This is a simple example of calling a lua function that returns a table and retrieving the values out of the table

testluatable.c

#include <stdio.h>
#include <string.h>
#include <assert.h>
#include "lua.h"
#include "lualib.h"
#include "lauxlib.h"

char *luacode =
"function f ( )                 \n"
"    print(\"hello\\n\")        \n"
"    return {  a=1.0, b=3.0 }   \n"
"end                            \n"
;

void CheckLuaErr(lua_State *L, int err, int line)
{
    if (err != 0)
    {
        printf("line %d: lua error %d\n", line, err);
        printf("%s\n", lua_tostring(L, -1));
        lua_pop(L, 1);  // pop error message from the stack
    }
}

int main(int argc, char *argv[])
{
    lua_State *L = luaL_newstate();
    
    luaL_openlibs(L);
    int err = luaL_loadbuffer(L, luacode, strlen(luacode), "testscript");
    CheckLuaErr(L, err, __LINE__);
    //Have to run the script once so the global variables (including the function
    // names) are loaded
    //ref: http://www.troubleshooters.com/codecorn/lua/lua_c_calls_lua.htm
    err = lua_pcall(L, 0, 0, 0);
    CheckLuaErr(L, err, __LINE__);
    
    lua_getglobal(L, "f");  //Call the function f(), expecting 1 table returned
    err = lua_pcall(L, 0, 1, 0);
    CheckLuaErr(L, err, __LINE__);
    assert ( lua_istable(L, -1) );
    //-1 is now a reference to the table

    //Use gettable
    lua_pushstring(L, "a");
    lua_gettable(L, -2);
    float a = lua_tonumber(L, -1);
    lua_pop(L, 1);
    
    //Or getfield
    lua_getfield(L, -1, "b");
    float b = lua_tonumber(L, -1);
    
    lua_pop(L, 1);  // pop table from the stack
    
    printf("a = %f, b= %f\n",a,b);
    
    lua_close (L);
    
    return 0;
}