Lua has a very simple C API. You can run Lua code and access lua objects from C. Similarly, you can access C functions and libraries from Lua. In this post, we’ll primarily look at how to expose C functions to lua.

Install Lua

See full list on github.com.
You can change these numbers by calling luagc in C or collectgarbage in Lua. You can also use these functions to control the collector directly (e.g., stop and restart it). 2.5.1 – Garbage-Collection Metamethods. You can set garbage-collector metamethods for tables and, using the C API, for full userdata (see §2.4).
Aug 17, 2016 C API - luanext traversal of 'array' table. I have a small question regarding the Lua C API's 'luanext' function when used on a certain subset of objects: tables that are made like arrays, with sequential integer keys. Is the order of iteration guaranteed to go from '0' to 'N' in this case, or am I still subject to jumping around in.

We associate the metatable entries for push and pop with two static C functions llistpush and llistpop. This functions must be of type luaCFunction. That is they should take a luaState. as paramter and return an integer. That’s how the Lua communicates with C, via the luaState and its stack. The functions themselves are quite.

Let’s start off with the installation of lua. You can install lua using system package managers as

Mac:

Ubuntu:

You can also build lua from source. However, you have to build lua as a shared library and this can be a bit tricky.

Lua Stack

C Language Api

Communication between lua and C is held through stacks with each element in stack representing lua value (nil, number, string, etc.).

For example, a C wrapped function c_swap is called in lua as:

In C, you will receive arguments in a stack as

You will then read the arguments from the stack as (indexing for this stack starts with 1 like in lua):

This is actually unsafe because these calls will fail if value in the stack at the specified index is not a number. Following code uses auxiliary library lauxlib.h and is safer; it will raise errors instead of segfaulting.

Then, you will process the arguments and push the results to stack.

Stack will now look like

Since you need to communicate the number of return value to lua, you will return 2. Lua will then know that top 2 values in the stack are the returned values.

The stack we’ve described is not global. Each function has its own stack.

Code

Let’s put all this together into main.c. main() function illustrates how to run lua code from C.

And build and execute as If you have installed lua with apt-get, build using gcc main.c -o swap -llua5.2 -I/usr/include/lua5.2/

which should print

Lua Table Size

Library

In the above code, you’ve not really used lua interpretor; you’ve run the lua code in C itself.You might rather want to create a module which you can load into a lua interpretor using

To do this, you’ll have to register your functions by creating a array of luaL_Reg and a function luaopen_mylib.

Let’s do this right away by editing main.c. We will also add additional mysin function.

⊕ This code works only for lua 5.2. For lua 5.1, please use luaL_register(L, 'mylib', mylib); instead of luaL_newlib(L, mylib);

Create a loadable .so file with

Lua C Api Push Array

⊕ If you have installed lua with apt-get, build using gcc main.c -shared -o mylib.so -fPIC -llua5.2 -I/usr/include/lua5.2/

You can now load this module in lua.

Now, you should be able to wrap any C library by writing a lua wrapper.We still have some caveats:

We will have to write a wrapping function for each of the functions in your C library to ‘parse’ the arguments(luaL_checknumber etc.). This is repetitive and can be a potential source for bugs.
C doesn’t have classes. What if your library is in C++? You cannot easily wrap classes like above. You will have to mess with metatables and so on.

A solution to both of these is to use Swig. Swig allows you to wrap C/C++ classes/functions into many languages like python, lua, java quite easily. In a later post, we will see how to use swig and wrap a simple library.

This first edition was written for Lua 5.0. Rumus lingkaran luas keliling. While still largely relevant for later versions, there are some differences.
The fourth edition targets Lua 5.3 and is available at Amazon and other bookstores.
By buying the book, you also help to support the Lua project.

Programming in Lua

Part IV. The C APIChapter 27. Techniques for Writing C Functions

27.2 – String Manipulation

When a C function receives a string argument from Lua,there are only two rules that it must observe:Not to pop the string from the stack while accessing itand never to modify the string.

Things get more demanding when a C functionneeds to create a string to return to Lua.Now, it is up to the C code to take care ofbuffer allocation/deallocation, buffer overflow, and the like.Nevertheless, the Lua API provides some functionsto help with those tasks.

The standard API provides support for two of the most basic string operations:substring extraction and string concatenation.To extract a substring,remember that the basic operation lua_pushlstringgets the string length as an extra argument.Therefore, if you want to pass to Lua a substring of a string sranging from position i to j (inclusive),all you have to do isAs an example, suppose you want a functionthat splits a string accordingto a given separator (a single character)and returns a table with the substrings.For instance, the callshould return the table {'hi', ', 'there'}.We could write a simple implementation as follows.It needs no extra buffersand puts no constraints on the size of the strings it can handle.

To concatenate strings,Lua provides a specific function in its API,called lua_concat.It is equivalent to the . operator in Lua:It converts numbers to strings and triggers metamethodswhen necessary.Moreover, it can concatenate more than two strings at once.The call lua_concat(L, n) will concatenate (and pop) then values at the top of the stackand leave the result on the top.

Another helpful function is lua_pushfstring:It is somewhat similar to the C function sprintf,in that it creates a string according to a format stringand some extra arguments.Unlike sprintf, however, you do not need to provide a buffer.Lua dynamically creates the string for you,as large as it needs to be.There are no worries about buffer overflow and the like.The function pushes the resulting string on the stackand returns a pointer to it.Currently, this function accepts only thedirectives %% (for the character `%´),%s (for strings), %d (for integers),%f (for Lua numbers, that is, doubles),and %c (accepts an integer and formats it as a character).It does not accept any options (such as width or precision).

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G Series Lua Api

Both lua_concat and lua_pushfstringare useful when we want to concatenate only a few strings.However,if we need to concatenate many strings (or characters) together,a one-by-one approach can be quite inefficient,as we saw in Section 11.6.Instead,we can use the buffer facilities provided by the auxiliary library.Auxlib implements these buffers in two levels.The first level is similar to buffers in I/O operations:It collects small strings (or individual characters) in a local bufferand passes them to Lua (with lua_pushlstrin

Lua C Api Push Table