I'm trying to diagnose strange GHC linking behavior which, as far as I can tell, only occurs on macOS. I feel a bit out of my league debugging this, so I'm hoping someone knows what is causing this.
When building the Haskell libffi library [1], it will link pass -lffi to GHC. Typically, most installations of libffi will include both a static library (libffi.a) and a shared library (libffi.{so,dylib,dll}). My impression is that when the linker finds both, it will default to the shared library. This is definitely the case on Linux and Windows, at least.
An exception to this rule is macOS, however. On macOS, building libffi always appears to default to linking against the static version of libffi, even when a dynamic version is also available. To reproduce this phenomenon, check out libffi [1] and run the following commands:
$ brew install libffi # If it is not already installed
$ cabal build ctime
$ otool -L $(cabal list-bin ctime)
The output reveals that the compiled executable does not dynamically link against libffi.dylib:
$ otool -L $(cabal list-bin ctime)
/Users/rscott/Documents/Hacking/Haskell/libffi/dist-newstyle/build/x86_64-osx/ghc-8.10.7/libffi-examples-0.1/x/ctime/build/ctime/ctime:
/usr/lib/libiconv.2.dylib (compatibility version 7.0.0, current version 7.0.0)
/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1281.100.1)
/usr/lib/libcharset.1.dylib (compatibility version 2.0.0, current version 2.0.0)
This is exceedingly strange, since my Hombrew installation does in fact provide libffi.dylib:
$ ls -alh ~/Software/homebrew/Cellar/libffi/3.4.2/lib/
total 232
drwxr-xr-x 6 rscott 1340850540 192B Aug 7 09:38 .
drwxr-xr-x 11 rscott 1340850540 352B Aug 7 08:51 ..
-rw-r--r-- 1 rscott 1340850540 70K Aug 7 08:51 libffi.8.dylib
-r--r--r-- 1 rscott 1340850540 41K Jun 28 2021 libffi.a
lrwxr-xr-x 1 rscott 1340850540 14B Jun 28 2021 libffi.dylib -> libffi.8.dylib
drwxr-xr-x 3 rscott 1340850540 96B Aug 7 08:51 pkgconfig
What's more, this only seems to happen when building the libffi library in particular. If I try building another library that has a C extra-libraries dependency, such as HsOpenSSL, then I can find its dynamic dependencies (libssl.dylib and libcrypto.dylib) using otool:
$ otool -L dist-newstyle/build/x86_64-osx/ghc-8.10.7/HsOpenSSL-0.11.7.2/build/test-dsa/test-dsa
dist-newstyle/build/x86_64-osx/ghc-8.10.7/HsOpenSSL-0.11.7.2/build/test-dsa/test-dsa:
/usr/local/opt/openssl@1.1/lib/libssl.1.1.dylib (compatibility version 1.1.0, current version 1.1.0)
/usr/local/opt/openssl@1.1/lib/libcrypto.1.1.dylib (compatibility version 1.1.0, current version 1.1.0)
/usr/lib/libiconv.2.dylib (compatibility version 7.0.0, current version 7.0.0)
/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1281.100.1)
/usr/lib/libcharset.1.dylib (compatibility version 2.0.0, current version 2.0.0)
I'm at a bit of a loss trying to figure out why this only happens for libffi, and only on macOS. To make things even stranger, this only seems to happen when using GHC. If I try to compile a simple libffi C program using -lffi, for instance, then otool finds libffi.dylib. Is there something about GHC's linking behavior that would cause this?
Ryan S.
-----