How to wrap a C function whose parameters are pointer to structs, so that it can be called from Lua?

Question

I have the follwing C function. How should I wrap it so it can be called from a Lua script?

typedef struct tagT{
    int a ; 
    int b ;
} type_t;

int lib_a_f_4(type_t *t)
{
     return t->a * t->b ;
}

I know how to wrapr it if the function parameter type were int or char *. Should I use table type for a C structure?

EDIT: I am using SWIG for the wraping , according to this doc, It seems that I should automatically have this funtion new_type_t(2,3) , but it is not the case.

If you wrap a C structure, it is also mapped to a Lua userdata. By adding a metatable to the userdata, this provides a very natural interface. For example,

struct Point{ int x,y; };

is used as follows:

p=example.new_Point() p.x=3 p.y=5 print(p.x,p.y) 3 5

Similar access is provided for unions and the data members of C++ classes. C structures are created using a function new_Point(), but for C++ classes are created using just the name Point().

Answer 1

I put this together in a hurry. It compiled; then I did a few last-minute edits. I hope it's close to the right thing. Go through the Lua manual and look at all the unfamiliar funtions.

#include <lua.h>
#include <lauxlib.h>

const char *metaname = "mine.type_t"; // associated with userdata of type type_t*

typedef struct tagT{
    int a ; 
    int b ;
}type_t;


int lib_a_f_4(type_t *t)
{
     return t->a * t->b ;
}

static int lua_lib_a_f_4(lua_State *L) {
  type_t *t = luaL_checkudata(L, 1, metaname);  // check argument type
  lua_pushnumber(L, (lua_Number)lib_a_f_4(t));
  return 1;
}

static int lua_new_t(lua_State *L) { // get Lua to allocate an initialize a type_t*
  int a = luaL_checkint(L, 1);
  int b = luaL_checkint(L, 2);
  type_t *t = lua_newuserdata(L, sizeof(*t));
  luaL_getmetatable(L, metaname);
  lua_setmetatable(L, -2);
  t->a = a;
  t->b = b;
  return 1;
}

static const struct luaL_reg functions[] = {
  { "lib_a_f_4", lua_lib_a_f_4 },
  { "new_t", lua_new_t },
  { NULL, NULL }
};

int mylib_open(lua_State *L) {
  luaL_register(L, "mylib", functions);
  luaL_newmetatable(L, metaname);
  lua_pop(L, 1);
  return 1;
}

//compile and use it in lua
root@pierr-desktop:/opt/task/dt/lua/try1# gcc -shared -o mylib.so -I/usr/include/lua5.1/ -llua *.c -ldl
root@pierr-desktop:/opt/task/dt/lua/try1# lua
Lua 5.1.3  Copyright (C) 1994-2008 Lua.org, PUC-Rio
> require("mylib")
> t=mylib.new_t(2,3)
> mylib.lib_a_f_4(t)
> print(mylib.lib_a_f_4(t))
6
> 

Answer 2

Solved.

1. Should also add the type definition in the example.i file , just include the .h is not enough.

   %module example 
   %{
     #include "liba.h"
   %}
   
   
   void lib_a_f_1(void);
   int  lib_a_f_2(int a, int b);
   int lib_a_f_3(const char *s);
   int lib_a_f_4(struct Point *t);
   
   
   struct Point{
     int a;
     int b;
   };
   

2. use example.Point() ,not example.new_Point() (SWIG Version 1.3.35)

     example.Point()
     f=example.Point()
     f.a=2
     f.b=3
     example.lib_a_f_4(f)
     print(example.lib_a_f_4(f))