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I'm using stack to build projects and find that the generated executables are pretty large (10M) even for simple programs. In order to reduce the executable size, how can i get stack to build shared libraries and generate executables that dynamically link to shared libraries?
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Dynamic linking consists of compiling and linking code into a form that is loadable by programs at run time as well as link time. The ability to load them at run time is what distinguishes them from ordinary object files. Various operating systems have different names for such loadable code: UNIX: Sharable Libraries.
Dynamic libraries are linked during the execution of the final executable. Only the name of the dynamic library is placed in the final executable. The actual linking happens during runtime, when both executable and library are placed in the main memory.
With dynamic linking, external symbols referenced in user code and defined in a shared library are resolved by the loader at load time. When you compile a program that uses shared libraries, they are dynamically linked to your program by default.
In load-time dynamic linking executable is linked to the DLL Library while in Runtime dynamic linking no executable was linked o any DLL.
Add '-dynamic' to ghc-options in your .cabal file.
For example, in a project generated by stack new simple-example simple, the simple executable was 1.0M before adding '-dynamic' to ghc-options. It reduced to 12K after the change.
Here's the relevant part of the simple-example.cabal:
executable simple-example
hs-source-dirs: src
main-is: Main.hs
default-language: Haskell2010
build-depends: base >= 4.7 && < 5
ghc-options: -dynamic
Build with stack build (no options are required).
To show which libraries it's dynamically linked against, you can use the ldd tool (or otool -L on Mac).
$ otool -L .stack-work/install/x86_64-osx/lts-6.10/7.10.3/bin/simple-example
.stack-work/install/x86_64-osx/lts-6.10/7.10.3/bin/simple-example:
@rpath/libHSbase-4.8.2.0-HQfYBxpPvuw8OunzQu6JGM-ghc7.10.3.dylib (compatibility version 0.0.0, current version 0.0.0)
@rpath/libHSinteger-gmp-1.0.0.0-2aU3IZNMF9a7mQ0OzsZ0dS-ghc7.10.3.dylib (compatibility version 0.0.0, current version 0.0.0)
@rpath/libHSghc-prim-0.4.0.0-8TmvWUcS1U1IKHT0levwg3-ghc7.10.3.dylib (compatibility version 0.0.0, current version 0.0.0)
@rpath/libHSrts-ghc7.10.3.dylib (compatibility version 0.0.0, current version 0.0.0)
@rpath/libffi.dylib (compatibility version 7.0.0, current version 7.2.0)
/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 1226.10.1)
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