If ANSI C++ doesn't support multithreading, how can unmanaged C++ apps be multithreaded?

Question

I have heard that C++ offers no native support for multithreading. I assume that multithreaded C++ apps depended on managed code for multithreading; that is, for example, a Visual C++ app used MFC or .NET or something along those lines to provide multithreading capability. I further assume that some or all of those managed-code capabilities are unavailable to unmanaged applications. But I have read about unmanaged multithreaded applications. How is this possible? Which of my assumptions is false?

Answer 1

It is wholly up to the operating system to provide support for multi-threading. On Windows, the necessary functionality is available via the Win32 API. Frameworks such as MFC provide wrappers over the low-level threading functions to simplify things, while of course .NET/CLR has its own managed interface for accessing Win32 multi-threading capabilities.

A good explanation is offerred in this article (Multithreading in C++).

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.

Answer 2

Some of your assumptions are not quite right. The operating system (I'm talking about win32 since you mention .NET) provides support for threading. There are lots of good threading libs. that build ontop of the OS functionality in C++ to make multithreading "easier" :) -- pthreads for example. Here is more at MSDN.

Answer 3

Multithreading in C++ does not require managed code.

In very much the same way that C++ does not provide native support for displaying graphics or emitting sounds or reading input from a mouse, the operating system that's being used will provide a C++ API for utilizing these features.

It's not a matter of C++ not being able to do it. It simply hasn't been written into the C++ standard yet.

Answer 4

The ISO standard for the programming language C++ neither defines nor prohibits multithreading. An implementation is allowed to provide extensions if it wishes. A program is allowed to use implementation extensions if it wishes, and then the program will only run on systems that provide those extensions.

For comparison, the ISO standard for the programming language C++ neither defines nor prohibits the use of a mouse. A program is allowed to use implementation extensions and then it will only run on systems that provide those extensions. For another comparison, the ISO standard for C++ neither defines nor prohibits UTF-8, so your program can depend on Latin-1 and then your program will only run on systems that provide Latin-1.

Answer 5

Unlike Java, which defined language constructs and JVM specs, the C++ standard is oblivious to threading (so is C). As far as these languages are concerned, anything thread-related consists of function calls to OS functionality. Libraries compiled for multithreading simply make sense of the same calls, but from a language perspectives they are plain old code.

Answer 6

Native C++ does not offer "built in" multithreading support simply because it was not intended to, or in fact, needed. Your misconception is that this is a fault, while it is in fact a strength of the language. By being "oblivious" to multithreading, C++ seamlessly integrates with the MT support offered by the OS your code will compile and run on, thereby offering much more flexibility and efficiency than if it came with it's own "MT baggage" so to speak. You mention MFC and .NET as examples - be aware that these libraries/wrappers are merely a layer over basic Win32 API's. Using C++ as intended will provide you with efficient code that will run multithrededly on ANY OS, as long as you seperate the logic from the OS-specific MT API calles (i.e thread creation etc), so that porting between OS's is greatly facilitated.

Answer 7

I think you misunderstand the definition of 'managed' code. 'Managed' code is a Microsoft-specific term meaning code that is uses the .NET framework and thus is subject to the various aspects of .NET. 'Unmanaged' code means code that runs outside that and does not operate through the .NET layer. MFC code is 'unmanaged'; it's merely a spectacularly bad wrapper for the ubiquitous Win32 API (which isn't even the lowest level API available on Windows).

The .NET libraries (including multithreading) are almost all, at some level, interfaces for the more basic system APIs used by traditional, 'unmanaged' applications. There is, generally speaking, no functionality available to 'managed' code that cannot be replicated in 'unmanaged' code with sufficient effort, though the reverse is not true (this is called the abstraction penalty, if you wish to know more). While it may be easier to do in 'managed' code, that's just because somewhere, some 'unmanaged' code is doing it for you, more or less. In the case of a threading API, it is in turn an interface to the operating system kernel, which itself accesses the processor's capabilities to allow a process to run in multiple places concurrently (if using multiple cores; if not, then it's just a pseudo-concurrency).

The C++ standard currently provides no definition of threads (the upcoming C++1x standard does). There are a number different threading libraries available, including those provided by Win32 and MFC, the pthreads library found on POSIX systems, and Boost.Thread, which will use the platform's local threading library.

Answer 8

The next C++ standard (named c++0x) will have support for multithreading. Will include atomic operations.