> On Sep 14, 2016, at 3:00 PM, Craig A. Berry <craigberry_at_mac.com> wrote:
>
>
> On Sep 14, 2016, at 01:23 PM, "Peter White (jawhite2)" <jawhite2_at_cisco.com> wrote:
>
>> Hi there,
>>
>> On macOS Sierra (v10.12) and iOS 10, the function clock_gettime has been introduced in the Darwin kernel. When building libcurl on macOS 10.12 with Xcode 8 and iOS10 SDK, the configure script will return yes for “monotonic clock_gettime” (around Line 20434 of configure).
>>
>> However, when using libcurl in an application on an earlier version of iOS, this will cause a run time crash due to a missing symbol for clock_gettime.
>>
>> No crash occurs on an iPhone 6 running iOS10, but on an iPad running iOS9.3.5, a crash occurs due to a missing symbol for clock_gettime.
>>
>> OS: macOS Sierra (v10.12). Darwin kernel version 16.0.0
>> Libcurl version build: v7.49.1
>> Building libcurl for iOS 9.0 with Xcode 8 GM seed.
>>
>> Configure output with Xcode 8 (on macOS 10.12)
>> ==========
>> configure:20433: checking for monotonic clock_gettime
>> configure:20466: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang -c -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS10.0.sdk/ -miphoneos-version-min=9.0 -arch armv7 -mthumb -DNDEBUG -isystem /Volumes/local/JenkinsSlave/trunk/dependencies/openssl/include -Qunused-arguments -Os conftest.c >&5
>> configure:20466: $? = 0
>> configure:20468: result: yes
>> ==========
>
> Since you are specifying -miphoneos-version-min=9.0 it really shouldn't be able to see functions that don't exist on that version. Check time.h and see if Apple forgot to put a version check around the prototype for clock_gettime().
>
>>
>> Configure output with Xcode 7 (on OSX 10.11)
>> ==========
>> configure:20466: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang -c -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS9.0.sdk/ -miphoneos-version-min=9.0 -arch armv7 -mthumb -isystem /Volumes/local/JenkinsSlave/trunk/dependencies/openssl/include -Qunused-arguments -Os conftest.c >&5
>> conftest.c:72:15: warning: implicit declaration of function 'clock_gettime' is invalid in C99 [-Wimplicit-function-declaration]
>> (void)clock_gettime(CLOCK_MONOTONIC, &ts);
>> ^
>> conftest.c:72:29: error: use of undeclared identifier 'CLOCK_MONOTONIC'
>> (void)clock_gettime(CLOCK_MONOTONIC, &ts);
>> ^
>> 1 warning and 1 error generated.
>> configure:20466: $? = 1
>> =========
>>
>> I changed “curl_func_clock_gettime” to “no” in configure as a quick workaround and the libcurl binary didn’t cause any issues in either iOS9 or iOS10.
>>
>> Although not tested yet, I believe the same issue would occur when building libcurl on macOS 10.12 and using that binary on OSX 10.11 or lower.
>>
>> Best,
>> Peter
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This is actually fairly nasty (and does affect OSX as well as iOS)

Apple did in fact declare the function with the appropriate availability, however they did so in a weakly linked manner. This means that even if you specify -miphoneos-version-min=9.0 or -mmacosx-version-min=10.11, clock_gettime will still be declared at compile time. The only point that you can know if the function is available is by checking if clock_gettime == null at runtime.

See: https://opensource.apple.com/source/xnu/xnu-3247.1.106/EXTERNAL_HEADERS/Availability.h
And: http://clang.llvm.org/docs/AttributeReference.html#availability

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