.NET/Java PDF, Tiff, Barcode SDK Library

The lock function operates in a fashion similar to using. The purpose of the function is to lock a section of code so that only one thread can pass through it at a time. This issue arises in multithreaded programming because a thread can context switch at any time, leaving operations that should have been atomic half done. This is controlled by locking on an object; the idea is that as soon as the lock is taken, any thread attempting to enter the section of code will be blocked until the lock is released by the thread that holds it. Code protected in this way is sometimes called a critical section. This is achieved by calling System.Threading.Monitor.Enter at the start of the code that is to be protected and System.Threading.Monitor.Exit at the end; it is important to guarantee that Monitor.Exit is called, or this could lead to threads being locked forever. The lock function is a nice way to ensure that Monitor.Exit is always called if Monitor.Enter has been called. It takes two parameters; the first is the object to be locked on,

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Figure 1-29. Motor breakdown The NXT Rotation Sensor works on the same principle as the RCX Rotation Sensor, only on a smaller scale, as seen in the close-up photo in Figure 1-30.

and the second is a function that contains the code to be protected. This function should take unit as its parameter, and it can return any value. The following example demonstrates the subtle issues involved in locking. It needs to be quite long and has been deliberately written to exaggerate the problem of context switching. The idea is that two threads will run at the same time, both trying to write the console. The aim of the sample is to write the string "One ... Two ... Three ... " to the console atomically; that is, one thread should be able to finish writing its message before the next one starts. The example has a function, called makeUnsafeThread, that creates a thread that will not be able to write to the console atomically and a second one, makeSafeThread, that writes to the console atomically by using a lock. #light open System open System.Threading // function to print to the console character by character // this increases the chance of there being a context switch // between threads. let printSlowly (s : string) = s.ToCharArray() |> Array.iter print_char

Web development with JavaScript is no longer as painful as it was even a few short years ago. Today, modern browsers are remarkably similar in their implementation of the W3C DOM standards and JavaScript. Some workarounds are still necessary, though mostly because of Internet Explorer. The tips offered in this appendix will help you write JavaScript that will work correctly on most modern browsers. By following the techniques outlined here, you ll save yourself countless hours of debugging and testing. Happy scripting!

// create a thread that prints to the console in an unsafe way let makeUnsafeThread() = new Thread(fun () -> for x = 1 to 100 do printSlowly "One ... Two ... Three ... " print_newline() done) // the object that will be used as a lock let lockObj = new Object() // create a thread that prints to the console in a safe way let makeSafeThread() = new Thread(fun () -> for x = 1 to 100 do // use lock to ensure operation is atomic lock lockObj (fun () -> printSlowly "One ... Two ... Three ... " print_newline() ) done)

The programming language you choose is often a matter of personal preference. This book is mostly about building electronic hardware, but software is always an essential ingredient of any robotic project. Before tackling the projects in this book, you should be comfortable programming the NXT using just the software that comes with the MINDSTORMS kit known as NXT-G. We provide a brief survey of the major languages available to program the NXT in this chapter. Because this book relies exclusively on the NXT-G and an alternative language called NXC for its programming, we will only discuss them in detail.