int lua_sethook (lua_State *L, lua_Hook f, int mask, int count);

Sets the debugging hook function.

Argument f is the hook function. mask specifies on which events the hook will be called: it is formed by a bitwise or of the constants LUA_MASKCALL, LUA_MASKRET, LUA_MASKLINE, and LUA_MASKCOUNT. The count argument is only meaningful when the mask includes LUA_MASKCOUNT. For each event, the hook is called as explained below:

• The call hook: is called when the interpreter calls a function. The hook is called just after Lua enters the new function, before the function gets its arguments.
• The return hook: is called when the interpreter returns from a function. The hook is called just before Lua leaves the function. You have no access to the values to be returned by the function.
• The line hook: is called when the interpreter is about to start the execution of a new line of code, or when it jumps back in the code (even to the same line). (This event only happens while Lua is executing a Lua function.)
• The count hook: is called after the interpreter executes every count instructions. (This event only happens while Lua is executing a Lua function.)

A hook is disabled by setting mask to zero.