Are Standard C and Boost Vectors Thread-Safe for Concurrent `push_back()` Operations?

Are Standard C Vectors and Boost Vectors Thread-Safe?

Question:

Can multiple threads simultaneously call the push_back() method on a shared std::vector object without compromising thread safety? Or, does the user need to implement additional synchronization mechanisms?

Answer:

Contrary to popular assumptions, both the standard C vector (std::vector) and the Boost vector (boost::vector) provide limited thread safety guarantees in accordance with the C standard.

Thread Safety Guarantees:

1. Concurrent Readers: Multiple threads can concurrently read the same container without the need for synchronization.
2. Exclusive Writing: When one thread is writing to the container, no other threads can modify or even read the container simultaneously.

These guarantees may not align with typical expectations for thread safety, but they are sensible considering the design of standard containers, which prioritizes efficient access in single-threaded scenarios. Incorporating locking mechanisms into their methods would hinder this efficiency.

External Locking:

To ensure full thread safety when multiple threads access containers concurrently, external synchronization mechanisms must be implemented. The specific requirements are outlined in section 17.6.4.10 [res.on.objects] paragraph 1 of the C standard.

Boost Vector Considerations:

The thread safety guarantees for the Boost vector are expected to be identical to those of the standard vector, given their similar interfaces. However, external locking is still necessary to guarantee safe concurrent access.

Conclusion:

While standard C and Boost vectors provide certain thread safety guarantees, they are limited. For complete thread safety in multithreaded scenarios, external synchronization mechanisms must be employed to prevent data races.

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