Thinking In C++. Volume 2: Practical Programming

A conventional function cannot continue performing its operations and at the same time return control to the rest of the program. In fact, this sounds like an impossibility, as if the CPU must be in two places at once, but this is precisely the illusion that concurrency provides.

You can also use concurrency to optimize throughput. For example, you might be able to do important work while you’re stuck waiting for input to arrive on an I/O port. Without threading, the only reasonable solution is to poll the I/O port, which is awkward and can be difficult.

If you have a multiprocessor machine, multiple threads can be distributed across multiple processors, which can dramatically improve throughput. This is often the case with powerful multiprocessor web servers, which can distribute large numbers of user requests across CPUs in a program that allocates one thread per request.

One thing to keep in mind is that a program that has many threads must be able to run on a single-CPU machine. Therefore, it must also be possible to write the same program without using any threads. However, multithreading provides an important organizational benefit: The design of your program can be greatly simplified. Some types of problems, such as simulation—a video game, for example—are difficult to solve without support for concurrency.

The threading model is a programming convenience to simplify juggling several operations at the same time within a single program: The CPU will pop around and give each thread some of its time. Each thread has the consciousness of constantly having the CPU to itself, but the CPU’s time is actually sliced among all the threads. The exception is a program that is running on multiple CPU. But one of the great things about threading is that you are abstracted away from this layer, so your code does not need to know whether it is actually running on a single CPU or many. Thus, using threads is a way to create transparently scalable programs—if a program is running too slowly, you can easily speed it up by adding CPUs to your computer. Multitasking and multithreading tend to be the most reasonable ways to utilize multiprocessor systems.

Threading can reduce computing efficiency somewhat in single-CPU machines, but the net improvement in program design, resource balancing, and user convenience is often quite valuable. In general, threads enable you to create a more loosely coupled design; otherwise, parts of your code would be forced to pay explicit attention to tasks that would normally be handled by threads.

Concurrency in C++

When the C++ standards committee was creating the initial C++ standard, a concurrency mechanism was explicitly excluded, because C didn’t have one and also because there were a number of competing approaches to implementing concurrency. It seemed too much of a constraint to force programmers to use only one of these.

The alternative turned out to be worse, however. To use concurrency, you had to find and learn a library and deal with its idiosyncrasies and the uncertainties of working with a particular vendor. In addition, there was no guarantee that such a library would work on different compilers or across different platforms. Also, since concurrency was not part of the standard language, it was more difficult to find C++ programmers that also understood concurrent programming.

Another influence may have been the Java language, which included concurrency in the core language. Although multithreading is still complicated, Java programmers tend to start learning and using it from the beginning.

The C++ standards committee is seriously considering the addition of concurrency support to the next iteration of C++, but at the time of this writing it is unclear what the library will look like. Therefore, we decided to use the ZThread library as the basis for this chapter. We preferred the design, and it is open-source and freely available at http://zthread.sourceforge.net. Eric Crahen of IBM, the author of the ZThread library, was instrumental in creating this chapter.[125]

This chapter uses only a subset of the ZThread library, in order to convey the fundamental ideas of threading. The ZThread library contains significantly more sophisticated thread support than is shown here, and you should study that library further in order to fully understand its capabilities.

Installing ZThreads

Please note that the ZThread library is an independent project and is not supported by the authors of this book; we are simply using the library in this chapter and cannot provide technical support for installation issues. See the ZThread web site for installation support and error reports.

The ZThread library is distributed as source code. After downloading it from the ZThread web site, you must first compile the library, and then configure your project to use the library.