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September 2002

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91 discussions

Dynamic linking again
by Duncan Coutts 01 Sep '02

01 Sep '02

So here's another idea. ghc's linker provides (via the ffi) a command addDLL to load dynamic libraries. It uses dlopen on unix and LoadLibrary on win32. Now, with dlopen if you pass it NULL rather than the filename of a library it will give you back a handle to itself - the program that called it. If you've linked your program with the right incantations this gives you access to all the symbols in the main program. So if this method works it would be the easiest way to get plugins working. No need for me to modify ghc (I think)! There do seem to be some problems with using -rdynamic with ghc however: http://www.haskell.org/pipermail/gtkhs/2002-March/000253.html As it is noted, it produces large executibles since all symbols get exported. I've not yet been able to find a way to limit the symbols that are exported. I believe that a similar thing is avaliable on windows since glib's GModule library claims to provide a portable interface for plugins on Linux/Sun HP-UX and Windows. http://developer.gnome.org/doc/API/glib/glib-dynamic-loading-of-modules.html The rightlinker incantations in this case are -rdynamic or perhaps -export-dynamic. So says the man page for ld, though there are some better explanations elsewhere: http://www.tldp.org/HOWTO/Program-Library-HOWTO/shared-libraries.html#AEN93 Specifically this bit: In some cases, the call to gcc to create the object file will also need to include the option ``-Wl,-export-dynamic''. Normally, the dynamic symbol table contains only symbols which are used by a dynamic object. This option (when creating an ELF file) adds all symbols to the dynamic symbol table (see ld(1) for more information). You need to use this option when there are 'reverse dependencies', i.e., a DL library has unresolved symbols that by convention must be defined in the programs that intend to load these libraries. For ``reverse dependencies'' to work, the master program must make its symbols dynamically available. Note that you could say ``-rdynamic'' instead of ``-Wl,export-dynamic'' if you only work with Linux systems, but according to the ELF documentation the ``-rdynamic'' flag doesn't always work for gcc on non-Linux systems. So can anyone poke holes in my grand plan or should I give it a go? Duncan

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