Pass a table from C to Lua and let the script modify it.

lua_passing_table_as_reference.c

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

#include <stdio.h>

static const char *_script =
    "function init(t)\n"
    "  t['width'] = 320\n"
    "  t['height'] = 240\n"
    "end\n";

int main(int argc, char *argv[])
{
    lua_State *L = luaL_newstate();

    // Load and evaluate the script.
    (void)luaL_dostring(L, _script);

    // Create a a table on the stack, and populate with a single key/value pair.
    lua_newtable(L);                // [] -> [T]
    lua_pushstring(L, "width");     // [T] -> [T S]
    lua_pushnumber(L, 0.0);         // [T S] -> [T S N]
    lua_rawset(L, -3);              // [T S N] -> [T]

    // Get a reference to the global function to be called, a copy the table
    // reference to pass it as an argument. Then call the function.
    lua_getglobal(L, "init");       // [T] -> [T F]
    lua_pushvalue(L, -2);           // [T F] -> [T F T]
    lua_call(L, 1, 0);              // [T F T] -> [T]

    // Dump the table content.
    lua_pushnil(L);                 // [T] -> [T N]
    while (lua_next(L, -2)) {       // [T N] -> [T K V]
        printf("t[%s]=%f\n", lua_tostring(L, -2), lua_tonumber(L, -1));
        lua_pop(L, 1);              // [T K V] -> [T K]
    }

    // Dispose the table from the stack.
    lua_pop(L, 1);                  // [T] -> []

    // So long, and thanks for all the fish...
    lua_close(L);

    return EXIT_SUCCESS;
}