An Overview of the C API

Lua is an embedded language
There are two kinds of interactions between Lua and C code:
- As a stand-alone language that can be extended with native C code (the C code is the library), this makes Lua an extensible language
- As a library to integrate into C projects to extend them (Lua is the library), this makes Lua an extension language
The difference between the two kinds of interactions is the language which has the control
The Lua interpreter uses Lua as a library
The C API is the set of functions that allow C code to interact with Lua, it comprises a set of functions to:
- read and write Lua global variables
- call Lua functions
- run pieces of Lua code
- register C functions to be callable from within Lua
The C API follows the modus operandi of C, which means that we must care about several inconveniences, including:
- type checking
- error recovery
- memory-allocation errors
Anything that can be done within Lua can also be done with the C API, however it may be more lengthy (common tasks may involve several API calls)
The major component in the communication between Lua and C is an omnipresent virtual stack

The file lua.h defines the basic functions provided by Lua, it includes functions to do the following:
- create a new Lua environment
- invoke Lua functions (eg. lua_pcall)
- read and write global variables in the Lua environment
All functions in lua.h are prefixed with lua_
The header fule lauxlib.h defines the functions provided by the auxillary library (auxlib)
All functions in lauxlib.h are prefixed with luaL_
They use the basic functions defined in lua.h to provide a higher abstraction level
The Lua library keeps all its state in the dynamic structure lua_State
New states can be created by the luaL_newstate function
The standard libraries are not loaded by default, this can be done with luaL_openlibs
The function luaL_loadstring compiles Lua code, and pushes the resulting function to the stack, returning nonzero on error.

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