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Is it possible to pass char* between 2 applications using a custom message in SendMessage? I know it is possible using WM_COPYDATA, but I want to know if I can send this using custom message(WM_USER + ..)

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WM_COPYDATA has been specifically invented because what you ask is not feasible directly. This because different applications live in different address spaces, so a pointer passed from an application has no meaning in another one.

WM_COPYDATA deals with the problem by using some IPC mechanism under the hood, which is what you usually do when you want to share data with another application; viable options are the usual ones: pipes, shared memory & co, have a look here to see what Windows provides.

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@David: no problem, I'll remove mine. :) By the way, you may be interested to have a look at the series about GlobalAlloc on Raymond Chen's blog, it describes in detail all the transitions it traveled through before becoming just a wrapper for LocalAlloc; quite an interesting read if you like an historical approach to Win32. :)
I'm sure I have read it once upon a time, and it seems I need to revise my knowledge!!
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I'm nearly certain that's not possible. The other app won't be able to read directly from a pointer to the first app's heap/stack.

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Here is good article which will help you in interprocess communication. It was helpful for me in sending string between two application.

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