How do you make a program to interface with another?

In C++ (the programming language I am most familiar with), how easy or hard would it be to make a program which interacted with another program. Say for example, how easy would it be to write a program which interacted with msn to send out a nudge or a predefined message to a certain contact at a regular time interval. How easy would it be to write a program which could cause another program to send out different predefined messages (continuing the idea with msn), depending on what the contact in the conversation wrote?

How exactly would/could all the previously mentioned things be done?

This will be enough to get you started...

The software should have assemblies in it's install directories which I assume manage their protocol connections and will allow you to interface.

Actually, for example, what you'd really want to do, for AOL Instant Messanger is not interface with the application, but learn the protocol it communicates with. In it's case it's Oscar Protocol, which is NOT open source but has been reverse engineered and documentation can be found around the net which describes it.

You would write a piece of software that accesses those assemblies to its underlying functionality via some exploit, or write the software to access its protocol and use the appropriate requests through the application to connect an account and send information across to any given user...

This should be enough for you to get started, I refuse to divulge more since it better be a damn learning experience for you. So I hope no one else makes it too easy on you. Since obviously your intent is to screw around with people immaturely. via "how easy would it be to write a program which interacted with msn to send out a nudge or a predefined message to a certain contact at a regular time interval." PSCH! Lame.

lol, actually, I have no intention of using this program to annoy people by sending them regular nudges, I was merely using that as an example to try and get my point across. What I Really want to do is to make something which is very long and boring (and which happens over a network obviously) automated. but anyway, thanks for the info.

edit: By the way in my initial post I spoke about sending a nudge at regular intervals, it does also imply that my real intention for this program involves sending information at regular intervals.

Well if you mean interacting with a program as in, actually using the program with another program, you would need to learn the Win32 API with C++, and learn about FindWindow which will help you select the proper GUI HWND's (AKA the GUI parts of the program) and interact with them according to the type of GUI it is, and use the corresponding API in msdn.library.com.

If you mean interacting with the program via a protocol, which would be the best, you would need to know that specific programs protocol, or if you own source code for both programs then you can have them communicate with each other either by OS WinAPI messages using SendMessage, or you can use TCP or UDP internet protocols to communicate as well.

However, I think usually the GUI integration is easiest. However it takes a long time to learn everything about that subject as documentation is not that abundant on it.

just learn socket programming.

finche_88 i gues you wuld like to cheat at some cind of game like second life or something:)? if you send 2 many same info thay can find that out and kick you out of the game.

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Originally Posted by ziga

finche_88 i gues you wuld like to cheat at some cind of game like second life or something:)? if you send 2 many same info thay can find that out and kick you out of the game.

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lol, that wasn't the initial use of it, but good idea though...*Finchie_88 plots* (just kidding :P)
As for the info. Thank you for giving me initial information. If I have more difficulties I will probably post here again.