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           #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.


           seek offset with the process that called popen(), if the original  pro-
           cess  has done a buffered read, the command's input position may not be
           as expected.  Similarly, the output from a command opened  for  writing
           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)

    GNU 2013-04-19 POPEN(3)


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