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Darwin/MacOSX Support - December 16, 2003
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=========================================
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GC_init() MUST be called before calling any other GC functions. This
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is necessary to properly register segments in dynamic libraries. This
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call is required even if you code does not use dynamic libraries as the
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dyld code handles registering all data segments.
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When your use of the garbage collector is confined to dylibs and you
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cannot call GC_init() before your libraries' static initializers have
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run and perhaps called GC_malloc(), create an initialization routine
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for each library to call GC_init():
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extern "C" void my_library_init() { GC_init(); }
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Compile this code into a my_library_init.o, and link it into your
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dylib. When you link the dylib, pass the -init argument with
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_my_library_init (e.g. gcc -dynamiclib -o my_library.dylib a.o b.o c.o
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my_library_init.o -init _my_library_init). This causes
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my_library_init() to be called before any static initializers, and
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will initialize the garbage collector properly.
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Note: It doesn't hurt to call GC_init() more than once, so it's best,
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if you have an application or set of libraries that all use the
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garbage collector, to create an initialization routine for each of
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them that calls GC_init(). Better safe than sorry.
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The incremental collector is still a bit flaky on darwin. It seems to
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work reliably with workarounds for a few possible bugs in place however
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these workaround may not work correctly in all cases. There may also
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be additional problems that I have not found.
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Thread-local GC allocation will not work with threads that are not
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created using the GC-provided override of pthread_create(). Threads
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created without the GC-provided pthread_create() do not have the
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necessary data structures in the GC to store this data.
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Implementation Information
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==========================
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Darwin/MacOSX support is nearly complete. Thread support is reliable on
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Darwin 6.x (MacOSX 10.2) and there have been reports of success on older
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Darwin versions (MacOSX 10.1). Shared library support had also been
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added and the gc can be run from a shared library. There is currently only
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support for Darwin/PPC although adding x86 support should be trivial.
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Thread support is implemented in terms of mach thread_suspend and
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thread_resume calls. These provide a very clean interface to thread
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suspension. This implementation doesn't rely on pthread_kill so the
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code works on Darwin < 6.0 (MacOSX 10.1). All the code to stop and
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start the world is located in darwin_stop_world.c.
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Since not all uses of the GC enable clients to override pthread_create()
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before threads have been created, the code for stopping the world has
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been rewritten to look for threads using Mach kernel calls. Each
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thread identified in this way is suspended and resumed as above. In
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addition, since Mach kernel threads do not contain pointers to their
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stacks, a stack-walking function has been written to find the stack
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limits. Given an initial stack pointer (for the current thread, a
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pointer to a stack-allocated local variable will do; for a non-active
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thread, we grab the value of register 1 (on PowerPC)), it
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will walk the PPC Mach-O-ABI compliant stack chain until it reaches the
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top of the stack. This appears to work correctly for GCC-compiled C,
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C++, Objective-C, and Objective-C++ code, as well as for Java
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programs that use JNI. If you run code that does not follow the stack
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layout or stack pointer conventions laid out in the PPC Mach-O ABI,
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then this will likely crash the garbage collector.
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The original incremental collector support unfortunatelly no longer works
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on recent Darwin versions. It also relied on some undocumented kernel
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structures. Mach, however, does have a very clean interface to exception
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handing. The current implementation uses Mach's exception handling.
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Much thanks goes to Andrew Stone, Dietmar Planitzer, Andrew Begel,
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Jeff Sturm, and Jesse Rosenstock for all their work on the
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Older Information (Most of this no longer applies to the current code)
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======================================================================
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While the GC should work on MacOS X Server, MacOS X and Darwin, I only tested
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I've added a PPC assembly version of GC_push_regs(), thus the setjmp() hack is
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no longer necessary. Incremental collection is supported via mprotect/signal.
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The current solution isn't really optimal because the signal handler must decode
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the faulting PPC machine instruction in order to find the correct heap address.
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Further, it must poke around in the register state which the kernel saved away
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in some obscure register state structure before it calls the signal handler -
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needless to say the layout of this structure is no where documented.
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Threads and dynamic libraries are not yet supported (adding dynamic library
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support via the low-level dyld API shouldn't be that hard).
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The original MacOS X port was brought to you by Andrew Stone.
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Note from Andrew Begel:
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One more fix to enable gc.a to link successfully into a shared library for
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MacOS X. You have to add -fno-common to the CFLAGS in the Makefile. MacOSX
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disallows common symbols in anything that eventually finds its way into a
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shared library. (I don't completely understand why, but -fno-common seems to
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work and doesn't mess up the garbage collector's functionality).
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Jeff Sturm and Jesse Rosenstock provided a patch that adds thread support.
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GC_MACOSX_THREADS should be defined in the build and in clients. Real
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dynamic library support is still missing, i.e. dynamic library data segments
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are still not scanned. Code that stores pointers to the garbage collected
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heap in statically allocated variables should not reside in a dynamic
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library. This still doesn't appear to be 100% reliable.
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Brian Alliet contributed dynamic library support for MacOSX. It could also
added
removed
doc/README.darwin
