#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 ||

       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.  In the event of an error, these functions set
       errno to indicate the cause of the error.

       The popen() function does not set errno if memory allocation fails.  If
       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 3.54 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                               2013-04-19                          POPEN(3)
Man Pages Copyright Respective Owners. Site Copyright (C) 1994 - 2019 Hurricane Electric. All Rights Reserved.