Question

When trying to dump a function’s bytecode to a buffer, I was surprised to see the Lua stack change size. Docs say that lua_dump should not push nor pop the stack. Some functions do and some do not.

This does:

function f() print "x" end

This does not:

function f() return "x" end

lua_dump in both cases returns 0 (no error).

I assume it has something to do with the use of the print function, but I can’t find any docs describing what is going on or what might be contained in the userdata. ..or whether that bytecode is going to be usable without it.

sample code:

static int base64_write(lua_State* L, unsigned char* str, size_t len, 
        struct luaL_Buffer *buf)
{
    unsigned int idx;
    char code[5];
    for (idx=0; idx<len; idx++){
        sprintf(code, "\\%03d", (unsigned int) str[idx]);
        //printf(code);
        luaL_addlstring(buf, code, 4);
    }
    //printf("\n");
    return 0;
}

void dump_function(lua_State* L){
    int stack_sz;
    int res;
    luaL_Buffer buf;

    luaL_buffinit(L, &buf);

    // ** test 1 - works as expected
    lua_settop(L,0);
    luaL_dostring(L, "function f() return 'x' end");
    lua_getglobal(L, "f");
    printf("stack sz: %i\n", lua_gettop(L));
    res = lua_dump(L, (lua_Writer)base64_write, &buf);
    printf("stack sz: %i\n", lua_gettop(L));

    // ** test 2 - not as expected
    lua_settop(L,0);
    luaL_dostring(L, "function f() print 'x' end");
    lua_getglobal(L, "f");
    printf("stack sz: %i\n", lua_gettop(L));
    res = lua_dump(L, (lua_Writer)base64_write, &buf);
    printf("stack sz: %i\n", lua_gettop(L));  // why is this 2?
    printf("type:%s\n", lua_typename(L, lua_type(L, -1))); // why userdata?
}

/* ----- Registration array ----- */
static const luaL_Reg R[] = {
    {"test",    dump_function},
    {NULL,      NULL} /* sentinel */
};

/* ----- Registration function ----- */

LUALIB_API int register_lua_tserialize(lua_State *L)
{
    lua_newtable(L); 
    lua_setglobal(L, "test"); 
    lua_getglobal(L, "test"); 
    luaL_setfuncs(L, R, 0); 
    lua_settop(L, 0);
    return 1;
}

Answer 1

It’s because of the usage of the buffer object. See the documentation at http://pgl.yoyo.org/luai/i/luaL_Buffer

You need to call luaL_pushresult to get rid of the intermediate buffer stack items.