{"id":1208,"date":"2013-09-05T01:00:00","date_gmt":"2013-09-04T23:00:00","guid":{"rendered":"https:\/\/www.fussylogic.co.uk\/blog\/?p=1208"},"modified":"2013-09-04T15:48:19","modified_gmt":"2013-09-04T14:48:19","slug":"lua-and-c","status":"publish","type":"post","link":"https:\/\/www.fussylogic.co.uk\/blog\/?p=1208","title":{"rendered":"Lua and C++"},"content":{"rendered":"

I\u00e2\u20ac\u2122ve not yet had a project that\u00e2\u20ac\u2122s needed an embedded scripting language. When I do, I think I\u00e2\u20ac\u2122ll reach for Lua<\/a>.<\/p>\n

$ apt-get install lua5.2 liblua5.2-dev<\/code><\/pre>\n
Command line lua:<\/p>\n
$ lua\nLua 5.2.2  Copyright (C) 1994-2013 Lua.org, PUC-Rio\n> print("Hello, World") \nHello, World<\/code><\/pre>\n
Embedded lua:<\/p>\n
#include <lua5.2\/lua.hpp><\/span>\n\nint<\/span> main()\n{\n    \/\/ Create a Lua virtual machine<\/span>\n    lua_State *l = luaL_newstate();\n\n    \/\/ Make all the lua libs to the virtual machine<\/span>\n    luaL_openlibs(l);\n\n    \/\/ Test it<\/span>\n    luaL_dostring(l, "print(<\/span>\\"<\/span>Helua, World!<\/span>\\"<\/span>)"<\/span>);\n    cout << lua_tostring(l, -1<\/span>);\n\n    lua_close(l);\n}<\/code><\/pre>\n
I\u00e2\u20ac\u2122m not sure it gets much easier.<\/p>\n
Calling C from Lua<\/h2>\n
Obviously the main reason for embedding a scripting language is to make custom routines from your application available to the script.<\/p>\n
int<\/span> x = 0<\/span>;\n\nint<\/span> customFunction(lua_State *L) {\n    \/\/ Number of elements on the parameter stack<\/span>\n    int<\/span> n = lua_gettop(L);\n    if<\/span>( n == 0<\/span> )\n        return<\/span> 0<\/span>;\n\n    for<\/span>(int<\/span> i = 0<\/span>; i < n; i++) {\n        if<\/span>( !lua_isnumber(L, i+1<\/span>) ) {\n            lua_pushstring(L, "incorrect arguments"<\/span>);\n            lua_error(L);\n        }\n        x += lua_tointeger(L, i+1<\/span>);\n    }\n\n    \/\/ return x<\/span>\n    lua_pushinteger(L, x);\n    return<\/span> 1<\/span>;\n}<\/code><\/pre>\n
Then register it and call it.<\/p>\n
int<\/span> main()\n{\n    \/\/ Create a Lua virtual machine<\/span>\n    lua_State *l = luaL_newstate();\n\n    lua_register(l, "customFunction"<\/span>, customFunction);\n\n    \/\/ Test it<\/span>\n    luaL_dostring(l, "return customFunction(1)"<\/span>);\n    std::cout << lua_tointeger(l, -1<\/span>) << endl;\n\n    lua_close(l);\n}<\/code><\/pre>\n
It took me quite a while to realise that \u00e2\u20ac\u0153return<\/code>\u00e2\u20ac\u009d was needed inside the luaL_dostring()<\/code> parameter. It seems that the string being evaluated is treated as being inside some anonymous function and we are effectively calling that entire function. Now I understand that, the following from the Lua reference manual is clearer:<\/p>\n
\n
int luaL_dofile (lua_State *L, const char *filename);<\/code><\/p>\n
Loads and runs the given file. It is defined as the following macro:<\/p>\n
 (luaL_loadfile(L, filename) || lua_pcall(L, 0, LUA_MULTRET, 0))<\/code><\/pre>\n
It returns false if there are no errors or true in case of errors.<\/p>\n<\/blockquote>\n
luaL_loadfile()<\/code> uses lua_load()<\/code> which says this:<\/p>\n
\n
Loads a Lua chunk (without running it). If there are no errors, lua_load pushes the compiled chunk as a Lua function on top of the stack.<\/p>\n<\/blockquote>\n
So the string we give to luaL_dostring()<\/code> becomes a function on the stack, which is what lua_pcall()<\/code> calls \u00e2\u20ac\u00a6 calls<\/em> means it must return. That\u00e2\u20ac\u2122s in addition to our customFunction()<\/code> which itself returns to this anonymous function.<\/p>\n
Calling Lua from C<\/h2>\n
The reverse will also be useful; when you want to let your script react to events generated by the C\/C++ application.<\/p>\n
int<\/span> main()\n{\n    \/\/ Create a Lua virtual machine<\/span>\n    lua_State *l = luaL_newstate();\n\n    \/\/ Define the lua callback in the state; note that this doesn't<\/span>\n    \/\/ call the function, it creates the reference to it in the<\/span>\n    \/\/ global table<\/span>\n    luaL_dostring(L, R"***(<\/span>\n        function onEvent(a)\n            return<\/span> a+10<\/span>\n        end\n    )***"));<\/span>\n\n    \/\/ Random number<\/span>\n    int<\/span> x = 0x7673f562<\/span>;\n\n    \/\/ What function do we want to call?  Find it and push it onto<\/span>\n    \/\/ the stack<\/span>\n    lua_getglobal(L, "onEvent"<\/span>);\n    \/\/ Then push the arguments<\/span>\n    lua_pushnumber(L, x);\n    \/\/ Call it, telling Lua it has one argument, one result<\/span>\n    lua_call(L, 1<\/span>, 1<\/span>);\n    \/\/ Fetch the result<\/span>\n    int<\/span> y = lua_tointeger(L, -1<\/span>);\n    \/\/ Restore the stack<\/span>\n    lua_pop(L, 1<\/span>);\n\n    cout << "LUA.onEvent()"<\/span> = y << endl;\n}<\/code><\/pre>\n","protected":false},"excerpt":{"rendered":"
I\u00e2\u20ac\u2122ve not yet had a project that\u00e2\u20ac\u2122s needed an embedded scripting language. When I do, I think I\u00e2\u20ac\u2122ll reach for Lua. $ apt-get install lua5.2 liblua5.2-dev Command line lua: $ lua Lua 5.2.2 Copyright (C) 1994-2013 Lua.org, PUC-Rio > print("Hello, World") Hello, World Embedded lua: #include <lua5.2\/lua.hpp> int main() { \/\/ Create a Lua virtual\u2026 Read More »<\/a><\/span><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":[],"categories":[1],"tags":[66,109,6],"_links":{"self":[{"href":"https:\/\/www.fussylogic.co.uk\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1208"}],"collection":[{"href":"https:\/\/www.fussylogic.co.uk\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.fussylogic.co.uk\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.fussylogic.co.uk\/blog\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.fussylogic.co.uk\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=1208"}],"version-history":[{"count":3,"href":"https:\/\/www.fussylogic.co.uk\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1208\/revisions"}],"predecessor-version":[{"id":1211,"href":"https:\/\/www.fussylogic.co.uk\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1208\/revisions\/1211"}],"wp:attachment":[{"href":"https:\/\/www.fussylogic.co.uk\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1208"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.fussylogic.co.uk\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1208"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.fussylogic.co.uk\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1208"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}