Are C 11 Thread-Local Variables Considered Static by Default?

Are Thread-Local Variables in C 11 Automatically Static?

In the context of thread safety and code organization, thread-local variables have gained prominence in C 11. These variables provide per-thread storage, ensuring that different threads operating on the same code have access to unique instances of the variable.

When defining a thread-local variable, it's essential to understand its implications and possible equivalence to static variables. This distinction can be particularly relevant when migrating existing code from a single-threaded to a multithreaded environment.

,Is There a Difference?

The C 11 Standard clarifies this question: "When thread_local is applied to a variable of block scope, the storage-class-specifier static is implied if it does not appear explicitly." This means that the following two code segments are equivalent:

<code class="cpp">void f() {
    thread_local vector<int> V;
    V.clear();
    ... // use V as a temporary variable
}</code>

<code class="cpp">void f() {
    static thread_local vector<int> V;
    V.clear();
    ... // use V as a temporary variable
}</code>

However, it's crucial to note that thread-local variables are distinct from static variables:

• static variables have static storage duration, meaning they exist for the entire lifetime of the program.
• thread_local variables have thread storage duration, meaning they exist only within the thread in which they are created.

This distinction is significant because thread-local variables ensure that each thread has its own instance of the variable, preventing conflicts between threads. Static variables, on the other hand, are shared among all threads.

Implications for Code Migration

When migrating code from a single-threaded to a multithreaded environment, it's essential to replace static variables with thread-local variables to avoid potential conflicts and ensure data consistency.

In the case of the code snippet provided, the original STATIC vector V can be safely replaced with thread_local vector V, as the code logic relies on a per-thread instance of the vector for intermediate values.

Summary

In C 11, thread_local variables are not automatically static, but the storage-class-specifier static is implied when thread_local is applied to a block-scoped variable. However, thread-local variables have a distinct thread storage duration, which is different from the static storage duration of static variables. When migrating code to a multithreaded environment, it's crucial to replace static variables with thread-local variables to ensure data consistency and avoid conflicts between threads.

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