Are IEnumerable Linq methods thread-safe?

The interface IEnumerable<T> is not thread safe. See the documentation on, which states: An enumerator remains valid as long as the collection remains unchanged. If changes are made to the collection, such as adding, modifying, or deleting elements, the enumerator is irrecoverably invalidated and its behavior is undefined. The enumerator does not have exclusive … Read more

Alignment requirements for atomic x86 instructions vs. MS’s InterlockedCompareExchange documentation?

x86 does not require alignment for a lock cmpxchg instruction to be atomic. However, alignment is necessary for good performance. This should be no surprise, backward compatibility means that software written with a manual from 14 years ago will still run on today’s processors. Modern CPUs even have a performance counter specifically for split-lock detection … Read more

Does it make any sense to use the LFENCE instruction on x86/x86_64 processors?

Bottom line (TL;DR): LFENCE alone indeed seems useless for memory ordering, however it does not make SFENCE a substitute for MFENCE. The “arithmetic” logic in the question is not applicable. Here is an excerpt from Intel’s Software Developers Manual, volume 3, section 8.2.2 (the edition 325384-052US of September 2014), the same that I used in … Read more

Which types on a 64-bit computer are naturally atomic in gnu C and gnu C++? — meaning they have atomic reads, and atomic writes

The answer from the point of view of the language standard is very simple: none of them are “definitively automatically” atomic. First of all, it’s important to distinguish between two senses of “atomic”. One is atomic with respect to signals. This ensures, for instance, that when you do x = 5 on a sig_atomic_t, then … Read more

How to declare a vector of atomic in C++

As described in this closely related question that was mentioned in the comments, std::atomic<T> isn’t copy-constructible, nor copy-assignable. Object types that don’t have these properties cannot be used as elements of std::vector. However, it should be possible to create a wrapper around the std::atomic<T> element that is copy-constructible and copy-assignable. It will have to use … Read more