument is invalid, and this condition	is detected, errno  is
       set to EINVAL.

       If pclose() cannot obtain the child status, errno is set	to ECHILD.

CONFORMING TO
       POSIX.2

BUGS
       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 expected.	Similarly, the output from a command opened for	 writ-
       ing  may	 become	 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
       failure	to  execute command, or	an immediate exit of the command.  The
       only hint is an exit status of 127.

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

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

BSD MANPAGE			  1998-05-07			      POPEN(3)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | BUGS | HISTORY | SEE ALSO

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