Hello. I am trying to write a game engine with Lua+Allegro. I've got a lot of it done, with the exception of keyboard input. I am clueless as to how to get the keyboard presses in Allegro and C into a Lua script, so that the Lua script can just call something like

isKeyDown(KEY_ESC)

(obviously, isKeyDown would be a C function that lua can call).

Could someone help me with this?

I think what you want is a callback for Lua.

ie:
OnKeyDown(int key)

That you implement in Lua. But I don't know Lua, so I'm just throwing stuff out there.

Read the manual and Programming In Lua. Especially the parts on registering C functions for Lua.

What I was thinking of is more of a function that returns a boolean and accepts the key to check, like the key[] array in Allegro does, only from within Lua. I'd like to avoid callbacks, and I don't know how to do callbacks in Lua.

edit:

I have read the manual, and can register functions and call them from Lua. That works. What I want to know is how to get the constants KEY_* into Lua so that when I call a isKeyDown or similar function it can accept these and return the boolean.

The same way you would register any other global value.

So, if I understand this correctly, I should set the KEY_* variables after initializing the Lua VM, as such:

lua_pushinteger(lvm,KEY_A);
lua_setglobal(lvm,"KEY_A");
lua_pushinteger(lvm,KEY_B);
lua_setglobal(lvm,"KEY_B");
lua_pushinteger(lvm,KEY_B);
lua_setglobal(lvm,"KEY_B");
//And so on

Then define and register the isKeyDown function:

static int isKeyDown(lua_State *l)
{
     int key=luaL_checkint(l,1);

     lua_pushboolean(lvm,KEY[key]);

     return 1;
}

//...VM is initialized, etc.

//Now we register the functions
lua_pushcfunction(lvm,isKeyDown);
lua_setglobal(lvm,"isKeyDown");

That part is fairly straightforward, thank you X-G. However, the setting of global KEY_* variables in the Lua VM is very tedious. Is there an elegant way of doing that?

I imagine you could loop through them all with some fancy for loop.

I found the defines in allegro/keyboard.h:

#define KEY_A                 __allegro_KEY_A
#define KEY_B                 __allegro_KEY_B
#define KEY_C                 __allegro_KEY_C
#define KEY_D                 __allegro_KEY_D
#define KEY_E                 __allegro_KEY_E

I wrote the following program to take this text and turn it into the lines of code I need:

Which, when I copy+paste the lines from the include file into stdin and redirect stdout to a file, gives me a file with the code neatly written for me.