How To Use Lua C Api

  1. How To Use Lua
  2. C++ With Lua
  3. How To Use Lua C Api Free

The ntopng core provides a Lua C API to interact with it. The API is internally usedby periodic scripts and GUI scripts to extract information or to apply configurationchanges.

Custom user scripts can use the API, for example, to provide new data visualizationsor extract the data to send it to an external program.

Since some API functions can return very complex objects, which will not be covered here,the suggestion is to use the utility function tprint to print out the result a the functionto figure out its format.

The Lua C API consists of two main objects:

  • the ntop object is used to access global ntopng functions, which arenot bound to a specific network interface.
  • the interface object is used to access a specific network interface functions.

The Application Program Interface. LRM §4 describes the C API for Lua. This is used by NodeMCU modules written in the C source language to interface to the the Lua runtime. The header file lua.h is used to define all API functions, together with their related types and constants. This section 4 forms the primary reference, though the NodeMCU.

How To Use Lua

For example, the flows information is a network interface specific information,so it’s available through the interface object API via interface.getFlowsInfo().

  • 1. ntop
  • 2. interface

In this short tutorial I'll show how to run Lua programs from C and C++ and howto expose functions to them. It's easy!

Update: The code in this post has been updated for Lua 5.2.4. I haven'tchecked if the Lua 5.3 C API is backwards-compatible with 5.2. All the codehere is available on GitHub.

The first program will just create a Lua state object and exit. It will be ahybrid between C and C++. Since the two languages must include different files,we need to discern between them by checking for the existence of the__cplusplus macro.

Notice that I'm being explicit about which version of Lua I'm using in thecode. If you trust that the Lua developers care about compatibility, you canjust #include <lua.hpp> and so on directly.

Lua

The purpose of the program is just to make sure that we can compile, link andrun it without errors.

You need to let the compiler know where it can find the include files and theLua shared library. The include files are usually located in/usr/local/include and the library files in /usr/local/lib. Search yoursystem directories if needed. To compile the above program, pass thedirectories with -I and -L, respectively.

You may swap out g++ with llvm-g++, or just c++, depending on yourcompiler. If you're using a C compiler, use gcc or llvm-gcc — butremember to rename the file to first.c.

Now try to run the program to make sure it doesn't segfault:

This one worked just fine.

Executing Lua programs from a host

The next step is to execute Lua programs from your C or C++ code. We'll createthe Lua state object as above, load a file from disk and execute it.

Put this into runlua.cpp or runlua.c:

You can reuse the compilation arguments from above:

or

Running Lua programs

Let's test this with some Lua programs. The first one prints the Lua versionand exits.

You may want to double-check that it works by running lua hello.lua. It maynot be important for this trivial program, but can become important when youtry more advanced ones.

Now try it with runlua:

You can even run bytecode-compiled programs:

We should also check that the error handling works. Put some garbage in a filecalled error.lua, for example

Running it produces

Convert lua to c

Calling C functions from Lua

C++ With Lua

It gets very interesting when Lua programs call back to your C or C++functions. We'll create a function called howdy that prints its inputarguments and returns the integer 123.

To be on the safe side, we'll declare C linkage for the function in the C++version of the program. This has to do with name mangling,but in this case, it really doesn't matter: Lua just receives a pointer to afunction, and that's that. But if you start using dynamic loading of sharedlibraries through dlopen and dlsym, this will be an issue. So let's do itcorrect from the start.

Copy the above program into a file called callback.cpp and add the howdyfunction.

We have to pass the address of this function to Lua along with a name. Put thefollowing line somewhere between the call to lua_newstate andluaL_loadfile:

Create a test program called callback.lua

Compile and test it

I told you it was easy!

What next?

Read the Lua C APIReference. You've learned enough now to get going with it. Did you see mynote about clearing the stack in howdy? You may want to investigate that.

Find out how to integrate Lua closures with your C functions.

If you want to hide or catch console output from Lua, you need to figure thatout as well. I once did it by trapping io.write(); I copied its code fromlualib.c and changed io_write to point to my own function. There isprobably a better way to do it, though. Doing so is useful for things like gameprogramming.

Use RAIIor smart pointers to manage resources like lua_State.

I also strongly recommend to try out LuaJIT.Calling into your functions there is even easier, using LuaJIT's foreignfunction library. I'll write a blog post on how todo that as well. In short, just create ordinary C functions, compile as ashared library, copy their signatures into pure Lua source code and hook themup with LuaJIT's FFIlibrary.

How To Use Lua C Api Free

LuaJIT runs between 10-20 and up to 135 times faster than interpreted Lua, soit's definitely worth it.