== C Struct to Lua table
=== Overview
For this tutorial we’ll change a C struct into a Lua table. The
structure we’ll be using won’t be the simplest structure you’ll come
across in the wild so hopefully the tutorial will do a little more than
just cover the basics. We’ll add the structures as `userdata` and not as
`lightuserdata`. Because of that, we won’t have to manage the memory
ourselves, instead we will let Lua’s GC handle it for us. Disclaimer:
* This turotial is not supposed to be a full dive into lua tables,
metatables and their implementation or behavior. The tutorial is meant
as an entry point into implementing custom Lua tables.
==== Yet Another One?
There are already a couple of tutorials on this, yes, but the ones I
managed to find were all targeting older versions of lua and as the Lua
devs have clearly stated, different Lua version are really different.
The other reason I wrote this is I needed a structures that had
structure members themselves and I couldn’t find a tutorial for that.
This tutorial will be targeting Lua 5.3. We’ll also be using a
not-so-simple structure to turn into a Lua table.
==== What you’ll need
* A working C compiler(I’ll be using clang)
* Make
* you can get the repo
https://github.com/bloodstalker/blogstuff/tree/master/src/cstruct2luatbale[here].
=== C Structs
First let’s take a look at the C structures we’ll be using. The primary
structure is called `a_t` which has, inside it, two more structures
`b_t` and `c_t`:
[source,c]
----
typedef struct {
uint64_t a_int;
double a_float;
char* a_string;
b_t* a_p;
c_t** a_pp;
} a_t;
----
[source,c]
----
typedef struct {
uint32_t b_int;
double b_float;
} b_t;
----
[source,c]
----
typedef struct {
char* c_string;
uint32_t c_int;
} c_t;
----
The structures are purely artificial.
=== First Step: Lua Types
First let’s take a look at `a_t` and decide how we want to do this.
`a_t` has five members:
* `a_int` which in Lua we can turn into an `integer`.
* `a_float` which we can turn into a `number`.
* `a_string` which will be a Lua `string`.
* `a_p` which is a pointer to another structure. As previously stated,
we will turn this into a `userdata`.
* `a_pp` which is a double pointer. We will turn this into a table of
`userdata`.
=== Second Step: Helper Functions
Now let’s think about what we need to do. First we need to think about
how we will be using our structures. For this example we will go with a
pointer, i.e., our library code will get a pointer to the structure so
we need to turn the table into `userdata`. Next, we want to be able to
push and pop our new table from the Lua stack. We can also use Lua’s
type check to make sure our library code complains when someone passes a
bad type. We will also add functions for pushing the structure arguments
onto the stack, a fucntion that acts as our constructor for our new
table(more on that later) and getter and setter methods to access our C
structures fields.
Let’s start: First we will write a function that checks the type and
returns the C structure:
[source,c]
----
static a_t* pop_a_t(lua_State* ls, int index) {
a_t* dummy;
dummy = luaL_checkudata(ls, index, "a_t");
if (!dummy) printf("error:bad type, expected a_t\n");
return dummy;
}
----
We check to see if the stack index we are getting is actually a userdata
type and then check the type of the userdata we get to make sure we get
the right userdata type. We check the type of the userdata by checking
its metatable. We will get into that later. This amounts to our ``pop''
functionality for our new type. Now let’s write a ``push'': The function
will look like this:
[source,c]
----
a_t* push_a_t(lua_State* ls) {
if (!lua_checkstack(ls, 1)) {
printf("o woe is me. no more room in hell...I mean stack...\n");return NULL;
}
a_t* dummy = lua_newuserdata(ls, sizeof(a_t));
luaL_getmetatable(ls, "a_t");
lua_setmetatable(ls, -2);
lua_pushlughtuserdata(ls, dummy);
lua_pushvalue(ls, -2);
lua_settable(ls, LUA_REGISTRYINDEX);
return dummy;
}
----
Notice that we reserve new memory here using `lua_newuserdata` instead
of `malloc` or what have you. This way we leave it up to Lua to handle
the GC(in the real world however, you might not have the luxury of doing
so). Now let’s talk about what we are actually doing here: First off we
reserve memory for our new table using `lua_newuserdata`. Then we get
and set the metatable that we will register later in the tutorial with
Lua for our newly constructed userdata. Setting the metatable is our way
of telling Lua what our userdata is, what methods it has along with some
customizations that we will talk about later. We need to have a method
of retrieving our full userdata when we need it. We do that by
registering our userdata inside `LUA_REGISTRYINDEX`. We will need a key.
