why is it thread-unsafe?

[kenxausten]

From the code, I can't find it is thread-unsafe, why is it thead-unsafe?

[zeromus]

There is no portable way to achieve thread-safety. Add thread safety yourself like you would to any other thread unsafe-thing, it should take about five minutes to make wrappers with locks.

[lfnoise]

Besides, but related to the portability issue, there’s no simple way to achieve thread safety and also preserve the real-time behavior of this allocator.

[tigerccx]

Has anyone benchmarked its performance after made thread safe? If so, how it is compared to 'malloc' and 'free'?

[lfnoise]

My opinion: If you use locks to achieve thread safety, then I'd say just use another allocator. The appeal of this allocator is that it is real time. Add locks and then it no longer is real time, so why prefer it? There are other (non-real-time) allocators have been optimized for multi-threaded performance.