gcc/boehm-gc/doc/README.darwin

Darwin/MacOSX Support - December 16, 2003
=========================================

Important Usage Notes
=====================

GC_init() MUST be called before calling any other GC functions. This 
is necessary to properly register segments in dynamic libraries. This
call is required even if you code does not use dynamic libraries as the
dyld code handles registering all data segments.

When your use of the garbage collector is confined to dylibs and you
cannot call GC_init() before your libraries' static initializers have
run and perhaps called GC_malloc(), create an initialization routine
for each library to call GC_init():

#include <gc/gc.h>
extern "C" void my_library_init() { GC_init(); }

Compile this code into a my_library_init.o, and link it into your
dylib. When you link the dylib, pass the -init argument with
_my_library_init (e.g. gcc -dynamiclib -o my_library.dylib a.o b.o c.o
my_library_init.o -init _my_library_init). This causes
my_library_init() to be called before any static initializers, and
will initialize the garbage collector properly. 

Note: It doesn't hurt to call GC_init() more than once, so it's best,
if you have an application or set of libraries that all use the
garbage collector, to create an initialization routine for each of
them that calls GC_init(). Better safe than sorry. 

The incremental collector is still a bit flaky on darwin. It seems to 
work reliably with workarounds for a few possible bugs in place however
these workaround may not work correctly in all cases. There may also
be additional problems that I have not found. 

Thread-local GC allocation will not work with threads that are not
created using the GC-provided override of pthread_create(). