Lua C API: Push pointers as a userdata

Question

I have a pointer address which points to a userdata. I retrieved the address like so:

*(uint32_t*)lua_touserdata(L, -1)

Later on, I want to push that userdata back on top the stack by using the pointer. If this is possible, what must be done?

If you want to store a reference to a userdatum so you can push it back onto the Lua stack at a later time, I suggest using the functions luaL_ref and luaL_unref. — Personage
– Personage, Commented Apr 6, 2020 at 3:16

Caleb Loera · Accepted Answer · 2020-04-05 01:16:40Z

1

You could create a new user data after retrieving the pointer to the one you already created via lua_newuserdata(). Then set the underlying value of the pointer to the value of the underlying value of the first userdata. It should look something like this:

int *ud1 = lua_touserdata( L, -1 ); // Get userdata previously created

int *ud2 = lua_newuserdata( L, sizeof( int ) ); // Create new userdata
*ud2 = *ud1; // Set value of new userdata to the value of the previous userdata

// Userdata has been successfully "pushed"
assert(
        *( (int*) lua_touserdata( L, -1 ) ) == 
        *( (int*) lua_touserdata( L, -2 ) ) 
);

answered Apr 5, 2020 at 1:16

Caleb Loera

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2 Comments

Burner Dev Over a year ago

The assert passes for me. Am I incorrect to assume that I could continue adding to the stack after "*ud2 = *ud1;"? E.g lua_getfield(-1, "field")

Caleb Loera Over a year ago

Yes you are correct. What was done in the example I gave will not keep you from adding to the stack.

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