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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 || _XOPEN_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 writing.	 Since
       glibc 2.9, this argument can additionally include the letter '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 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(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 wait4(2) returns an error, or  some
       other error is detected.

       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.


       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)

       This page is part of release 3.22 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

GNU				  2008-10-10			      POPEN(3)

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