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

       popen, pclose - pipe stream to or from a process

       #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
	       || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

       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).

       popen():	 on  success,  returns a pointer to an open stream that can be
       used to read or write to the pipe; if  the  fork(2)  or	pipe(2)	 calls
       fail, or if the function cannot allocate memory, NULL is returned.

       pclose():  on  success,	returns	 the  exit  status  of the command; if
       wait4(2) returns an error, or some  other  error	 is  detected,	-1  is

       Both  functions	set  errno  to	an appropriate value in the case of an

       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.

       For   an	  explanation	of   the  terms	 used  in  this	 section,  see

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

       POSIX.1-2001, POSIX.1-2008.

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

       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.

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

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GNU				  2016-03-15			      POPEN(3)