How do I get the table length of a lua table in c?

Question

I have a c function that is called from lua. The first parameter is a table. That table is abused as an input array of numbers to an underlying api. So right now my code looks like this:

int n = 0;
lua_pushnil ( L );
while ( lua_next ( L, 2 ) ) {
   n++;
   lua_pop ( L, 1 );
}
int *flat = alloca ( n * 4 );
lua_pushnil ( L );
int i = 0;
while ( lua_next(L,2) ) {
   flat[i++] = (int)lua_tonumber( L, -1 ); 
   lua_pop ( L, 1 );
}

I typed the code blind, so please forgive errors. Also no error checking. But the problem is that I have to do the while loop twice. Is there an easy way to avoid that? I want to optimize for the case where the input is good - a table of ints.

Answer 1

The function you're looking for is unintuitive named lua_objlen, or in Lua 5.2, lua_len (there is a lua_rawlen if you wish to avoid metamethod invocations). It serves many roles (though some, like the length of a string, aren't very useful when you can just use lua_tolstring to get the string and its length), so you should be familiar with it.