Our online Lua Binding Generator is a good starting point for building your own Lua bindings. These bindings enable LSP pages and Lua scripts to call your own C functions.
A convenient way to test the examples below is to use the Mako Server. Mako can include Lua bindings either statically at compile time or dynamically at runtime. For dynamic loading, build the binding as a loadable Lua module, such as a DLL/shared library, and load it at runtime. The online Lua binding tutorial provides an introductory Lua binding tutorial and a walkthrough of loading modules dynamically, while the SOL2 C++ tutorial shows how to statically include Lua bindings at compile time. Note, for monolithic RTOS builds, Lua bindings must be included at compile time.
This page explains how to write manual Lua bindings, which are preferred for complex interactions such as working with objects on the C side. The Lua book provides a good introduction to designing Lua bindings.
This page explains:
- How to design a Lua interface to a C object, such as a handle in C code or a C++ class.
- How to release the global mutex for time-consuming code, enabling concurrent requests.
- How to call Lua code asynchronously from C code.
Object-Oriented Access and Releasing the Global Mutex
This tutorial shows how to write a Lua interface to a C object. You can use the same method to export a C++ class to Lua. The tutorial also shows how to release the global mutex for C functions that may take some time to complete, enabling the web server to accept other HTTP requests while the C function is running.
The tutorial requires that you have an understanding of Lua user data and Lua metatables as described in the Lua book. An older version of the book is available online where you can read the two sections Object-Oriented Access and metatables.
The Lua interface to the C object will be as follows:
Dynamically create a Lua object that represents the C object.
The C object is created by calling the open function in the global Lua2C table. The Lua2C table and the open function are both part of this exercise. In other words, we will extend the Lua engine with this functionality.
The dynamically created object will have two callable functions.
The "fast" and "slow" methods each increment a counter when called and return the number of times that method has been called. The "fast" function returns immediately, while the "slow" function takes 5 seconds to execute. The "slow" function therefore releases the global mutex while processing, enabling other threads in the server to execute.
Lua can return multiple values so both the "fast" and "slow" functions return two Lua integers: fast() returns fastCount, slowCount, and slow() returns slowCount, fastCount. Both methods require the userdata returned by Lua2C.open() as self; a different object raises a Lua argument error. open() takes no arguments and returns one userdata. These teaching methods have no operational error result. For application bindings, raise an error for incorrect API use and return nil, error for recoverable operational failures.
Creating the Lua2C table and binding the 'open' function
We must start by creating a C function that registers our Lua2C table.
The luaopen_Lua2C function is the only global function in the C file, and it is the function you must call from your startup code – i.e. from the code creating the main Lua state. The name of the function can be any name, but we have opted to use a naming convention that is compatible with the Lua function require. Our C file can be compiled as a standalone module on operating systems that support DLLs/shared libraries, and loaded on demand by calling require("Lua2C").
Notice how we set the last argument in function luaL_requiref to TRUE. This construction registers the Lua2C table as a global variable and makes it possible to reference this table directly in the global namespace, without having to call require("Lua2C") from the Lua code.
In a typical embedded system, function luaopen_Lua2C will be called just after you create the main Lua state, and the C file with the Lua2C bindings will be compiled and linked into the system. In other words, it will not be loaded dynamically as a dynamic library.
Function luaopen_Lua2C returns a value designed for dynamic loading via require. This return value is not needed when the code is linked directly with the server. After creating the main Lua state "L", call luaopen_Lua2C and pop the values pushed by luaopen_Lua2C off the stack. This keeps the Lua stack balanced.
In function luaopen_Lua2C, luaL_requiref takes function installLua2C as an argument. The installLua2C function installs the "open" function in a new Lua table and returns this table. Lua will then register this table as "Lua2C".
Lua code can now call Lua2C.open() and a call to this function from Lua ends up in the Lua binding Lua2C_open.
The key to understanding this code is to read the Lua documentation for luaL_newmetatable and review how metatable.__index = metatable works. This function creates a new table the first time it is called and retrieves the table on subsequent calls. The table is set as a metatable for the Lua userdata, i.e. for the C object Lua2C. The metatable binds the three Lua functions, "slow", "fast", and "__gc" to the dynamically created Lua2C object, making it possible for Lua code to use the userdata and call functions "fast" and "slow". The "__gc" function is run by Lua when the garbage collector reclaims the memory for the userdata object. The "__gc" function is the destructor.
Lua code can now call the "fast" and "slow" functions. The following will print out "1 0" and "2 0".
This Lua example triggers the C function Lua2C_fast twice.
Function Lua2C_fast verifies that the first stack argument is a Lua2C object, typecasts the Lua userdata object to a Lua2C object, increments the counter, and pushes the two return values onto the Lua stack.
Releasing the Global Mutex
Function Lua2C_slow is similar to function Lua2C_fast. It performs the same checking, increments the counter, and pushes the two values onto the stack. The "slow" function also sleeps for 5 seconds, simulating a lengthy C call.
This code fetches the server's main mutex and releases the mutex just before sleeping for 5 seconds. The mutex is reclaimed immediately after the sleep completes. You must make sure the mutex is locked when working with the Lua state L. The mutex is always locked when the Lua binding starts, and it must be locked again when the binding returns.
When an LSP page calls the "slow" function, it will block for 5 seconds. This enables other threads in the server's pool to execute. An introduction to the thread mechanism in the server can be found in the introduction under section Thread Mapping and Coroutines. Many of the Barracuda Application Server's Lua bindings release the mutex when calling code that can block. Examples of such functions are the functions in the SQL library and functions in the response object such as response:write.
