How to get the number of arguments a function in a table expect?

Question

I have functions inside a Lua table t and I learned that I can list all the functions the t contains by running the following script:

for k, v in pairs(t) do
  print(k, v)
end

It will for example print:

myfunction1 function: 0x107219805
myfunction2 function: 0x10721c194
myfunction3 function: 0x1071e067c

Now, I would like to know if it's possible to get each function's number of arguments it expects. So the result can be like the following:

myfunction1 function: 0x107219805 arguments: 3
myfunction2 function: 0x10721c194 arguments: 4
myfunction3 function: 0x1071e067c arguments: 5

Would this be possible in Lua? Or using the C API?

Answer 1

The call debug.getinfo(f) returns a table containing information about f. In particular,

• debug.getinfo(f).nparams gives the number of parameters in f
• debug.getinfo(f).isvararg tells whether f accepts a variable amount of arguments

Answer 2

For Lua 5.2+ see the answer of lhf.

This is a solution for Lua 5.1 only
(you can combine the two solutions into one)

local function get_numparams_isvararg(func)
   -- returns num_param (number), is_vararg (boolean)
   local s = string.dump(func)
   assert(s:sub(1, 6) == "\27LuaQ\0", "This code works only in Lua 5.1")
   local int_size = s:byte(8)
   local ptr_size = s:byte(9)
   local pos = 14 + ptr_size + (s:byte(7) > 0 and s:byte(13) or s:byte(12 + ptr_size)) + 2 * int_size
   return s:byte(pos), s:byte(pos + 1) > 0
end

Usage example:

local function f(a, b, c, ...)
end

local function g()
end

print(get_numparams_isvararg(f))  --> 3  true
print(get_numparams_isvararg(g))  --> 0  false