[eridius]

Apple can and does update APIs in ways that preserve old behavior for apps linked against older SDKs, specifically so old apps continue to work. They could do the same here with versioned symbols, if they wanted to switch mutexes to unfair-by-default. I believe they intentionally go with fair-by-default because it's the safe choice, the choice that guarantees program correctness, since most people don't think about this sort of thing and probably won't be able to figure out themselves when their locks should be fair or unfair. It's the same reason C11 and C++11 atomics default to sequential consistency when not otherwise specified; that's the slowest memory ordering, but it's the one that's guaranteed to be correct for all cases. If you know what you're doing you can override the default and pick something else.

[btschaegg]

> Apple can and does update APIs in ways that preserve old behavior for apps linked against older SDKs, specifically so old apps continue to work.

Fair point, although that still won't rule out applications that get updates (and thus still would have to be completely re-evaluated on the basis of such a case).

Also, wouldn't e.g. Homebrew also get those problems if you compile against the new SDK? (Non-Mac user here, so maybe I've got the wrong impressions on that...)

> I believe they intentionally go with fair-by-default because it's the safe choice [...]

Safety might be a big concern, but on the other hand, pthreads is a standard - if the article's right, and POSIX doesn't mandate fairness, you might still argue that this "addon" was better put into a custom solution than the other way around.

Then again, I've always suspected that strange implementations (or a total lack thereof) of POSIX must be one of the main reasons why Boost exists...

[eridius]

> Also, wouldn't e.g. Homebrew also get those problems if you compile against the new SDK?

Yes, but nearly all Homebrew software is cross-platform unix software, so that software will already have to deal with having unfair mutexes on Linux.

> … pthreads is a standard - if the article's right, and POSIX doesn't mandate fairness …

It doesn't mandate fairness, but it doesn't mandate unfairness either. Apple platforms defaulting to fair is no less conforming than Linux defaulting to unfair.

[btschaegg]

> Apple platforms defaulting to fair is no less conforming than Linux defaulting to unfair.

Of course, but it makes it less performant in many situations, while you can't count on that feature if you're coding against the standard (i.e. if for cross platform development; hence OP's blogpost). Which - in turn - explains why people are complaining about the performance, since they need to guarantee the fairness themselves anyway if they really need it.

On the other hand: If you're only coding against OSX, having the fairness in an extension shouldn't be a problem anyway.

Of course, this then facilitates other problems, if the standard is not evolving fast enough (the whole OpenGL extension mess would be a good example).

[eridius]

I don't think Apple makes decisions about their OS with the mindset of "what will someone who's writing a cross-platform CLI tool that happens to work on OS X expect?". Making mutexes unfair by default would make it behave more like other platforms that cross-platform tools expect, but at the cost of making software written specifically for OS X not be correct-by-default.

In general, when picking defaults and choosing between "correct" or "performant", the decision is usually made in favor of "correct", because incorrectness should always be an opt-in thing. I'm actually kind of surprised Linux and Windows default to unfair locks.

[btschaegg]

That's the point though, if you're Writing threading code specifically for Windows, you're not writing pthreads code. So all MS has to care about is that their custom threading abstraction is safe.

POSIX is more of a common denominator than a good choice in many usecases (you'll never get far if you're dealing with complicated file handling for example[1]), but it is an essential baseline for cross-platform development. If you're changing too much about it, you're burdening cross-platform devs to do system-specific implementations anyway, so there's not much point to it.

As a flipside, you'll also get devs who see a guarantee on one system, assume the standard guarantees it, and then make statements about their library that are untrue (i.e. "WTF::Lock is fair").

[1]: https://youtu.be/uhRWMGBjlO8

[eridius]

Just because you're using a POSIX API doesn't mean you're writing cross-platform code.