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)
end

I perform function call from c++ like this:

Compile it with g++ like this:

g++ -o last{,.cc} -llua -ldl

The results:

Result: -12.158958

Brief 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.

To allow function call from Lua script, your function must obey certain format. As variable passing from Lua to C and vice versa are through Lua Stack, therefore, the wrapper function must pass Lua State as the only parameter.
How about the real function call parameters? We can get it from the stack using lua_tonumber(), lua_tostring() etc. To return result of function call, we use lua_pushnumber(), lua_pushstring() etc. Because Lua function allows return more than 1 results, therefore you need tell Lua by returning an integer value.

We need to setup a library to contain your functions. By doing this, construct a static structure in array. Next, we load up our lib by calling luaL_openlib() and at last, to trigger a dofile().

Lets check out the sample codes on how to implement msleep function to Lua.

From the example above, we implemented msleep() that takes only 1 parameter, and we created a library “misc” in Lua. luaL_optint() is actually a clean version of doing lua_isinteger() and lua_tointeger(). luaL_optint() takes 3 parameter,

 luaL_optint(Lua State, Stack Number, Default Value)

If the param specified by lua function call is not integer, the default value will be assigned.

So let see how we can call that from Lua script.

for i=1,9,1 do
    io.write(string.format("[%d] Hello\n",i))
    misc.msleep(1000) -- sleep 1 sec
end

Check out for more details from Programming in Lua Chapter 26 [1].

And more Lua examples here, http://cc.byexamples.com/category/lua/.

Reference:
[1] Programming In Lua, Chapter 26 #Calling C from Lua

Value passing between c++ and Lua rely on Lua stack. Stack is a data structure based on the principle of Last In First Out (LIFO). This is very important keep in mind while coding with C API of Lua.

P.S: I am using Lua 5.1.3’s C API.

The Lua script “config.lua”:

var1=12
var2=34

Compilation line:

g++ -o aconf{,.cpp} -llua -ldl

if (luaL_loadfile(L, "config.lua") || lua_pcall(L, 0, 0, 0)) can be written as luaL_dofile(L,"config.lua"), it does the same thing. By calling lua_getglobal(L, "var1");, Lua will push the value of var1 to the stack. When execute getglobal for “var2″, again it will push var2’s value to the stack, var2 will now stack on top of var1.

The most top of the stack will be assign as logical address -1. Stack address -2 will be at below the stack -1. Therefore to access var1, you have to point to -2. For more info regarding Lua stack, can read HERE.

To check the value type in the stack -2 (var1) is it number, we can use this:
lua_isnumber(L, -2)

To access the value to number, we do this:
lua_tonumber(L, -2)

As lua_tonumber(L, -2) will return as double, therefore we must add (int) in front of the function call. Well, we should use lua_tointeger(L, -2) in this case.

Again, I can access var1 and var2 one by one, therefore you access both variables from the top stack.

When accessing Lua script and trigger exception, the error message will always push to the stack, that why We can print out the message when errors detected like this:

printf("error: %s", lua_tostring(L, -1));

Accessing simple variables is simple, but for accessing table data type, you need more steps, the Lua online ebook does cover that.

Lua Online Ebook: Programming in Lua

Auto-complete are useful if only we can customize it. Well, readline allows us do it by assign our own callback functions. First of all, you may want to read up the manual from HERE. It does provides a c sample codes as well but I find it too complicated, here I provide a simplified version that can be compiled under c++.

I store my ‘keywords’ targeted for auto-completion in an array cmd. Before calling readline(), I must point my completion callback function to rl_attempted_completion_function.

In my_completion(), if the state==1 meaning auto-completing the first word from readline(). If the state > 1 and my_completion() return NULL, filename auto-complete will be trigger automatically, which I find it damn irritating. So when state !=1, I disable it, and enable it back after next readline(). It is a dirty hack to me, I am still searching for a proper way to disable filename completion entirely.

I wrote xmalloc() myself because extern xmalloc doesn’t seems to work while compiling under c++.

To get better understanding of readline auto-complete, please read up the manual carefully and try it out yourself.

At last, hope you enjoy the post.

GNU readline library provides this feature, and it is pretty easy to use. Lets look at the example below:

I use rl_bind_key() to disable the tab key for file auto-complete that enabled by default. I will write another post for custom auto-complete using libreadline soon. add_history() is used to add the enterd inputs into the history list, and key bindings to arrow keys are enabled by default.

Compile it with g++ now:

g++ -o simple_rl{,.cpp} -lreadline

That is it, simple and nice :)

Lately I’ll been using STL Singleton template for my projects, I find it simple to implement and yet efficient.

Lets define the Singleton Template:

In order to turn your own class into singleton, you class must inherits from CSingleton like this:

Access MyClass variables and functions like this:

I find the appearance of MyClass::Instance() are ugyl and lengthy, therefore I like to define a nicer look of singleton class.

