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Lua will not catch the C exception. If you do not catch it, it will be passed up the call stack until it is either caught by some other block of code or causes the program to crash (unhandled exception). If the functions you expose to Lua call functions that can throw exceptions, you should handle them in that function.

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Programming In Lua : 23.1.1 - The Programming Language Lua

Sep 02, 2005 To start with the easiest, create a console project so that you can see the print outs from Lua. Lua problems with C. Lua is written in C, the whole Lua API is C based. Hence converting Lua into the C world would seem rather difficult, but Lua does provide abilities to do this. In Lua, though we don't have variable data types, we have three types based on the scope of the variable. Global variables − All variables are considered global unless explicitly declared as a local. Local variables − When the type is specified as local for a variable then its scope is limited with the functions inside their scope.

See full list on codeproject.com.

Please note that you do NOT need to read the previous article to understand this one, this article will only focus on the Lua VM and my very own Lua Decompiler.

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This article is part 2 of 4, you can find a complete overview of all the articles below:

Lua Compiled

During the compilation process, most of the original data gets lost. Programming languages that compile to virtual bytecode are designed for a virtual run-time environment, those languages are often called scripting languages. Most scripting languages like Lua, Python, Java, Javascript, and so on can be compiled to a virtual instruction set (aka virtual bytecode). Our goal for today is to turn Lua's virtual instructions back into human-readable code.

Lua Functions

Compiled Lua binaries are a collection of one or more Lua Functions, every Lua Function is made out of the following components:

Instructions
Upvalues
Constants
Debugging Info

Each of those will be explained in more detail.

Lua Instructions

Each Lua Function of course comes with a collection of instructions, these instructions are what we referred to as Lua Bytecodes. The instructions are specific to the Lua VM, and since this article is all about Lua version 5.1, we will continue to work with Lua 5.1 for the decompiler. The Lua instructions themselves are 4 bytes in size and contain the bytecode identifier (opcode) together with the value of registers A, B, and C.

Lua bytecodes are disassembled using the following snippet (src/Core/LuaInstruction.cs:136):

Lua Upvalues

Upvalues are commonly used to reference a local variable between multiple functions. These upvalues make it possible for a parent function to pass a reference to a local variable to one of their child functions. This may be a little bit tricky to understand, but it will become more clear when we get to the actual decompiling of the upvalues.

Lua Constants

Each Lua Function has an array of Lua Constants. Lua Constants are basiclly Objects in Lua, they can be a String, a Number (Float/Double), or just null. The number is either a float or a double depending on the compiler settings. This array of constants is saved inside the compiled Lua binary and will be accessed during runtime.

Debugging Info

Remember how compilers lose information when they compile files? Lua has a 'solution' for this, by default, Lua compilers will add additional data that is not necessary for the runtime environment but instead is used for us, humans, to help us debug our scripts when needed. This debugging information exists of local variable names, upvalues, and even line numbers. Debugging information will most likely not be present in virtualized scripts such as Luraph because the debugging info is not required for the run-time environment. Therefore I will not go into more detail nor will I support this in the decompiler (at the time of writing).

Lua VM

The Lua VM is the piece of software that is responsible for interpreting Lua bytecodes while also handling the memory. This piece of software can be found on GitHub (mirror) or somewhere on the official lua.org website. Maybe I should mention that the official Lua website is at lua.org, but I assume most people are familiar with GitHub, so my preference is the GitHub mirror.

VM in a nutshell

Since the complete Lua VM is a little out of scope for this article, I will just sum up a few basic things we need to know before we continue. The VM has 3 main registers, A, B, and C. Register A is 8-bit while registers B and C are 9-bits. Register B and C are combined to form the 18-bit register Bx. The final register is sBx, which is just a signed interpretation of the 18-bit register Bx.

One important thing to know is the usage of registers B and C, they are both 9-bit long while register A is only 8-bit long. This is done for the reason that registers B and C are often used to reference a Lua Constant, this is done by setting the most significant bit. Lucky for A, Lua instructions only use a maximum of 2 constants at a time, so register A shouldn't have to worry about his small size ;).

Another important fact you may want to know is that the sBx register is only used in the JMP instruction, because the only time Lua may need a negative number is whenever it performs a branch/jump.

Decoding

Before we can start decompiling we must first get a better understanding of what each Lua instruction represents. Each Lua instruction is unique and has its own logic, to give you an idea of how they work I will start with explaining a few common Lua instructions.

Example Instructions

The first thing you notice is that there are only two numbers after the instruction, and that is because the MOVE instruction only uses registers A and B. To have an overview of all the Lua bytecode you can take a look at the Lua's lopcodes.h on GitHub (mirror). Not only do we know which registers the MOVE uses, but also what it should be doing. For the MOVE instruction, it is R[A] := R[B], which means that it moves the value of register B into register A, meaning that MOVE 1 0 is just var1 = var0.

Believe it or not, but we have almost decompiled that MOVE instruction. The only thing that is missing is that we may need to prepend the local keyword for the first variable to indicate that var1 is a local variable that has not yet been used. And there we go, our first instruction, MOVE, can be decompiled like that. But unfortunately, not every instruction is as easy as the MOVE instruction, lets take a look at the CALL for example. Opcode CALL uses registers A, B and C in a more complex way, but fear not, it's actually pretty simple once you understand what is going on. To explain this one, I will