在c99中使用__thread
我想使用 __thread 存储类定义一些特定于线程的变量。但三个问题让我犹豫：
- 它真的是c99中的标准吗？或者更重要的是，编译器支持有多好？
- 变量会在每个线程中初始化吗？
- 非多线程程序是否将它们视为普通的旧全局变量？
I would like to define a few variables as thread-specific using the __thread storage class. But three questions make me hesitate:
- Is it really standard in c99? Or more to the point, how good is the compiler support?
- Will the variables be initialised in every thread?
- Do non-multi threaded programs treat them as plain-old-globals?
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回答您的具体问题：
__thread变量在每个新线程中都以其初始化值开始。除了实现 C/POSIX 之外（例如 errno 等），在我看来，线程本地存储类实际上不是很有用。它几乎是避免以上下文指针或类似形式干净地传递必要状态的拐杖。您可能认为它对于解决像
qsort这样不采用上下文指针的损坏接口很有用，但不幸的是，不能保证qsort会调用比较函数在调用 qsort 的同一线程中。它可能会分解作业并在多个线程中运行它。对于大多数其他可以使用这种解决方法的接口也是如此。To answer your specific questions:
__threadvariables start out with their initialized value in every new thread.Outside of implementing C/POSIX (e.g.
errno, etc.), thread-local storage class is actually not very useful, in my opinion. It's pretty much a crutch for avoiding cleanly passing around the necessary state in the form of a context pointer or similar. You might think it could be useful for getting around broken interfaces likeqsortthat don't take a context pointer, but unfortunately there is no guarantee thatqsortwill call the comparison function in the same thread that calledqsort. It might break the job down and run it in multiple threads. Same goes for most other interfaces where this sort of workaround would be possible.您可能想阅读以下内容：
http://www.akkadia.org/drepper/tls.pdf< /a>
1) MSVC 不支持 C99。 GCC 确实和其他编译器尝试 GCC 兼容性。
编辑编译器对
__thread支持的详细信息可在此处找到：http://chtekk.longitekk.com/index.php?/archives/2011/02/C8.html
2) 仅 C++ 支持初始化程序并且它必须是恒定的。
3）非多线程应用程序是单线程应用程序。
You probably want to read this:
http://www.akkadia.org/drepper/tls.pdf
1) MSVC doesn't support C99. GCC does and other compilers attempt GCC compatibility.
edit A breakdown of compiler support for
__threadis available here:http://chtekk.longitekk.com/index.php?/archives/2011/02/C8.html
2) Only C++ supports an initializer and it must be constant.
3) Non-multi-threaded applications are single-threaded applications.