[tor-bugs] #20235 [Core Tor/Tor]: Configure misidentifies clock_gettime and getentropy on macOS Sierra/XCode8 when compiling for iOS 8

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#20235: Configure misidentifies clock_gettime and getentropy on macOS Sierra/XCode8
when compiling for iOS 8
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 Reporter:  rainwolf                             |          Owner:
     Type:  defect                               |         Status:  new
 Priority:  Medium                               |      Milestone:  Tor:
                                                 |  0.2.9.x-final
Component:  Core Tor/Tor                         |        Version:  Tor:
                                                 |  0.2.8.8
 Severity:  Normal                               |     Resolution:
 Keywords:  Sierra, XCode 8, clock_gettime,      |  Actual Points:
  getentropy, libSystem.dylib 028-backport osx   |
Parent ID:                                       |         Points:
 Reviewer:                                       |        Sponsor:
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Comment (by rainwolf):

 Replying to [comment:1 nickm]:

 They mention [https://github.com/Homebrew/homebrew-core/issues/3727 here]
 that the new linker flag `-Wl,-no_weak_imports` should fail to link if the
 link would be a leak link.  (This happens when you  use the Apple-supplied
 headers and they indicate the symbol was added  after your current
 deployment target.)

 This should cause the tests for `clock_gettime` and `getentropy` to fail
 correctly if you were to target an OS version that doesn't supply them.

 > (#20241 is related to this, but not the same.)
 >
 > Does Apple provide a general way to target a specific version of their
 APIs, other than downgrading your developer tools?

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/20235#comment:3>
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