1 <?xml version="1.0" encoding="iso-8859-1"?>
2 <sect1 id="using-shared-libs">
3 <title>Using shared libraries</title>
4 <indexterm><primary>Shared libraries</primary><secondary>using</secondary></indexterm>
5 <indexterm><primary>Dynamic libraries</primary><secondary>using</secondary></indexterm>
8 On some platforms GHC supports building Haskell code into shared
9 libraries. Shared libraries are also sometimes known as dynamic
10 libraries, in particular on Windows they are referred to as dynamic link
15 Shared libraries allow a single instance of some pre-compiled code to be
16 shared between several programs. In contrast, with static linking the
17 code is copied into each program. Using shared libraries can thus save
18 disk space. They also allow a single copy of code to be shared in memory
19 between several programs that use it. Shared libraires are often used as
20 a way of structuring large projects, especially where different parts are
21 written in different programming languages. Shared libraries are also
22 commonly used as a plugin mechanism by various applications. This is
23 particularly common on Windows using COM.
27 In GHC version 6.12 building shared libraries is supported for Linux on
28 x86 and x86-64 architectures and there is partial support on Windows (see
29 <xref linkend="win32-dlls"/>). The crucial difference in support on
30 Windows is that it is not currently possible to build each Haskell
31 package as a separate DLL, it is only possible to link an entire Haskell
32 program as one massive DLL.
36 Building and using shared libraries is slightly more complicated than
37 building and using static libraries. When using Cabal much of the detail
38 is hidden, just use <literal>--enable-shared</literal> when configuring a
39 package to build it into a shared library, or to link it against other
40 packages built as shared libraries. The additional complexity when
41 building code is to distinguish whether the code will be used in a shared
42 library or will use shared library versions of other packages it depends
43 on. There is additional complexity when installing and distributing
44 shared libraries or programs that use shared libraries, to ensure that
45 all shared libraries that are required at runtime are present in suitable
50 <title>Building programs that use shared libraries</title>
52 To build a simple program and have it use shared libraries for the
53 runtime system and the base libraries use the
54 <literal>-dynamic</literal> flag:
56 ghc --make -dynamic Main.hs
58 This has two effects. The first is to compile the code in such a way
59 that it can be linked against shared library versions of Haskell
60 packages (such as base). The second is when linking, to link against
61 the shared versions of the packages' libraries rather than the static
62 versions. Obviously this requires that the packages were build with
63 shared libraries. On supported platforms GHC comes with shared
64 libraries for all the core packages, but if you install extra packages
65 (e.g. with Cabal) then they would also have to be built with shared
66 libraries (<literal>--enable-shared</literal> for Cabal).
71 <title>Building shared libraries</title>
73 To build some Haskell modules into a shared library use the
74 <literal>-dynamic</literal>, <literal>-fPIC</literal> and
75 <literal>-shared</literal> flags:
77 ghc --make -dynamic -shared -fPIC Foo.hs -o libfoo.so
79 As before, the <literal>-dynamic</literal> flag specifies that this
80 library links against the shared library versions of the rts and base
81 package. The <literal>-fPIC</literal> flag is required for all code
82 that will end up in a shared library. The <literal>-shared</literal>
83 flag specifies to make a shared library rather than a program. To make
84 this clearer we can break this down into separate compliation and link
87 ghc -dynamic -fPIC -c Foo.hs
88 ghc -dynamic -shared Foo.o -o libfoo.so
90 In principle you can use <literal>-shared</literal> without
91 <literal>-dynamic</literal> in the link step. That means to
92 statically link the rts all the base libraries into your new shared
93 library. This would make a very big, but standalone shared library.
94 Indeed this is exactly what we must currently do on Windows where
95 -dynamic is not yet supported (see <xref linkend="win32-dlls"/>).
96 On most platforms however that would require all the static libraries
97 to have been built with <literal>-fPIC</literal> so that the code is
98 suitable to include into a shared library and we do not do that at the
104 <title>Shared libraries that export a C API</title>
106 Building Haskell code into a shared library is a good way to include
107 Haskell code in a larger mixed-language project. While with static
108 linking it is recommended to use GHC to perform the final link step,
109 with shared libaries a Haskell library can be treated just like any
110 other shared libary. The linking can be done using the normal system C
114 It is possible to load shared libraries generated by GHC in other
115 programs not written in Haskell, so they are suitable for using as
116 plugins. Of course to construct a plugin you will have to use the FFI
117 to export C functions and follow the rules about initialising the RTS.
118 See <xref linkend="ffi-library"/>. In particular you will probably want
119 to export a C function from your shared library to initialise the
120 plugin before any Haskell functions are called.
125 <title>Shared libraries for Haskell packages</title>
127 When building Haskell packages as shared libraries to be used by other
128 Haskell programs there are certain conventions that must be followed.
129 These are handled by Cabal but for the details see <xref
130 linkend="building-packages"/>.
134 <sect2 id="finding-shared-libs">
135 <title>Finding shared libraries at runtime</title>
137 The primary difficulty with managing shared libraries is arranging
138 things such that programs can find the libraries they need at runtime.
139 The details of how this works varies between platforms, in particular
140 the three major systems: Unix ELF platforms, Windows and Mac OS X.
143 On Unix there are two mechanisms. Shared libraries can be installed
144 into standard locations that the dynamic linker knows about. For
145 example <literal>/usr/lib</literal> or
146 <literal>/usr/local/lib</literal> on most systems. The other mechanism
147 is to use a "runtime path" or "rpath" embedded into programs and
148 libraries themselves. These paths can either be absolute paths or on at
149 least Linux and Solaris they can be paths relative to the program or
150 libary itself. In principle this makes it possible to construct fully
151 relocatable sets of programs and libraries.
154 GHC has a <literal>-dynload</literal> linking flag to select the method
155 that is used to find shared libraries at runtime. There are currently
162 A system-dependent mode. This is also the default mode. On Unix
163 ELF systems this embeds rpaths into the shared library or
164 executable. In particular it uses absolute paths to where the
165 shared libraries for the rts and each package can be found.
166 This means the program can immediately be run and it will be
167 able to find the libraries it needs. However it may not be
168 suitable for deployment if the libraries are installed in a
169 different location on another machine.
177 This does not embed any runtime paths. It relies on the shared
178 libraries being available in a standard location or in a
179 directory given by the <literal>LD_LIBRARY_PATH</literal>
180 environment variable.
188 This mode generates a wrapper program which in turn calls the
189 real program (in the same directory but with a .dyn extension)
190 in such a way that it can find the shared libraries that it
191 needs. At the current time this mode is somewhat experimental.
196 To use relative paths for dependent libraries on Linux and Solaris you
197 can use the <literal>deploy</literal> mode and pass suitable a -rpath
200 ghc -dynamic Main.hs -o main -lfoo -L. -optl-Wl,-rpath,'$ORIGIN'
202 This assumes that the library <literal>libfoo.so</literal> is in the
203 current directory and will be able to be found in the same directory as
204 the executable <literal>main</literal> once the program is deployed.
205 Similarly it would be possible to use a subdirectory relative to the
206 executable e.g. <literal>-optl-Wl,-rpath,'$ORIGIN/lib'</literal>.