usehaskellfromc

write Haskell code, call it from C code Goal:

• show how to link to C program
• compile C program with gcc instead of Haskell compiler
• show how to produce Python wheel package that includes Haskell shared object
• show how to wrap Haskell shared object via cgo

This is Linux only!

I couldn't find a project that demonstrated how to use cabal to wrap Haskell code into as shared object I could link into a C program, so I created this.

compile

After running cabal new-build, three files required for the next step are produced in the dist-newstyle subdirectory.

• test.o
• Foo_stub.h
• libFoo.so.0.0.1

This repo uses GHC to compile the C source to a .o file, but the next version will show you how to use gcc alone.

link

Once you've copied test.o, Foo_stub.h, and libFoo.so.0.0.1 files into some handy directory, you will need to tell gcc to link to the libHSrts-ghc$VERSION shared object, and libFoo.so.0.0.1

For me, that's libHSrts-ghc8.4.1.so so the command below includes -lHSrts-ghc8.4.1.

There's one complication, gcc looks for libFoo.so not libFoo.so.0.0.1 so I created a symlink via ln -s libFoo.so.0.0.1 libFoo.so

The command below then uses -L to tell gcc to look for required shared objects in both /opt/ghc/lib/ghc-8.4.1/rts and the current directory.

gcc -o test test.o -L. -L /opt/ghc/lib/ghc-8.4.1/rts -lHSrts-ghc8.4.1 -lffi -lFoo -Wl,-rpath,'$ORIGIN'

run

You should now have a test binary, but it probably won't immediately run.

If you run the ldd command on your binary, you will see that when the binary starts, it will search for several shared objects, including libFoo.so.0 and libHSrts-ghc8.4.1.so. In a default system, ldd test will show those two .so files as not found.

You will also notice that the dynamic loader wants to see a file called libFoo.so.0, I created another symbolic link in the same directory: ln -s libFoo.so.0.0.1 libFoo.so.0

You can temporarily add new search locations by modifying the LD_LIBRARY_PATH environment variable like so:

LD_LIBRARY_PATH=.:/opt/ghc/lib/ghc-8.4.1/rts ./test

And it works!

citations

I put together this demo from these links:

• https://qnikst.github.io/posts/2018-05-02-cabal-foreign-library.html
• https://www.vex.net/~trebla/haskell/so.xhtml#cabal
• https://cabal.readthedocs.io/en/latest/developing-packages.html#foreign-libraries
• https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/ffi-chap.html