[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.

[btschaegg]

No, but nothing is stopping you from using another New&Better™ Apple API if you're not concerned by cross-platform compatibility. Having tons of vendor-specific code in your cross-platform software, on the other hand, is a pain and one of the main reason why standards like POSIX exist.

Conformance with a standard doesn't just happen on a whim - if you negate the benefits that come with having a standard in the first place, any criticism you'll get as the provider of said API you brought on yourself.

Edit: Also, I have to say: I really don't like the attitude of "there are other use cases, so the one you are referring to when voicing criticisms is invalid". This has never been a constructive use of anyone's time. As I said, there are very valid reasons for why the API works as it does, but that doesn't invalidate the argument that the basic decision might not have been the best one to begin with.