for simplicity’s sake we use the pointer that `lua_newuserdata` returned
as the key for each new full userdata. As for the value of the key, we
will use the full userdata itself. That’s why we are using
`lua_pushvalue`. Please note that lua doesn’t have a `push_fulluserdata`
function and we can’t just pass the pointer to our userdata as the key
since that would just be a lihgtuserdata and not a userdata so we just
copy the fulluserdata onto the stack as the value for the key. Lastly we
just set our key-value pair with `LUA_REGISTRYINDEX`.
Next we will write a function that pushes the fields of the structure
onto the stack:
[source,c]
----
int a_t_push_args(lua_State* ls, a_t* a) {
if (!lua_checkstack(ls, 5)) {
printf("welp. lua doesn't love you today so no more stack space for you\n");
return 0;
}
lua_pushinteger(ls, a->a_int);
lua_pushnumber(ls, a->a_float);
lua_pushstring(ls, a->a_string);
push_b_t(ls);
lua_pushlightuserdata(ls, a->a_pp);
return 5;
}
----
Notice that we are returning 5, since our new next function which is the
new function expects to see the 5 fields on top of the stack.
Next up is our new function:
[source,c]
----
int new_a_t(lua_State* ls) {
if (!lua_checkstack(ls, 6)) {
printf("today isnt your day, is it?no more room on top of stack\n");
return 0;
}
int a_int = lua_tointeger(ls, -1);
float a_float = lua_tonumber(ls, -2);
char* a_string = lua_tostring(ls, -3);
void* a_p = lua_touserdata(ls, -4);
void** a_pp = lua_touserdata(ls, -5);
lua_pop(ls, 5);
a_t* dummy = push_a_t(ls);
dummy->a_int = a_int;
dummy->a_float = a_float;
dummy->a_string = a_string;
dummy->a_p = a_p;
dummy->a_pp = a_pp;
return 1;
}
----
We just push an `a_t` on top of stack and then populate the fields with
the values already on top of stack. The fact that we wrote tha two
separate functions for pushing the arguments and returning a new table
instance means we can use `new_a_t` as a constructor from lua as well.
We’ll later talk about that.
=== Third Step: Setters and Getters
Now lets move onto writing our setter and getter functions. For the
non-userdata types its fairly straightforward:
[source,c]
----
static int getter_a_float(lua_State* ls) {
a_t* dummy = pop_a_t(ls, -1);
lua_pushnumber(ls, dummy->a_number);
return 1;
}
static int getter_a_string(lua_State* ls) {
a_t* dummy = pop_a_t(ls, -1);
lua_pushstring(ls, dummy->a_string);
return 1;
}
----
As for the setters:
[source,c]
----
static int setter_a_int(lua_State* ls) {
a_t* dummy = pop_a_t(ls, 1);
dummy->a_int = lua_checkinteger(ls, 2);
return 1;
}
----
Now for the 4th and 5th fields:
[source,c]
----
static int getter_a_p(lua_State *ls) {
a_t* dummy = pop_a_t(ls, 1);
lua_pop(ls, -1);
lua_pushlightuserdata(ls, dummy->a_p);
lua_gettable(ls, LUA_REGISTRYINDEX);
return 1;
}
----
For the sake of laziness, let’s assume `a_t->a_int` denotes the number
of entries in `a_t->a_pp`.
