> On 8 Oct 2016, at 05:01, Quincey Morris <quinceymor...@rivergatesoftware.com>
> wrote:
>
> On Oct 7, 2016, at 07:49 , Gerriet M. Denkmann <g...@mdenkmann.de> wrote:
>>
>> Is there a better way than this:
>> dsema = dispatch_semaphore_create( 0 );
>>
>> some loop to be counted
>> {
>> dispatch_semaphore_signal(dsema);
>> ….
>> }
>>
>> NSUInteger counter = 0;
>> for(;;)
>> {
>> long a = dispatch_semaphore_wait(dsema, DISPATCH_TIME_NOW);
>> if ( a != 0 ) break;
>> counter++;
>> };
>
> Er, I didn’t pay enough attention to this when you posted. No, this isn’t the
> correct approach. You’re supposed to be using the semaphore as a lock. In
> theory, you’d do it like this:
>
>> dsema = dispatch_semaphore_create (1);
>>
>> NSUInteger counter = 0;
>>
>> for …
>> {
>> …
>> dispatch_semaphore_wait (dsema, <forever>);
>> counter++;
>> dispatch_semaphore_signal (dsema);
>> }
>
> That is, the semaphore controls access to a pool of 1 resources (the resource
> being permission to increment the counter), and you wait on the resource to
> become available (“lock”), increment the counter, then release the resource
> (“unlock”).
This looks much better.
But, alas, it is also much slower: overhead almost 40 sec (whereas my
admittedly rather hackish way took less then half a second).
B.t.w.: I also tried NSLock - it is even slower (but not much) than
@synchronized.
My preferred way to count (not deprecated and fast, but, as you said, probably
not available in Swift) is:
#import <stdatomic.h>
atomic_uint_fast64_t counter;
atomic_fetch_add_explicit( &counter, 1, memory_order_relaxed );
I have no idea, whether atomic_uint_fast64_t is correct (there also is
atomic_uint_least64_t — what is the difference ?)
and what memory_order_relaxed means (Xcode suggested this to me while warning
about deprecation of OSAtomicIncrement64).
I looked at <http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf> (as
kindly mentioned by Alastair).
In “7.17.6 Atomic integer types” it says: “The semantics of the operations on
these types are defined in 7.17.7”.
But 7.17.7 does not mention any difference between atomic_uint_fast64_t and
atomic_uint_least64_t.
Kind regards,
Gerriet.
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