Lua stack dump for c++
sagasw
Any interactions between c++ and lua are going through lua stack. Aware of the stack status may help for debugging. I always do that, will a peek of the current Lua stack not only helps me debug my codes but also helps me figure out the ways how I can pass table from c++ to Lua and vice versa.
- void stackdump_g(lua_State* l)
- {
- int i;
- int top = lua_gettop(l);
- printf("total in stack %d\n",top);
- for (i = 1; i <= top; i++)
- { /* repeat for each level */
- int t = lua_type(l, i);
- switch (t) {
- case LUA_TSTRING: /* strings */
- printf("string: '%s'\n", lua_tostring(l, i));
- break;
- case LUA_TBOOLEAN: /* booleans */
- printf("boolean %s\n",lua_toboolean(l, i) ? "true" : "false");
- break;
- case LUA_TNUMBER: /* numbers */
- printf("number: %g\n", lua_tonumber(l, i));
- break;
- default: /* other values */
- printf("%s\n", lua_typename(l, t));
- break;
- }
- printf(" "); /* put a separator */
- }
- printf("\n"); /* end the listing */
- }
I usually interested on knowing how many blocks in my stack had been occupied and also each block’s variable type, if they are string, number or bool, I would like to know the value as well.
The usage is simple, you will only needs to pass in your current Lua State pointer.
The print line output may look like this:
total in stack 2
string: '5A5B5C8855778899'
number: 89
Accessing Lua global variables from c++
calling c++ function from Lua, implement sleep function
sagasw
Calling Lua function from c++ is very simple. Value passing between c++ and Lua goes through stack, Lua C API provides a convenience ways for you to call Lua function from C. To call Lua function, you need to specify:
1. Function Name.
2. Parameters of function call.
3. Return values expected ( Lua function support multiple results reture)
Let say my lua function name call f in last.lua, takes 2 parameters.
-- last.lua
function f (x, y)
return (x^2 * math.sin(y))/(1 - x)
endI perform function call from c++ like this:
- //last.cc
- # extern "C" {
- # #include "lua.h"
- # #include "lualib.h"
- # #include "lauxlib.h"
- # }
- int main()
- {
- double z;
- lua_State *L = lua_open();
- luaL_openlibs(L);
- if (luaL_loadfile(L, "last.lua") || lua_pcall(L, 0, 0, 0)) {
- printf("error: %s", lua_tostring(L, -1));
- return -1;
- }
- lua_getglobal(L, "f");
- if(!lua_isfunction(L,-1))
- {
- lua_pop(L,1);
- return -1;
- }
- lua_pushnumber(L, 21); /* push 1st argument */
- lua_pushnumber(L, 31); /* push 2nd argument */
- /* do the call (2 arguments, 1 result) */
- if (lua_pcall(L, 2, 1, 0) != 0) {
- printf("error running function `f': %s\n",lua_tostring(L, -1));
- return -1;
- }
- /* retrieve result */
- if (!lua_isnumber(L, -1)) {
- printf("function `f' must return a number\n");
- return -1;
- }
- z = lua_tonumber(L, -1);
- printf("Result: %f\n",z);
- lua_pop(L, 1);
- lua_close(L);
- return 0;
- }
Compile it with g++ like this:
g++ -o last{,.cc} -llua -ldlThe results:
Result: -12.158958Brief explanation of the C++ codes above:
First, I trigger lua_getglobal
to get the function name, then I push 2 parameters to stack. I make
lua_pcall by telling Lua I have 2 params and expect 1 value return.
Upon success, I retrieve the return value from the top of the stack.
calling c++ function from Lua, implement sleep function