The next events debug.sethook can hook into are function calls. These events fire every time a function is called. The event handler only needs to take one argument, a string. The value of this string will always be "call". To subscribe to function call events, provide debug.sethook with two arguments: the handler function, and the "c" string. Function callback hooks become much more powerful when combined with the info obtained from debug.getinfo. The following code demonstrates this:

function PrintV(x, y, z)
  local out = "(" .. x .. ","
  out = out .. ", " .. y
  out = out .. ", " .. z .. ")"
  return out
end

function MagnitudeSq(x, y, z)
  local magSq = x * x + y * y + z * z
  return magSq;
end

function trace(event)
  local info = debug.getinfo(2)
  if info.what == "Lua" then
    print ("event: " .. event)
    print (" function: " .. info.name)
    print (" defined on: " .. info.linedefined)
  end
end

debug.sethook(trace, "c")

local mSq = MagnitudeSq(9, 2, 6)
print ("Sqr mag: " .. mSq)
PrintV(9,2,6)