If I do xcrun --sdk macosx10.9 clang -o foo foo.c it builds for 10.9 (and gives me a compile error for using openat). If I do export SDKROOT=macosx10.9 then a plain compile: clang -o foo foo.c errors out with openat. The xcrun man page https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man1/xcrun.1.html says that the default SDK is the boot system. So this is just a bug in OSX, it seems. I guess the interesting questions are if others are seeing this and what to do about it.
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