[source,c]
----
static int getter_a_pp(lua_State* ls) {
a_t* dummy = pop_a_t(ls, 1);
lua_pop(ls, -1);
if (!lua_checkstack(ls, 3)) {
printf("sacrifice a keyboard to the moon gods or something... couldnt grow stack.\n");
return 0;
}
lua_newtable(ls);
for (uint64_t i = 0; i < dummy->a_int; ++i) {
lua_pushinteger(ls, i + 1);
if (dummy->a_pp[i] != NULL) {
lua_pushlightuserdata(ls, dummy->a_pp[i]);
lua_gettable(ls, LUA_REGISTRYINDEX);
} else {
lua_pop(ls, 1);
continue;
}
lua_settable(ls, -3);
}
return 1;
}
----
Since we register all our tables with `LUA_REGISTRYINDEX` we just
retreive the key which in our case, conviniently is the pointer to the
userdata and retrieve the value(our userdata). As you can see, for
setters we are assuming that the table itself is being passed as the
first argument(the `pop_a_t` line assumes that).
Our setters methods would be called like this in Lua:
[source,lua]
----
local a = a_t()
a:set_a_int(my_int)
----
The `:` operator in Lua is syntactic sugar. The second line from the
above snippet is equivalent to `a.set_a_int(self, my_int)`. As you can
see, the table itself will always be our first argument. That’s why our
assumption above will always be true if the lua code is well-formed.
We do the same steps above for `b_t` and `c_t` getter functions.
Now let’s look at our setters:
[source,c]
----
static int setter_a_string(lua_State *ls) {
a_t* dummy = pop_a_t(ls, 1);
dummy->a_string = lua_tostring(ls, 2);
lua_settop(ls, 1);
return 0;
}
static int setter_a_p(lua_State *ls) {
a_t* dummy = pop_a_t(ls, 1);
dummy->a_p = luaL_checkudata(ls, 2, "b_t");
lua_pop(ls, 1);
lua_settop(ls, 1);
return 0;
}
----
[source,c]
----
static int setter_a_pp(lua_State* ls) {
a_t* dummy = pop_a_t(ls, 1);
dummy->a_pp = lua_newuserdata(ls, sizeof(void*));
if (!lua_checkstack(ls, 3)) {
printf("is it a curse or something? couldnt grow stack.\n");
return 0;
}
int table_length = lua_rawlen(ls, 2);
for (int i = 1; i <= table_length; ++i) {
lua_rawgeti(ls, 2, i);
dummy->a_pp[i - 1] = luaL_checkudata(ls, -1, "c_t");
lua_pop(ls, 1);
}
return 0;
}
----
We are all done with the functions we needed for our new table. Now we
need to register the metatable we kept using:
== Fourth Step: Metatable
First, if you haven’t already, take a look at the chapter on metatable
and metamethods on pil https://www.lua.org/pil/13.html[here].
[source,c]
----
static const luaL_Reg a_t_methods[] = {
{"new", new_a_t},
{"set_a_int", setter_a_int},
{"set_a_float", setter_a_float},
{"set_a_string", setter_a_string},
{"set_a_p", setter_a_p},
{"set_a_pp", setter_a_pp},
{"a_int", getter_a_int},
{"a_float", getter_a_float},
{"a_string", getter_a_string},
{"a_p", getter_a_p},
{"a_pp", getter_a_pp},
{0, 0}};
static const luaL_Reg a_t_meta[] = {{0, 0}};
----
We just list the functions we want to be accessible inside Lua code. Lua
expects the C functions that we register with Lua to have the form
`(int)(func_ptr*)(lua_State*)`. Also, it’s a good idea to take a look at
the metatable events that Lua 5.3 supports
http://lua-users.org/wiki/MetatableEvents[here]. They provide
customization options for our new table type(as an example we get the
same functionality as C++ where we get to define what an operator does
for our table type).
