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



NAME
       popen, pclose - pipe stream to or from a process

SYNOPSIS
       #include <stdio.h>

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

       int pclose(FILE *stream);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       popen(), pclose():
	   _POSIX_C_SOURCE >= 2 || _XOPEN_SOURCE || _BSD_SOURCE ||
	   _SVID_SOURCE

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  type  argument is a pointer to a null-terminated string which must
       contain either the letter 'r' for reading or the letter 'w'  for	 writ-
       ing.   Since glibc 2.9, this argument can additionally include the let-
       ter 'e', which causes the close-on-exec flag (FD_CLOEXEC) to be set  on
       the  underlying	file  descriptor; see the description of the O_CLOEXEC
       flag in open(2) for reasons why this may be useful.

       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(3).  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 the stream reads the command's stan-
       dard output, and the command's standard input is the same  as  that  of
       the process that called popen().

       Note that output popen() streams are block 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).

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(2) returns an error, or some
       other error is detected.	 In the event of an error, these functions set
       errno to indicate the cause of the error.

ERRORS
       The popen() function does not set errno if memory allocation fails.  If
       the underlying fork(2) or pipe(2) fails, errno  is  set	appropriately.
       If  the type 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.

ATTRIBUTES
       For  an	explanation  of	 the  terms  used   in	 this	section,   see
       attributes(7).

       +------------------+---------------+---------+
       |Interface	  | Attribute	  | Value   |
       +------------------+---------------+---------+
       |popen(), pclose() | Thread safety | MT-Safe |
       +------------------+---------------+---------+

CONFORMING TO
       POSIX.1-2001, POSIX.1-2008.

       The 'e' value for type is a Linux extension.

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.

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

COLOPHON
       This  page  is  part of release 4.04 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest	  version     of     this    page,    can    be	   found    at
       http://www.kernel.org/doc/man-pages/.



GNU				  2015-08-08			      POPEN(3)