Multithreading is becoming an increasingly important part of modern programming. One reason for this is that multithreading enables a program to make the best use of available CPU cycles, thus allowing very efficient programs to be written. Another reason is that multithreading is a natural choice for handling event-driven code, which is so common in today's highly distributed, networked, GUI-based environments. Of course, the fact that the most widely used operating system, Windows, supports multithreading is also a factor. Whatever the reasons, the increased use of multithreading is changing the way that programmers think about the fundamental architecture of a program. Although C++ does not contain built-in support for multithreaded programs, it is right at home in this arena.
Because of its growing importance, this chapter explores using C++ to create multithreaded programs. It does so by developing two multithreaded applications. The first is a thread control panel, which you can use to control the execution of threads within a program. This is both an interesting demonstration of multithreading and a practical tool that you can use when developing multithreaded applications. The second example shows how to apply multithreading to a practical example by creating a modified version of the garbage collector from Chapter 2 that runs in a background thread.
This chapter also serves another purpose: it shows how adept C++ is at interfacing directly to the operating system. In some other languages, such as Java, there is a layer of processing between your program and the OS. This layer adds overhead that can be unacceptable for some types of programs, such as those used in a real-time environment. In sharp contrast, C++ has direct access to low-level functionality provided by the operating system. This is one of the reasons C++ can produce higher performance code.
What Is Multi-threading?
Before beginning, it is necessary to define precisely what is meant by the term multithreading. Multithreading is a specialized form of multitasking. In general, there are two types of multitasking: process-based and thread-based. A process is, in essence, a program that is executing. Thus, process-based multitasking is the feature that allows your computer to run two or more programs concurrently. For example, it is process-based multitasking that allows you to run a word processor at the same time you are using a spreadsheet or browsing the Internet. In process-based multitasking, a program is the smallest unit of code that can be dispatched by the scheduler.
A thread is a dispatchable unit of executable code. The name comes from the concept of a thread of execution. In a thread-based multitasking environment, all processes have at least one thread, but they can have more. This means that a single program can perform two or more tasks concurrently. For instance, a text editor can be formatting text at the same time that it is printing, as long as these two actions are being performed by two separate threads. The differences between process-based and thread-based multitasking can be summarized like this: Process-based multitasking handles the concurrent execution of programs. Thread-based multitasking deals with the concurrent execution of pieces of the same program.
In the preceding discussions, it is important to clarify that true concurrent execution is possible only in a multiple-CPU system in which each process or thread has unrestricted access to a CPU. For single CPU systems, which constitute the vast majority of systems in use today, only the appearance of simultaneous execution is achieved. In a single CPU system, each process or thread receives a portion of the CPU time, with the amount of time determined by several factors, including the priority of the process or thread. Although truly concurrent execution does not exist on most computers, when writing multithreaded programs, you should assume that it does. This is because you can't know the precise order in which separate threads will be executed, or if they will execute in the same sequence twice. Thus, its best to program as if true concurrent execution is the case. Multithreading Changes the Architecture of a Program
Multithreading changes the fundamental architecture of a program. Unlike a single-threaded program that executes in a strictly linear fashion, a multithreaded program executes portions of itself concurrently. Thus, all multithreaded programs include an element of parallelism. Consequently, a major issue in multithreaded programs is managing the interaction of the threads.
As explained earlier, all processes have at least one thread of execution, which is called the main thread. The main thread is created when your program begins. In a multithreaded program, the main thread creates one or more child threads. Thus, each multithreaded process starts with one thread of execution and then creates one or more additional threads. In a properly designed program, each thread represents a single logical unit of activity.
The principal advantage of multithreading is that it enables you to write very efficient programs because it lets you utilize the idle time that is present in most programs. Most I/O devices, whether they are network ports, disk drives, or the keyboard, are much slower than the CPU. Often, a program will spend a majority of its execution time waiting to send or receive data. With the careful use of multithreading, your program can execute another task during this idle time. For example, while one part of your program is sending a file over the Internet, another part can be reading keyboard input, and still another can be buffering the next block of data to send.
Why Doesn't C++ Contain Built-In Support for Multithreading?
C++ does not contain any built-in support for multithreaded applications. Instead, it relies entirely upon the operating system to provide this feature. Given that both Java and C# provide built-in support for multithreading, it is natural to ask why this isn't also the case for C++. The answers are efficiency, control, and the range of applications to which C++ is applied. Let's examine each.
By not building in support for multithreading, C++ does not attempt to define a one size fits all solution. Instead, C++ allows you to directly utilize the multithreading features provided by the operating system. This approach means that your programs can be multithreaded in the most efficient means supported by the execution environment. Because many multitasking environments offer rich support for multithreading, being able to access that support is crucial to the creation of high-performance, multithreaded programs.
Using operating system functions to support multithreading gives you access to the full range of control offered by the execution environment. Consider Windows. It defines a rich set of thread-related functions that enable finely grained control over the creation and management of a thread. For example, Windows has several ways to control access to a shared resource, including semaphores, mutexes, event objects, waitable timers, and critical sections. This level of flexibility cannot be easily designed into a language because the capabilities of operating systems differ. Thus, language-level support for multithreading usually means offering only a lowest common denominator of features. With C++, you gain access to all the features that the operating system provides. This is a major advantage when writing high-performance code.
C++ was designed for all types of programming, from embedded systems in which there is no operating system in the execution environment to highly distributed, GUI-based end-user applications and everything in between. Therefore, C++ cannot place significant constraints on its execution environment. Building in support for multithreading would have inherently limited C++ to only those environments that supported it and thus prevented C++ from being used to create software for nonthreaded environments.
In the final analysis, not building in support of multithreading is a major advantage for C++ because it enables programs to be written in the most efficient way possible for the target execution environment. Remember, C++ is all about power. In the case of multithreading, it is definitely a situation in which less is more. What Operating System and Compiler?
Because C++ relies on the operating system to provide support for multithreaded programming, it is necessary to choose an operating system as the target for the multithreaded applications in this chapter. Because Windows is the most widely used operating system in the world, it is the operating system used in this chapter. However, much of the information can be generalized to any OS that supports multithreading.
Because Visual C++ is arguably the most widely used compiler for producing Windows programs, it is the compiler required by the examples in this chapter. The importance of this is made apparent in the following section. However, if you are using another compiler, the code can be easily adapted to accommodate it.
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