Accessing the Body of a Function with Lua

Question

I'm going back to the basics here but in Lua, you can define a table like so:

myTable = {}
myTable [1] = 12

Printing the table reference itself brings back a pointer to it. To access its elements you need to specify an index (i.e. exactly like you would an array)

print(myTable )    --prints pointer
print(myTable[1])  --prints 12

Now functions are a different story. You can define and print a function like so:

myFunc = function() local x = 14 end     --Defined function
print(myFunc)                            --Printed pointer to function

Is there a way to access the body of a defined function. I am trying to put together a small code visualizer and would like to 'seed' a given function with special functions/variables to allow a visualizer to 'hook' itself into the code, I would need to be able to redefine the function either from a variable or a string.

Answer 1

You could accomplish this by creating an environment for each function (see setfenv) and using global (versus local) variables. Variables created in the function would then appear in the environment table after the function is executed.

env = {}
myFunc = function() x = 14 end
setfenv(myFunc, env)
myFunc()
print(myFunc)    -- prints pointer
print(env.x)     -- prints 14

Alternatively, you could make use of the Debug Library:

> myFunc = function() local x = 14 ; debug.debug() end
> myFunc()
> lua_debug> _, x = debug.getlocal(3, 1)
> lua_debug> print(x) -- prints 14

It would probably be more useful to you to retrieve the local variables with a hook function instead of explicitly entering debug mode (i.e. adding the debug.debug() call)

There is also a Debug Interface in the Lua C API.

Answer 2

Check out Lua Introspective Facilities in the debugging library.

The main introspective function in the debug library is the debug.getinfo function. Its first parameter may be a function or a stack level. When you call debug.getinfo(foo) for some function foo, you get a table with some data about that function. The table may have the following fields:

The field you would want is func I think.

Answer 3

Using the debug library is your only bet. Using that, you can get either the string (if the function is defined in a chunk that was loaded with 'loadstring') or the name of the file in which the function was defined; together with the line-numbers at which the function definition starts and ends. See the documentation.

Here at my current job we have patched Lua so that it even gives you the column numbers for the start and end of the function, so you can get the function source using that. The patch is not very difficult to reproduce, but I don't think I'll be allowed to post it here :-(