ceph/src/boost/tools/build/src/engine/boehm_gc/doc/README.darwin

   1 Darwin/MacOSX Support - December 16, 2003
   2 =========================================
   3
   4 Important Usage Notes
   5 =====================
   6
   7 GC_init() MUST be called before calling any other GC functions. This
   8 is necessary to properly register segments in dynamic libraries. This
   9 call is required even if you code does not use dynamic libraries as the
  10 dyld code handles registering all data segments.
  11
  12 When your use of the garbage collector is confined to dylibs and you
  13 cannot call GC_init() before your libraries' static initializers have
  14 run and perhaps called GC_malloc(), create an initialization routine
  15 for each library to call GC_init():
  16
  17 #include <gc/gc.h>
  18 extern "C" void my_library_init() { GC_init(); }
  19
  20 Compile this code into a my_library_init.o, and link it into your
  21 dylib. When you link the dylib, pass the -init argument with
  22 _my_library_init (e.g. gcc -dynamiclib -o my_library.dylib a.o b.o c.o
  23 my_library_init.o -init _my_library_init). This causes
  24 my_library_init() to be called before any static initializers, and
  25 will initialize the garbage collector properly.
  26
  27 Note: It doesn't hurt to call GC_init() more than once, so it's best,
  28 if you have an application or set of libraries that all use the
  29 garbage collector, to create an initialization routine for each of
  30 them that calls GC_init(). Better safe than sorry.
  31
  32 The incremental collector is still a bit flaky on darwin. It seems to
  33 work reliably with workarounds for a few possible bugs in place however
  34 these workaround may not work correctly in all cases. There may also
  35 be additional problems that I have not found.
  36
  37 Thread-local GC allocation will not work with threads that are not
  38 created using the GC-provided override of pthread_create(). Threads
  39 created without the GC-provided pthread_create() do not have the
  40 necessary data structures in the GC to store this data.
  41
  42
  43 Implementation Information
  44 ==========================
  45 Darwin/MacOSX support is nearly complete. Thread support is reliable on
  46 Darwin 6.x (MacOSX 10.2) and there have been reports of success on older
  47 Darwin versions (MacOSX 10.1). Shared library support had also been
  48 added and the gc can be run from a shared library. There is currently only
  49 support for Darwin/PPC although adding x86 support should be trivial.
  50
  51 Thread support is implemented in terms of mach thread_suspend and
  52 thread_resume calls. These provide a very clean interface to thread
  53 suspension. This implementation doesn't rely on pthread_kill so the
  54 code works on Darwin < 6.0 (MacOSX 10.1). All the code to stop and
  55 start the world is located in darwin_stop_world.c.
  56
  57 Since not all uses of the GC enable clients to override pthread_create()
  58 before threads have been created, the code for stopping the world has
  59 been rewritten to look for threads using Mach kernel calls. Each
  60 thread identified in this way is suspended and resumed as above. In
  61 addition, since Mach kernel threads do not contain pointers to their
  62 stacks, a stack-walking function has been written to find the stack
  63 limits. Given an initial stack pointer (for the current thread, a
  64 pointer to a stack-allocated local variable will do; for a non-active
  65 thread, we grab the value of register 1 (on PowerPC)), it
  66 will walk the PPC Mach-O-ABI compliant stack chain until it reaches the
  67 top of the stack. This appears to work correctly for GCC-compiled C,
  68 C++, Objective-C, and Objective-C++ code, as well as for Java
  69 programs that use JNI. If you run code that does not follow the stack
  70 layout or stack pointer conventions laid out in the PPC Mach-O ABI,
  71 then this will likely crash the garbage collector.
  72
  73 The original incremental collector support unfortunatelly no longer works
  74 on recent Darwin versions. It also relied on some undocumented kernel
  75 structures. Mach, however, does have a very clean interface to exception
  76 handing. The current implementation uses Mach's exception handling.
  77
  78 Much thanks goes to Andrew Stone, Dietmar Planitzer, Andrew Begel,
  79 Jeff Sturm, and Jesse Rosenstock for all their work on the
  80 Darwin/OS X port.
  81
  82 -Brian Alliet
  83 brian@brianweb.net
  84
  85 gc_cpp.h usage
  86 ==============
  87
  88 Replacement of operator new and delete is apparently not supported with
  89 dynamic libraries.  This means that applications using gc_cpp.h
  90 (including the built-in test) will probably not work correctly with
  91 the collector in a dynamic library, unless special care is taken.
  92
  93 See
  94 http://article.gmane.org/gmane.comp.programming.garbage-collection.boehmgc/1421
  95 for some details.
  96
  97 - Hans Boehm (based on information from Andrew Begel)
  98
  99
 100 Older Information (Most of this no longer applies to the current code)
 101 ======================================================================
 102
 103 While the GC should work on MacOS X Server, MacOS X and Darwin, I only tested
 104 it on MacOS X Server.
 105 I've added a PPC assembly version of GC_push_regs(), thus the setjmp() hack is
 106 no longer necessary. Incremental collection is supported via mprotect/signal.
 107 The current solution isn't really optimal because the signal handler must decode
 108 the faulting PPC machine instruction in order to find the correct heap address.
 109 Further, it must poke around in the register state which the kernel saved away
 110 in some obscure register state structure before it calls the signal handler -
 111 needless to say the layout of this structure is no where documented.
 112 Threads and dynamic libraries are not yet supported (adding dynamic library
 113 support via the low-level dyld API shouldn't be that hard).
 114
 115 The original MacOS X port was brought to you by Andrew Stone.
 116
 117
 118 June, 1 2000
 119
 120 Dietmar Planitzer
 121 dave.pl@ping.at
 122
 123 Note from Andrew Begel:
 124
 125 One more fix to enable gc.a to link successfully into a shared library for
 126 MacOS X. You have to add -fno-common to the CFLAGS in the Makefile. MacOSX
 127 disallows common symbols in anything that eventually finds its way into a
 128 shared library. (I don't completely understand why, but -fno-common seems to
 129 work and doesn't mess up the garbage collector's functionality).
 130
 131 Feb 26, 2003
 132
 133 Jeff Sturm and Jesse Rosenstock provided a patch that adds thread support.
 134 GC_MACOSX_THREADS should be defined in the build and in clients.  Real
 135 dynamic library support is still missing, i.e. dynamic library data segments
 136 are still not scanned.  Code that stores pointers to the garbage collected
 137 heap in statically allocated variables should not reside in a dynamic
 138 library.  This still doesn't appear to be 100% reliable.
 139
 140 Mar 10, 2003
 141 Brian Alliet contributed dynamic library support for MacOSX.  It could also
 142 use more testing.