Hi! I was trying to use the ghc on a system with OS X 10.6 to compile for both 10.6 and 10.5 (and hopefully 10.7). In the past (with ghc-7.2.2 IIRC) I had luck with if os(darwin) cc-options: -isysroot /Developer/SDKs/MacOSX10.5.sdk -mmacosx-version-min=10.5 ld-options: -isysroot /Developer/SDKs/MacOSX10.5.sdk -mmacosx-version-min=10.5 in the cabal file, but with ghc-7.4.1 (from Haskell-Platform-2012.2.0.0) I get linker errors: Undefined symbols: "_fopen$DARWIN_EXTSN", referenced from: _procRtsOpts in libHSrts.a(RtsFlags.o) _initProfiling2 in libHSrts.a(ProfHeap.o) _exitHpc in libHSrts.a(Hpc.o) _startupHpc in libHSrts.a(Hpc.o) "_fopen$UNIX2003", referenced from: _loadArchive in libHSrts.a(Linker.o) _addDLL in libHSrts.a(Linker.o) "_kevent64", referenced from: _s3VN_info in libHSbase-4.5.0.0.a(KQueue__166.o) ld: symbol(s) not found If I remove the backward-compatibility flags from the cabal file, everything works fine. (I assume it won't run on OS X 10.5 then, of course.) If I understand correctly the RTS and the base library in ghc-7.4.1 depend on a newer version of the OS X SDK (>= 10.6). Is there an easy way to work around this version constraint? If not, does that mean, ghc-7.4.1 does not support OS X 10.5? Thanks, Sönke