The unix package states:

build-depends: base >=4.1 && <4.3

And while it does compile with base 4.1, using ghc-6.10, nothing can successfully link against it.

The symptoms are:

/usr/lib64/ghc-6.10.4/base-4.1.0.0/libHSbase-4.1.0.0.a(PrelIOUtils.o): In function `__hscore_d_name':
(.text+0x1c0): multiple definition of `__hscore_d_name'
/home/duncan/.cabal/lib/unix-2.4.0.0/ghc-6.10.4/libHSunix-2.4.0.0.a(dirUtils.o):dirUtils.c:(.text+0x0): first defined here
/usr/lib64/ghc-6.10.4/base-4.1.0.0/libHSbase-4.1.0.0.a(PrelIOUtils.o): In function `__hscore_free_dirent':
(.text+0x4f0): multiple definition of `__hscore_free_dirent'
/home/duncan/.cabal/lib/unix-2.4.0.0/ghc-6.10.4/libHSunix-2.4.0.0.a(dirUtils.o):dirUtils.c:(.text+0x10): first defined here
/home/duncan/.cabal/lib/unix-2.4.0.0/ghc-6.10.4/libHSunix-2.4.0.0.a(IO.o): In function `sk63_info':
(.text+0x30c4): undefined reference to `fcntl_read'
/home/duncan/.cabal/lib/unix-2.4.0.0/ghc-6.10.4/libHSunix-2.4.0.0.a(IO.o): In function `sk6A_info':
(.text+0x31f0): undefined reference to `fcntl_write'
/home/duncan/.cabal/lib/unix-2.4.0.0/ghc-6.10.4/libHSunix-2.4.0.0.a(IO.o): In function `sk8h_info':
(.text+0x36cc): undefined reference to `fcntl_read'
/home/duncan/.cabal/lib/unix-2.4.0.0/ghc-6.10.4/libHSunix-2.4.0.0.a(IO.o): In function `sl4O_info':
(.text+0x3a52): undefined reference to `fcntl_lock'
/home/duncan/.cabal/lib/unix-2.4.0.0/ghc-6.10.4/libHSunix-2.4.0.0.a(IO.o): In function `sl6S_info':
(.text+0x3cc2): undefined reference to `fcntl_lock'
/home/duncan/.cabal/lib/unix-2.4.0.0/ghc-6.10.4/libHSunix-2.4.0.0.a(IO.o): In function `sl9I_info':
(.text+0x402a): undefined reference to `fcntl_lock'

There are two problems here, the duplicate __hscore_* symbols and and the missing fcntl_* symbols.

The __hscore_d_name C function really is duplicated. There is one copy in base/include/HsBase.h in ghc-6.10, there is another copy in cbits/dirUtils.c in unix-2.4.0.0. Clearly what has happened is that the function from base has been moved into the unix package. However that means when building the unix package with the older base then we get both. The solution is to rename the one in the unix package. Ideally we could limit the visibility of symbols somehow.

The fcntl_read are not defined in the unix package. They are defined in HsBase.h in ghc-6.12 but not in 6.10. Hence they are also missing when unix-2.4.0.0 is built against the older base. The solution is to define them locally.

Originally filed as Cabal ticket: http://hackage.haskell.org/trac/hackage/ticket/620