From: Anderson <ealmeida AT sigmanet DOT com DOT br>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: How to switch popen read/write modes?
Date: Tue, 28 Nov 2000 20:48:53 -0200
Organization: Pennsylvania College of Technology
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To: djgpp AT delorie DOT com
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Hans-Bernhard Broeker wrote:

> > I'm trying out with popen function. It says (at help file) that it can
> > open a program for reading and writing
> > handle = popen ("program", "rb+")
> 
> In what help file did you find that? It's untrue, for every popen()
> implementation I've seen. The only 'mode' arguments for popen() are
> "r" and ". I.e. the DJGPP docs may have a little flaw, here, where
> they say:
> 
>         The MODE is the same as for `fopen' (*note fopen::.).
> 
> That's not true.

Oh, I think that's it, then. 

Just as you said, I've searched for popen, and help says the mode is the
same as for fopen.

So, I'm going to post the problem I was working on, maybe you guys can
give some ideas (I'm counting on you! :).

I was trying to control a "child" program within a "parent" one, sending
to it messages and reading it's output, without the user even noticing
the existence of "child". It worked fine for writing OR reading, but I
couldn't figure it out why it was not working for both (thank you, now I
know !)

Here is the code:

================= child ================
// child.cpp: must be called within parent

#include <iostream.h>
#include <conio.h>
#include <string.h>
#include <stdio.h>
#include <fstream.h> //ifstream, ofstream

int main()
{
  clrscr();
  char buf[100];
  ofstream out ("out.txt");

  int a;
  cout<<"CHILD: Enter a number: ";
  cin >> a;
  cout <<"CHILD: Enter some words: ";
  gets (buf); // somebody said me I must use fgets -- why?

  // Sending results to screen
  cout << "\nCHILD: The squared number is: " << a*a << endl;
  cout << "CHILD: The words were: "<< buf; 

  // Sending results to file "out.txt"
  out << "\nThe squared number is " << a*a <<endl;
  out << "The words were "<< buf;
  return 0;
}

=========== parent =====================
// PARENT: Calls child program, send values to it
// and checks for screen output. "Emulates a user"

#include <stdio.h>
#include <iostream.h>
#include <conio.h>
#include <string.h>
#include <fstream.h>


int main()
{
  clrscr();
  FILE* handle;
  char buf[100];

  handle = popen("child", "wt+"); // + opens for both write/read.

  // Writing test: int & string
  int a;
  cout <<"PARENT: Enter a number: ";
  cin >> a;
  fprintf (handle,"%i\n",a);  // \n = pushes ENTER into child

  cout << "PARENT: Enter some words: ";
  gets (buf); // fgets ?
  fprintf (handle, "%s\n", buf); // \n = pushes ENTER

  rewind (handle); // Prepares to change for reading

  // How can I read the output of child?

  // Testing reading
  while (fgets (buf,100,handle))
    cprintf ("line obtained= %s", buf);

  getch();
  pclose(handle);

  return 0;
}
-------------------------------------------
It's possible to solve this problem in a reasonable simple way?
Thank you very much!


Sincerely,
Anderson.