I would like to write a "simple" programming language which can call functions written in C. For example, I would like to integrate the language with the socket library. 

What is the easiest way to call C functions from a custom programming language?

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Note: This is similar to [How do multiple-languages interact in one project?](https://stackoverflow.com/questions/636841/how-do-multiple-languages-interact-in-one-project/636851), but I hope that this question focuses more on the easiest way to *design* a language for interoperability.