Now we move on to registering our metatable with Lua:
[source,c]
----
int a_t_register(lua_State *ls) {
lua_checkstack(ls, 4);
lua_newtable(ls);
luaL_setfuncs(ls, a_t_methods, 0);
luaL_newmetatable(ls, "a_t");
luaL_setfuncs(ls, a_t_methods, 0);
luaL_setfuncs(ls, a_t_meta, 0);
lua_pushliteral(ls, "__index");
lua_pushvalue(ls, -3);
lua_rawset(ls, -3);
lua_pushliteral(ls, "__metatable");
lua_pushvalue(ls, -3);
lua_rawset(ls, -3);
lua_setglobal(ls, "a_t");
return 0;
}
----
Please note that we are registering the metatable as a global. It is
generally not recommended to do so.Why you ask? Adding a new enrty to
the global table in Lua means you are already reserving that keyword, so
if another library also needs that key, you are going to have lots of
fun(the term `fun` here is borrowed from the Dwarf Fortress literature).
Entries in the global table will require Lua to look things up in the
global table so it slows things down a bit, though whether the slow-down
is signifacant enough really depends on you and your requirements.
We are almost done with our new table but there is one thing remaining
and that is our table doesn’t have a cozy constructor(Cozy constructors
are not a thing. Seriously. I just made it up.). We can use our `new`
function as a constructor, we have registered it with our metatable, but
it requires you to pass all the arguments at the time of construction.
Sometimes it’s convinient to hold off on passing all or some of the args
at construction time, mostly because you are writing a library and your
power users will do all sorts of unconventional and crazy/creative
things with your library.
Remember metatable events? That’s what we’ll use. Lua metatables support
something called metatable events. Eeach event has a string key and the
value is whatever you put as the value. The values are used whenever
that event happens. Some the events are:
* `__call`
* `__pairs`
* `__sub`
* `__add`
* `__gc` The `__sub` event is triggered when your table is the operand
of a suntraction operator. `__gc` is used when lua want to dispose of
the table so if you are handling the memory yourself, in contrast to
letting Lua handle it for you, here’s where you free memory. The events
are a powerful tool that help us customize how our new table behaves.
For a constructor, we will use the `__call` event. That means when
someone calls our metatable in Lua, like this(call event is triggered
when our table is called, syntactically speaking):
[source,lua]
----
local a = a_t()
----
`a` will become a new instance of our table. We can add a value for our
metatable’s `__call` key from either Lua or C. Since we are talking
about Lua and haven’t almost written anything in Lua, let’s do it in
Lua:
[source,lua]
----
setmetatable(a_t, {__call =
function(self, arg1, arg2, arg3, arg4, arg5)
local t = self.new(arg1, arg2, arg3, arg4, arg5)
return t
end
}
)
----
We use our `new` method which we previously registered for our
metatable. Note that Lua will pass `nil` for the argument if we don’t
provide any. That’s how our cozy constructor works.
=== Final Words
The tutorial’s goal is to show you one way of doing the task and not
necessarily the best way of doing it. Besides, depending on your
situation, you might want to do things differently so by no means is
this tutorial enough. It’s an entry level tutorial. Any feedback,
suggestions and/or fixes to the tutorial is much appreciated.
=== Shameless Plug
I needed to turn a C struct into a lua table for an application I’m
working https://github.com/bloodstalker/mutator/tree/master/bruiser[on].
Further down the line, I needed to do the same for a lot more C structs
with the possibility of me having to do the same for a lot more C
structs. I just couldn’t bring myself to do it manually for that many C
structs so I decided to work on a code generator that does that for me.
The result is https://github.com/bloodstalker/luatablegen[luatablegen].
`luatablegen` is a simple script that takes the description of your C
structures in an XML file and generates the C code for your new tables
and metatables. It does everything we did by hand automatically for us.
`lautablegen` is in its early stages, so again, any feedback or help
will be appreciated.
timestamp:1705630055
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