Hi Joe,
>> I just realised, that the problem is slightly worse than I originally
>> described. I believed that successful calls in between the actual call the
>> programmer wanted to make and capturing `errno` are not a problem.
>>
>> But POSIX seems to suggest [4] that "The setting of errno after a successful
>> call to a function is unspecified unless the description of that function
>> specifies that errno shall not be modified." . The Linux man page [5] also
>> mentions that "a function that succeeds is allowed to change errno."
>>
>> To me this means that the issue is wider than just ARC. I think the problem
>> extends to memory allocations on the heap. Failed memory allocations aren't
>> a problem because they are terminal in Swift. However, _successful_ memory
>> allocations might be a problem because the malloc(3) that the Swift compiler
>> will use is absolutely free to set errno to 0 (or any other value in fact)
>> indicating success. (Said that at least malloc doesn't change `errno` on the
>> macOS or Linux I tested today, we probably shouldn't rely on that though.)
>>
>> This makes it even more unpredictable to the programmer what a use of
>> `errno` in Swift will return. IMHO it shouldn't be exported to Swift as its
>> value is undefined almost(?) everywhere.
>>
>> [4]: http://pubs.opengroup.org/onlinepubs/9699919799/functions/errno.html
>> [5]: http://man7.org/linux/man-pages/man3/errno.3.html
>
> Swift's implicit allocations don't directly use malloc/free.
Ah right, that's good to know. But still, "explicit" allocation by creating an
object does, right?
Like
class Foo {}
let x = Foo()
does allocate through _swift_allocObject_ -> swift_slowAlloc -> malloc .
> If there is a platform where a successful malloc dirties errno, we should
> avoid doing so in the Swift runtime's entry points.
ok, agreed, should there be a platform where malloc that dirties `errno` it
could be worked around by saving and restoring the `errno`.
Cheers,
Johannes
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