debug.sethook ([thread,] hook, mask [, count])
Sets the given function as a hook. The string mask and the number count describe when the hook will be called. The string mask may have the following characters, with the given meaning:
"c": The hook is called every time Lua calls a function;"r": The hook is called every time Lua returns from a function;"l": The hook is called every time Lua enters a new line of code.
With a count different from zero, the hook is called after every count instructions.
When called without arguments, debug.sethook ([thread,] hook, mask [, count]) turns off the hook.
When the hook is called, its first parameter is a string describing the event that has triggered its call: "call", "return" (or "tail return"), "line", and "count". For line events, the hook also gets the new line number as its second parameter. Inside a hook, you can call getinfo with level 2 to get more information about the running function (level 0 is the getinfo function, and level 1 is the hook function), unless the event is "tail return". In this case, Lua is only simulating the return, and a call to getinfo will return invalid data.