Passing a table to C Func and back to Lua code

GregBUG

(feel free to move this topic if isn't in the correct category)

ok. i start to play with plugins...
this is the lua code...
with some var, a table...

call a C function "tntengine.alphamorph(color, ferro, ciccio)" this do some on the parametres passed and modify directly some filed of the table... then release 3 values...
this example works fine but...

require "tntparticlesengine"   color = 1000 ferro = 2000 ciccio = {Nome = "ciao", xPos = 10, yPos = 20, Color = 1.212, linee = {}, spazi = 23 }   a,b,c =tntengine.alphamorph(color, ferro, ciccio)   print("-----") print(a) print(b) print(c) print(ciccio.xPos) print(ciccio.yPos) print("-----")

#include "lua.h" #include "lauxlib.h" #include "gideros.h"   static int alpha_morph(lua_State *L) { double color = lua_tonumber(L, -3); double ferro = lua_tonumber(L, -2);   lua_pushstring(L, "Color"); lua_gettable(L, -2); double spazi = lua_tonumber(L, -1); lua_pop(L, 1);   color++; ferro--;   lua_pushstring(L, "xPos"); lua_pushnumber(L, 256); lua_settable(L, -3);   lua_pushstring(L, "yPos"); lua_pushnumber(L, 156); lua_settable(L, -3);   lua_pushnumber(L, color); lua_pushnumber(L, ferro); lua_pushnumber(L, spazi); return 3; }   static const struct luaL_Reg tntEngineFuncs[] = { { "alphamorph", alpha_morph }, { NULL, NULL } };   LUALIB_API int luaopen_tntparticlesengine(lua_State *L) { luaL_register(L, "tntengine", tntEngineFuncs); return 1; }

stack index of parametres passed in the func are:

for 1^st "color" is -3 for "ferro" is -2

but why for the table "ciccio" is -2 and not -1 ?

i miss something... how work exactly the stack with simple parametres and tables ?

i pass an object how can access to it ?

there is a better way to change directly a value in the table than this :

lua_pushstring(L, "xPos"); lua_pushnumber(L, 256); lua_settable(L, -3);

what means really " lua_settable(L, -3);"

thanks.

and sorry for my continuous questions ...
but the possibility of using the plugin really interests me ...
...when I have more experience I hope to give something back to communities...

Gianluca.

Caroline

lua_pushstring(L, "Color"); lua_gettable(L, -2);

-1, -2, -3 etc is the top of the "stack". You push things on the stack, and pop them off.

When you first go in, the table ciccio is presumably at -1, then you push the string "Color", so "Color is at -1 and ciccio is at -2 now.

Have you read the Ultimate Guide, Chapter 8?
http://giderosmobile.com/DevCenter/index.php?title=Ultimate_Guide_to_Gideros_Studio

8.2 has a table indicating what 1,2,3 and -1,-2,-3 are.

It is a very complex subject, and I am quite happy to acknowledge that I may not have presented it completely the best. However, it will be very useful to test it on an actual unsuspecting user , so that we can make the Guide the best.

The code in the guide is a bit fragmented, but you can download a working plugin with the Gideros project, and this has a few examples of various things you can do.

http://www.giderosmobile.com/forum/discussion/433/sample-iphone-plugin

I did write it for iOS though. Perhaps I should put in my signature that I know nothing about Android (insert dunce emoticon here )

GregBUG

yes, Caroline. i feel so stupid some times! (or often? )
I read the manual but in a hurry and at night... better to make things very quietly ...

you have presented this subject very clearly!
but it's my desire to learn that I care... and the fact that I do not understand English well ... (my mother language is Italian) and use google translator!

PS: your example (and that one posted by MikeHart) is very good but i don't understand well Objective C.

BTW, now the "stack" is clear... i need to make some practice with it! now!...

ps: I'll probably still have some "stupid" questions...
I warned you!


thanks.

ciao!
Gianluca.

atilim

Hey don't forget me

To access the parameters that are passed to a C function, it's better to use positive indices. I mean instead of

double color = lua_tonumber(L, -3); double ferro = lua_tonumber(L, -2);

use

double color = lua_tonumber(L, 1); double ferro = lua_tonumber(L, 2);

Also, it's better to use luaL_checknumber instead of lua_tonumber so that if the function parameter cannot be converted to number, Lua gives an understandable error.

http://pgl.yoyo.org/luai/i/lua_tonumber
http://pgl.yoyo.org/luai/i/luaL_checknumber

Also using lua_setfield and lua_getfield is easier for changing a field of a table. I mean, instead of

lua_pushstring(L, "xPos"); lua_pushnumber(L, 256); lua_settable(L, -3);

you can write:

lua_pushnumber(L, 256); lua_setfield(L, -2, "xPos");

Both ways are the same.

http://pgl.yoyo.org/luai/i/lua_setfield
http://pgl.yoyo.org/luai/i/lua_settable

best,

atilim

Also similar to lua_setfield/lua_getfield, there are lua_rawgeti and lua_rawseti to easily change the fields of a table with integer keys:

http://pgl.yoyo.org/luai/i/lua_rawgeti
http://pgl.yoyo.org/luai/i/lua_rawseti

Caroline

To access the parameters that are passed to a C function, it's better to use positive indices.

Why "better"? I tend to find working backwards from the top easier than relying on what's on the bottom.

(But then I am backwards about many things.)

Your other points are good ones. I always forget to use the checknumber instead of tonumber.

GregBUG

Hey don't forget me

Never! You're always in my mind!

thanks for the tips.!

atilim

Because if someone passes more parameters than you've expected, using negative indices returns wrong parameters. I mean if the function is called as:

a,b,c =tntengine.alphamorph(color, ferro, ciccio, 10, 20, 30)

using negative indices refers to 10, 20, 30 instead of color, ferro, ciccio.

Caroline

Hey don't forget me

Never! You're always in my mind!

Mine too - the Ultimate Guide personified .

atilim

I usually use positive indices while accessing function parameters and negative indices for all the other cases.

Caroline

Because if someone passes more parameters than you've expected, using negative indices returns wrong parameters.

Good point - I've always been careful that I've sent the right parameters, but I guess that if a plugin is for redistribution, then it should be as robust as possible. Thanks.

atilim

@GregBUG @Caroline ;;)