Each native thread in the server's thread pool is mapped to one Lua coroutine stack, enabling the server to handle many concurrent Lua stacks at the same time. The next section explains how to map a coroutine stack onto a native thread when calling Lua from C code.
Download Example Code
Download the source code for this example.
Automatic Mutex Wrapper Generation
Creating mutex-release code for many lengthy function calls is tedious. You may also have existing Lua bindings, or Lua bindings from the Internet, that you want to adapt quickly. Real Time Logic provides two scripts that automate this process.
mutex.lua: Mutex wrapper script generator
This script creates C code that wraps around the original functions. Each wrapper function releases the mutex, calls the original function, and locks the mutex when the original function returns.
Let's say you have the following function and you want to wrap this function with another function that releases the mutex while it runs.
Create a file such as functions.txt and put all the function declarations you want to wrap into this file. In our case, we put the function declaration above into this file. The next step is to create the wrapper code. Run the mutex script as follows:
lua mutex.lua functions.txt my-generated-wrapper my-header.h
The Lua script mutex.lua parses the function declarations in functions.txt and produces my-generated-wrapper.h and my-generated-wrapper.c. The last argument passed into mutex.lua (my-header.h) is a header file that provides the original function declarations. This file is included in the output (my-generated-wrapper.h).
my-generated-wrapper.h:
my-generated-wrapper.c:
fext.lua: Function declaration extraction script
The file that lists all functions (functions.txt) can be created manually, or you can use fext.lua to parse your header files and automatically produce functions.txt.
$lua fext.lua Usage: lua fext.lua [-p pattern] [-c cdata-blob-file] [-e exclude-file] input-files....
The optional pattern (-p) means that only files matching this pattern should be included. See Lua patterns for details.
You do not want to create wrappers for functions not being used by your Lua bindings. The optional -c option is for loading a "blob file" that is a concatenation of all C files that are part of your Lua binding library. When a function is found in the header file(s), the "blob file" is searched. The function is only included in functions.txt if it is found in the "blob file". You can, for example, create this file as follows: cat *.c > c-data-blob.txt
You may also want to limit the set of functions included. The -e option is for a file that lists all functions that should be excluded.
Download mutex.lua and fext.lua
For additional examples on how to use these two Lua scripts, see the build scripts (build.sh) for the MongoDB and PostgreSQL Modules.
Calling Lua Code Asynchronously from C Code
The Barracuda App Server enables multiple threads to call and execute Lua code. In the previous section, we explained how threads in the thread pool can simultaneously execute in the Lua VM instance by using coroutine thread stacks. Only one thread can execute at any time in the VM, but Lua scripts can call Lua bindings (C functions), which can then release the global mutex protecting the Lua VM. This construction enables multiple threads to cooperatively run the Lua VM instance.
Easy Solution
Calling Lua asynchronously from C code is more complicated than calling C code from Lua. The recommended design is explained below, but you may first want to consider an easier alternative. The Barracuda App Server provides an API for creating native threads in Lua. Lua code running in the native thread can repeatedly call a Lua-to-C function that blocks until data is available. The C side of the binding releases the global mutex as explained in the mutex section, uses an OS primitive to wait for data, reclaims the mutex, and returns the data to Lua. This pattern simulates an asynchronous call while keeping the design simpler.
Recommended Solution
Let's revisit the diagram from the Thread Mapping introduction.
Suppose you have an asynchronous source that generates two integers, and you want to call a Lua function and pass those integers as arguments when the event occurs. If your C startup code created an LThreadMgr instance, you can use the Lua Thread Library to achieve this.
The following must be in your startup code. See Xedge's C code for an example.
The following C example passes two integers to a global Lua function onNativeEvent(value1, value2). Define that function before submitting events. The event source must be a normal native thread, not an interrupt handler. Stop the event source before destroying ltMgr or the Lua VM.
ThreadJob_lcreate(size, callback) returns a job pointer or NULL on allocation failure. It allocates at least sizeof(ThreadJob) bytes; extra payload storage is uninitialized. After LThreadMgr_run, the manager owns the job. Its return is TRUE when an idle worker was signalled and FALSE when the job was queued for a busy pool. Both mean acceptance. Pending jobs may be discarded during shutdown, so keep their payload cleanup requirements simple.
References: LThreadMgr, ThreadJob, ThreadMutex, and SoDispMutex. See the Lua documentation for lua_pushinteger and lua_pcall.
The C file AsynchLua.c in the Xedge's source code directory shows how to implement a complete example. Here is a copy of the AsynchLua.c file:
Initializing the TPM
The example code led.c includes the xedgeOpenAUX() function, which shows how to add additional secrets to the TPM and enable a device-unique key.
Using request/response in Lua Bindings
You may be porting an existing web application to the Barracuda App Server, or you may simply need the server's request or response object in your Lua binding. The following example shows how to write a Lua binding that can use the request and response objects.
This Lua binding uses function baluaENV_checkcmd for extracting the HttpCommand argument. In Lua the request/response objects are the same and represent the HttpCommand object.
Lua code can now call the function as follows:
Notice the baluaENV prefix in function baluaENV_checkcmd. The ENV prefix means that the function requires a special environment defined for the Barracuda App Server. The environment is provided as a C Closure. We must set this environment when we register the Lua binding.
In the first example above, we used the macro luaL_newlib to register the Lua bindings in function installLua2C. This macro is defined as:
Notice that the macro passes in the value zero as the number of up-values to function luaL_setfuncs.
The macro luaL_newlib cannot be used for this example since we need to push the Barracuda App Server environment onto the stack and let function luaL_setfuncs set this environment for the Lua bindings. Instead of calling macro luaL_newlib, we do the following:
Download Example Code
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