CODEWITHSUNDEEP
functiondebuggingluafunction-pointersvisualizer
user29053
user29053

Reputation:

Accessing the Body of a Function with Lua

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.

Upvotes: 11

Views: 14407

Answers (4)

Jan de Vos
Jan de Vos

Reputation: 3858

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 :-(

Upvotes: 5

Alexander Gladysh
Alexander Gladysh

Reputation: 41383

There is no way to get access to body source code of given function in plain Lua. Source code is thrown away after compilation to byte-code.

Note BTW that function may be defined in run-time with loadstring-like facility.

Partial solutions are possible — depending on what you actually want to achieve.

You may get source code position from the debug library — if debug library is enabled and debug symbols are not stripped from the bytecode. After that you may load actual source file and extract code from there.

You may decorate functions you're interested in manually with required metadata. Note that functions in Lua are valid table keys, so you may create a function-to-metadata table. You would want to make this table weak-keyed, so it would not prevent functions from being collected by GC.

If you would need a solution for analyzing Lua code, take a look at Metalua.

Upvotes: 11

Judge Maygarden
Judge Maygarden

Reputation: 27573

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.

Upvotes: 0

Nick Van Brunt
Nick Van Brunt

Reputation: 15464

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.

Upvotes: 5

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