How to make a (compiled) code that other programmers can use (cross-platform)?

[tom64]

(C++ question) I need to be able make a compiled code (like a .dll?) which other programmers can use on linux, win,, mac, etc.

The compiled code would simply do calculations and spit out an answer in memory.

I need it to have certain functions that they can easily call and understand (without actually seeing the source).

Does anyone know if this is possible and how?

Thanks in advance,
Tom.

[mateandmetal]

You cant

Windows uses .dll libraries
Mac uses dylib ? I guess 
Linux uses .so files

You must compile the libraries for the specific OS (and architecture)