Is Meyers' Singleton Pattern Thread-Safe in C 11 and Later?

Thread Safety of Meyers' Singleton Pattern Implementation

This implementation of Meyers' Singleton pattern using lazy initialization raises questions about its thread safety.

Meyers' Singleton Implementation

The provided code for the Singleton pattern is as follows:

static Singleton& instance()
{
     static Singleton s;
     return s;
}

Thread Safety Analysis

In C 11, this implementation is thread safe due to the guarantee in §6.7 [stmt.dcl] p4:

"If control enters the declaration concurrently while the variable is being initialized, the concurrent execution shall wait for completion of the initialization."

Compilers like GCC (since version 4.3) and Visual Studio (since 2015) support this feature.

C 03 Implementation

In C 03, the code was not thread safe. Meyers' article "C and the Perils of Double-Checked Locking" discusses such concerns. Full locking around the instantiating method was recommended to guarantee concurrency, while double-checked locking variants might encounter race conditions on specific architectures without memory barriers.

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