OpenMP和C99数据(OpenMP and C99 data)

OpenMP和C99数据(OpenMP and C99 data)

OpenMP如何处理并行部分内声明的数据？ 在C99之前，我将使用private（）子句来指定线程局部数据，例如

int i, x; #pragma omp parallel for private(x) for (i=0; i<n; i++) { x=i; }

现在C99允许混合数据和代码，我更喜欢在使用它们之前声明变量。 在循环范围内声明数据是否保证它是线程私有的？ 例如，以下是否有效？

#pragma omp parallel for for (int i=0; i<n; i++) { int x=i; }

我尝试添加private（x）以防万一，但我的编译器对象（可能因为x尚未声明）。

How does OpenMP deal with data declared inside a parallel section? Before C99 I would use the private() clause to specify thread-local data, e.g.

int i, x; #pragma omp parallel for private(x) for (i=0; i<n; i++) { x=i; }

Now that C99 allows mixing of data and code, I prefer to declare variables just before using them. Does declaring data within the scope of the loop guarantee it is thread-private? For example, is the following valid?

#pragma omp parallel for for (int i=0; i<n; i++) { int x=i; }

I tried adding private(x) just in case, but my compiler objects (probably since x isn't declared yet).

最满意答案

某些变量（包括在并行构造中声明的变量）具有预定的数据共享属性（例如，您不能将它们声明为shared或private ）。 这些在OMP3标准的2.9.1.1节中定义。

在这种情况下， OpenMP Standard 3.0,2.9.1.1 :( p78，第12行）“在构造内的范围内声明的具有自动存储持续时间的变量是私有的。” 我很确定在OpenMP中一直都是这样。 所以是的，在你的C99例子中，i和x是私有的; 另一方面，我理解相同的部分，如果x被声明为静态，它将被共享。 我认为在这方面，它或多或少都符合你的期望。

Some variables, including those declared within a parallel construct, have data-sharing attributes that are predetermined (eg, you can't declare them shared or private). Those are defined in section 2.9.1.1 in the OMP3 standard.

In this case, OpenMP Standard 3.0, 2.9.1.1: (p78, line 12) "Variables with automatic storage duration that are declared in a scope inside the construct are private." I'm pretty sure it's always been this way in OpenMP. So yes, in your C99 example, i and x are private; on the other hand, I understand that same section to say that if x was declared static, it'd be shared. I think in this respect, it more or less does what you'd expect.