#define MYCLASS MyClass::Instance()

So my source code looks cleaner.

Anyway, this is just my preference. Enjoy yourself with STL Singleton Template.

What is so special about Lua?
Lua is Fast, Light-weight and Embeddable.

Lua can be embedded into c and c++ programs and Lua core are statically complied with your c++ programs, meaning your c++ program itself are now becoming Lua interpreter that execute Lua scripts. To extend your c++ application to execute Lua script is simple, lets check it out.

Before you start, please download and install Lua from HERE.
Disclaimer: My example are based on Lua 5.1.3.

1. Create a simple Lua script and name it as foo.lua.

io.write("Please enter your name: ")
name = io.read() -- read input from user
print ("Hi " .. name .. ", enjoy hacking with Lua");

2. Write a cpp program clua.cpp to execute foo.lua.

Lua API are in c format, in order to make it work with C++ you need to extern “C”. luaL_openlibs(L) loading up all the basic libs such as IO, String, Math etc. I believe if you are using Lua 5.0, you have to replace luaL_open(L) to load the libs one by one like this:

      luaopen_base(L);
      luaopen_table(L);
      luaopen_io(L);
      luaopen_string(L);
      luaopen_math(L);

3. Compile the clua.cpp with g++.

g++ -o clua{,.cpp} -llua -ldl

IMPORTANT! You need to link you program with libdl besides liblua. I believe the use of luaL_openlibs() are calling dlopen, dlclose, dlerror, dlsym which needs libdl.

Else you may get linking error like this:

/usr/local/lib/liblua.a(loadlib.o): In function `ll_loadfunc':
loadlib.c:(.text+0x917): undefined reference to `dlsym'
loadlib.c:(.text+0x924): undefined reference to `dlerror'
loadlib.c:(.text+0x9fc): undefined reference to `dlopen'
loadlib.c:(.text+0xa11): undefined reference to `dlerror'
/usr/local/lib/liblua.a(loadlib.o): In function `gctm':
loadlib.c:(.text+0x101e): undefined reference to `dlclose'
collect2: ld returned 1 exit status

4. Execute your c++ app

$ ./clua
Please enter your name: surface
Hi surface, enjoy hacking with Lua

I am done with Lua in C++.

Cool isn’t it?

Lets try to change foo.lua into this:

io.write("Please enter your name: ")
name = io.read()
io.write("Hi " .. string.format("\27\91\1;38;40m%s\27\91\0;47;40m",name) .. ", enjoy hacking with Lua\n");

Now, run ./clua again! You name will be in RED, check out text color example HERE

Ofcause, in order to really extend your c++ apps to Lua script, you need more than lua_dofile(), you need to allow Lua script calling your c++ functions, you may need to access variables from Lua to c++ and vice versa. Well, mean while I am still learning, I will share more when I learn more tricks!

Reference and Tutorial!
Lua provides excellent documentation:

I am dilemma here whether should I post this at http://linux.byexamples.com or http://cc.byexamples.com. As an introduction post for Lua programming, I will put it on both blogs. For future post, if I write about Lua scripting, I will post at http://linux.byexamples.com and if I write about Lua C++ API, it will be at http://cc.byexamples.com.

Hope you enjoy this post, and start to script with Lua.
@lightstar: In case you are reading this, I would like to say thank you for introduce this wonderful language to me, I enjoy it very much!

For example, you write a callback timer. It allows you to specified the duration and what function to call, and the function will be callback accordingly. “Run myfunction() every 10 seconds for 5 times”

Or you can create a function directory, passing a list of function name and ask the library to callback accordingly. “Callback success() if success, callback fail() if failed.”

Lets look at a simple function pointer example

How to pass argument to callback function?
Observered that function pointer to implement callback takes in void *, which indicates that it can takes in any type of variable including structure. Therefore you can pass in multiple arguments by structure.

curl is a command line tool for transferring files with URL syntax, supporting FTP, FTPS, HTTP, HTTPS, SCP, SFTP, TFTP, TELNET, DICT, LDAP, LDAPS and FILE. curl supports SSL certificates, HTTP POST, HTTP PUT, FTP uploading, HTTP form based upload, proxies, cookies, user+password authentication (Basic, Digest, NTLM, Negotiate, kerberos…), file transfer resume, proxy tunneling and a busload of other useful tricks.

Quote from http://curl.haxx.se/

Advantages of using libcurl:

You do not need to read up RFC to code for HTTP post or ftp client, you just need to download libcurl-dev and try it out.

Let me show you one of the simple example source code from curl official site.

Check out the libcurl tutorial and the sample source codes.

Recently I found a better way, and with this function, you can create a better time stamp, yet the way of construct the time stamp is far more simple compared to my previous post.

Yes, that’s it, and the output is show as below:

Wed, 26 Sep 2007 23:54:50 +0800