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On some platforms GHC supports building Haskell code into shared libraries. Shared libraries are also sometimes known as dynamic libraries, in particular on Windows they are referred to as dynamic link libraries (DLLs).
Shared libraries allow a single instance of some pre-compiled code to be shared between several programs. In contrast, with static linking the code is copied into each program. Using shared libraries can thus save disk space. They also allow a single copy of code to be shared in memory between several programs that use it. Shared libraries are often used as a way of structuring large projects, especially where different parts are written in different programming languages. Shared libraries are also commonly used as a plugin mechanism by various applications. This is particularly common on Windows using COM.
In GHC version 6.12 building shared libraries is supported for Linux (on x86 and x86-64 architectures). GHC version 7.0 adds support on Windows (see Building and using Win32 DLLs), FreeBSD and OpenBSD (x86 and x86-64), Solaris (x86) and Mac OS X (x86 and PowerPC).
Building and using shared libraries is slightly more complicated than building and using static libraries. When using Cabal much of the detail is hidden, just use --enable-shared when configuring a package to build it into a shared library, or to link it against other packages built as shared libraries. The additional complexity when building code is to distinguish whether the code will be used in a shared library or will use shared library versions of other packages it depends on. There is additional complexity when installing and distributing shared libraries or programs that use shared libraries, to ensure that all shared libraries that are required at runtime are present in suitable locations.
To build a simple program and have it use shared libraries for the runtime system and the base libraries use the -dynamic flag:
ghc --make -dynamic Main.hs
This has two effects. The first is to compile the code in such a way that it can be linked against shared library versions of Haskell packages (such as base). The second is when linking, to link against the shared versions of the packages’ libraries rather than the static versions. Obviously this requires that the packages were built with shared libraries. On supported platforms GHC comes with shared libraries for all the core packages, but if you install extra packages (e.g. with Cabal) then they would also have to be built with shared libraries (--enable-shared for Cabal).
You can build Haskell code into a shared library and make a package to be used by other Haskell programs. The easiest way is using Cabal, simply configure the Cabal package with the --enable-shared flag.
If you want to do the steps manually or are writing your own build system then there are certain conventions that must be followed. Building a shared library that exports Haskell code, to be used by other Haskell code is a bit more complicated than it is for one that exports a C API and will be used by C code. If you get it wrong you will usually end up with linker errors.
In particular Haskell shared libraries must be made into packages. You cannot freely assign which modules go in which shared libraries. The Haskell shared libraries must match the package boundaries. The reason for this is that GHC handles references to symbols within the same shared library (or main executable binary) differently from references to symbols between different shared libraries. GHC needs to know for each imported module if that module lives locally in the same shared lib or in a separate shared lib. The way it does this is by using packages. When using -dynamic, a module from a separate package is assumed to come from a separate shared lib, while modules from the same package (or the default “main” package) are assumed to be within the same shared lib (or main executable binary).
Most of the conventions GHC expects when using packages are described in Building a package from Haskell source. In addition note that GHC expects the .hi files to use the extension .dyn_hi. The other requirements are the same as for C libraries and are described below, in particular the use of the flags -dynamic, -fPIC and -shared.
Building Haskell code into a shared library is a good way to include Haskell code in a larger mixed-language project. While with static linking it is recommended to use GHC to perform the final link step, with shared libraries a Haskell library can be treated just like any other shared library. The linking can be done using the normal system C compiler or linker.
It is possible to load shared libraries generated by GHC in other programs not written in Haskell, so they are suitable for using as plugins. Of course to construct a plugin you will have to use the FFI to export C functions and follow the rules about initialising the RTS. See Making a Haskell library that can be called from foreign code. In particular you will probably want to export a C function from your shared library to initialise the plugin before any Haskell functions are called.
ghc --make -dynamic -shared -fPIC Foo.hs -o libfoo.so
As before, the -dynamic flag specifies that this library links against the shared library versions of the rts and base package. The -fPIC flag is required for all code that will end up in a shared library. The -shared flag specifies to make a shared library rather than a program. To make this clearer we can break this down into separate compilation and link steps:
ghc -dynamic -fPIC -c Foo.hs ghc -dynamic -shared Foo.o -o libfoo.so
In principle you can use -shared without -dynamic in the link step. That means to statically link the rts all the base libraries into your new shared library. This would make a very big, but standalone shared library. On most platforms however that would require all the static libraries to have been built with -fPIC so that the code is suitable to include into a shared library and we do not do that at the moment.
If your shared library exports a Haskell API then you cannot directly link it into another Haskell program and use that Haskell API. You will get linker errors. You must instead make it into a package as described in the section above.
The primary difficulty with managing shared libraries is arranging things such that programs can find the libraries they need at runtime. The details of how this works varies between platforms, in particular the three major systems: Unix ELF platforms, Windows and Mac OS X.
On Unix there are two mechanisms. Shared libraries can be installed into standard locations that the dynamic linker knows about. For example /usr/lib or /usr/local/lib on most systems. The other mechanism is to use a “runtime path” or “rpath” embedded into programs and libraries themselves. These paths can either be absolute paths or on at least Linux and Solaris they can be paths relative to the program or library itself. In principle this makes it possible to construct fully relocatable sets of programs and libraries.
GHC has a -dynload linking flag to select the method that is used to find shared libraries at runtime. There are currently two modes:
To use relative paths for dependent libraries on Linux and Solaris you can pass a suitable -rpath flag to the linker:
ghc -dynamic Main.hs -o main -lfoo -L. -optl-Wl,-rpath,'$ORIGIN'
This assumes that the library libfoo.so is in the current directory and will be able to be found in the same directory as the executable main once the program is deployed. Similarly it would be possible to use a subdirectory relative to the executable e.g. -optl-Wl,-rpath,'$ORIGIN/lib'.
This relative path technique can be used with either of the two -dynload modes, though it makes most sense with the deploy mode. The difference is that with the deploy mode, the above example will end up with an ELF RUNPATH of just $ORIGIN while with the sysdep mode the RUNPATH will be $ORIGIN followed by all the library directories of all the packages that the program depends on (e.g. base and rts packages etc.) which are typically absolute paths. The unix tool readelf --dynamic is handy for inspecting the RPATH/RUNPATH entries in ELF shared libraries and executables.
The standard assumption on Darwin/Mac OS X is that dynamic libraries will be stamped at build time with an “install name”, which is the full ultimate install path of the library file. Any libraries or executables that subsequently link against it (even if it hasn’t been installed yet) will pick up that path as their runtime search location for it. When compiling with ghc directly, the install name is set by default to the location where it is built. You can override this with the -dylib-install-name option (which passes -install_name to the Apple linker). Cabal does this for you. It automatically sets the install name for dynamic libraries to the absolute path of the ultimate install location.