This first edition was written for Lua 5.0. While still largely relevant for later versions, there are some differences.
The fourth edition targets Lua 5.3 and is available at Amazon and other bookstores.
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Part IV. The C API Chapter 27. Techniques for Writing C Functions

27 – Techniques for Writing C Functions

Both the official API and the auxiliary library provide several mechanisms to help writing C functions. In this chapter, we cover special mechanisms for array manipulation, for string manipulation, and for storing Lua values in C.