Print( 'timing of Lua wait function: ', ostimevrbl ); end wait(0.5); Examples of Lua wait. Given below are the examples mentioned: Example #1. The Lua wait function with millisecond example and output. Code: function wait(millisecond) local ostimevrbl = os.time + millisecond print( 'timing of Lua wait function: ', ostimevrbl ); end wait(0.5); wait(1). In this tutorial we go over calling Lua functions from C.Twitter: https://twitter.com/KarmaKilledTC. In the example above we're doing these things: creating and setting up Lua VM as shown on the first example. Getting and pushing a Lua function from global Lua table onto virtual stack. Pushing string 'Hello C API' as an input argument onto the virtual stack. Instructing VM to call a function with one argument which is already on the stack.

This first edition was written for Lua 5.0. 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 24. An Overview of the C API

Lua is an embedded language.That means that Lua is not a stand-alone package,but a library that can be linked with other applicationsso as to incorporate Lua facilities into these applications.

You may be wondering:If Lua is not a stand-alone program,how come we have been using Lua stand alone through the whole book?The solution to this puzzle is the Lua interpreter(the executable lua).This interpreter is a tiny application(with less than five hundred lines of code)that uses the Lua library to implement the stand-alone interpreter.This program handles the interface with the user,taking her files and strings to feed them to the Lua library,which does the bulk of the work (such as actually running Lua code).

This ability to be used as a library to extend an applicationis what makes Lua an extension language.At the same time, a program that uses Lua can register newfunctions in the Lua environment;such functions are implemented in C (or another language)and can add facilities that cannot bewritten directly in Lua.This is what makes Lua an extensible language.

These two views of Lua(as an extension language and as an extensible language)correspond to two kinds of interaction between C and Lua.In the first kind, C has the control and Lua is the library.The C code in this kind of interactionis what we call application code.In the second kind, Lua has the control and C is the library.Here, the C code is called library code.Both application code and library code usethe same API to communicate with Lua,the so called C API.

The C API is the set of functions that allow C codeto interact with Lua.It comprises functions to read and write Lua global variables,to call Lua functions,to run pieces of Lua code,to register C functions so that they can later be called by Lua code,and so on.(Throughout this text, the term 'function' actually means'function or macro'.The API implements several facilities as macros.)

The C API follows the C modus operandi,which is quite different from Lua.When programming in C,we must care about type checking (and type errors),error recovery, memory-allocation errors,and several other sources of complexity.Most functions in the API do not check the correctness of their arguments;it is your responsibility to make sure that the argumentsare valid before calling a function.If you make mistakes,you can get a 'segmentation fault' error or something similar,instead of a well-behaved error message.Moreover, the API emphasizes flexibility and simplicity,sometimes at the cost of ease of use.Common tasks may involve several API calls.This may be boring,but it gives you full control over all details,such as error handling, buffer sizes, and the like.

As its title says,the goal of this chapter is to give an overview of what isinvolved when you use Lua from C.Do not bother understanding all the details of what is going on now.Later we will fill in the details.Nevertheless,do not forget that you can find more details about specific functionsin the Lua reference manual.Moreover, you can find several examples of the use of the API in theLua distribution itself.The Lua stand-alone interpreter (lua.c)provides examples of application code,while the standard libraries (lmathlib.c, lstrlib.c, etc.)provide examples of library code.

From now on, we are wearing a C programmers' hat.When we talk about 'you', we mean you when programming in C,or you impersonated by the C code you write.

A major component in the communication between Lua and C is anomnipresent virtual stack.Almost all API calls operate on values on this stack.All data exchange from Lua to C and from C to Lua occursthrough this stack.Moreover, you can use the stack to keep intermediateresults too.The stack helps to solve two impedance mismatches between Lua and C:The first is caused by Lua being garbage collected,whereas C requires explicit deallocation;the second results from the shock between dynamic typing in Luaversus the static typing of C.We will discuss the stack in more detail in Section 24.2.

Programming in Lua

Part IV. The C APIChapter 26. Calling C from Lua

26.1 – C Functions

As a first example,let us see how to implement a simplified version ofa function that returns the sine of a given number(a more professional implementation should check whetherits argument is a number):Any function registered with Lua must have this same prototype,defined as lua_CFunction in lua.h:From the point of view of C,a C function gets as its single argument the Lua stateand returns (in C) an integer with the number ofvalues it is returning (in Lua).Therefore, the function does not need to clear the stack beforepushing its results.After it returns,Lua automatically removes whatever is in the stack below the results.

Lua C Function Example

Before we can use this function from Lua,we must register it.We do this magic with lua_pushcfunction:It gets a pointer to a C function and createsa value of type 'function' to represent this function inside Lua.A quick-and-dirty way to test l_sin is to putits code directly into the file lua.cand add the following lines right after the call to lua_open:The first line pushes a value of type function.The second line assigns it to the global variable mysin.After these modifications,you rebuild your Lua executable;then you can use the new functionmysin in your Lua programs.In the next section,we will discuss better ways to link new C functions with Lua.

Lua C Function Example Java

For a more professional sine function,we must check the type of its argument.Here, the auxiliary library helps us.The luaL_checknumber function checks whether a givenargument is a number:In case of errors, it throws an informative error message;otherwise, it returns the number.The modification in our function is minimal:With the above definition,if you call mysin('a'), you get the messageNotice how luaL_checknumber automatically fills the messagewith the argument number (1),the function name ('mysin'),the expected parameter type ('number'),and the actual parameter type ('string').

Lua C Function Examples

As a more complex example,let us write a function thatreturns the contents of a given directory.Lua does not provide this function in its standard libraries,because ANSI C does not have functions for this job.Here, we will assume that we have a POSIX compliant system.Our function, dir,gets as argument a string with the directory pathand returns an array with the directory entries.For instance, a call dir('/home/lua') may ret