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2.7.1 threadprivate Directive

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The threadprivate directive makes the named file-scope, namespace-scope, or static block-scope variables specified in the variable-list private to a thread. variable-list is a comma-separated list of variables that do not have an incomplete type. The syntax of the threadprivate directive is as follows:

#pragma omp threadprivate(variable-list) new-line

Each copy of a threadprivate variable is initialized once, at an unspecified point in the program prior to the first reference to that copy, and in the usual manner (i.e., as the master copy would be initialized in a serial execution of the program). Note that if an object is referenced in an explicit initializer of a threadprivate variable, and the value of the object is modified prior to the first reference to a copy of the variable, then the behavior is unspecified.

As with any private variable, a thread must not reference another thread's copy of a threadprivate object. During serial regions and master regions of the program, references will be to the master thread's copy of the object.

After the first parallel region executes, the data in the threadprivate objects is guaranteed to persist only if the dynamic threads mechanism has been disabled and if the number of threads remains unchanged for all parallel regions.

The restrictions to the threadprivate directive are as follows:

  • A threadprivate directive for file-scope or namespace-scope variables must appear outside any definition or declaration, and must lexically precede all references to any of the variables in its list.

  • Each variable in the variable-list of a threadprivate directive at file or namespace scope must refer to a variable declaration at file or namespace scope that lexically precedes the directive.

  • A threadprivate directive for static block-scope variables must appear in the scope of the variable and not in a nested scope. The directive must lexically precede all references to any of the variables in its list.

  • Each variable in the variable-list of a threadprivate directive in block scope must refer to a variable declaration in the same scope that lexically precedes the directive. The variable declaration must use the static storage-class specifier.

  • If a variable is specified in a threadprivate directive in one translation unit, it must be specified in a threadprivate directive in every translation unit in which it is declared.

  • A threadprivate variable must not appear in any clause except the copyin, copyprivate, schedule, num_threads, or the if clause.

  • The address of a threadprivate variable is not an address constant.

  • A threadprivate variable must not have an incomplete type or a reference type.

  • A threadprivate variable with non-POD class type must have an accessible, unambiguous copy constructor if it is declared with an explicit initializer.

The following example illustrates how modifying a variable that appears in an initializer can cause unspecified behavior, and also how to avoid this problem by using an auxiliary object and a copy-constructor.

int x = 1;
T a(x);
const T b_aux(x); /* Capture value of x = 1 */
T b(b_aux);
#pragma omp threadprivate(a, b)

void f(int n) {
   x++;
   #pragma omp parallel for
   /* In each thread:
   * Object a is constructed from x (with value 1 or 2?)
   * Object b is copy-constructed from b_aux
   */
   for (int i=0; i<n; i++) {
      g(a, b); /* Value of a is unspecified. */
   }
}

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