4. Advanced Custom Functions

Arguments

int Arguments

The get_args() function exists which returns a tuple of a maximum of 7 arguments (integers only, but some data will be pointers), you can return arguments using push_args().

module.functions["sum"] = function()
   local a, b = module.util.get_args() 
   push_args(a+b)
end

float & double Arguments

Floating point numbers can be used as arguments.

module.functions["floating_sum"] = function()
   local a, b = module.util.get_f_args() -- notice the _f_, it will return normal luau numbers.
   push_f_args(a+b)
end

String Arguments

Strings are simply a integer that represents a pointer in module.memory <buffer> which holds the needed string.

module.functions["print_upper"] = function()
   local pointer = module.util.get_args()
   local str = module.util.read_string(pointer)
   
   print(str:upper())
end

Returning Strings

Getting strings back requires using malloc to write the string to a new location in memory.

const char * upperString(const char *);
int printf(const char *, ...);

int main() {
    const char *orig = "Hello, World!";
    const char *dest = upperString(orig);
    printf("%s: %s", orig, dest);

    return 0;
}

local module = require("./module")

module.functions["upperString"] = function()
   local readPointer = module.util.get_args()
   local str = module.util.read_string(readPointer)

   -- allocate new string
   local newString = str:upper().."\0" -- append null terminator, so reader knows where to stop
   local writePointer = module.util.malloc(#newString+1) -- find out where to place it

   buffer.writestring(module.memory, writePointer, newString) -- write to memory

   module.util.push_args(writePointer) -- return where we wrote to
end
module()

Example Functions

Much more utilities are provided for more advanced functions. Some of the built in functions will now be provided as examples.

printf

    ["printf"] = function()
        local args = {get_args()}
        local fmt = args[1]
        table.remove(args, 1)
        print(format_string(read_string(fmt), args))
    end,

• read_string is a function that is exposed to convert a string inside the memory buffer to a Luau string. It works by taking in a start pointer (int) and reading until it finds a null terminator (\0).
• format_string is a function that is meant to act like string.format but have support for:
  • formatting pointers into %s (with read_string)
  • converting ints into floats for %f

memcpy

     ["memcpy"] = function()
        local dest, src, count = get_args()

        buffer.copy(memory, dest, memory, src, count)
    end,

This is a simple wrapper for buffer.copy.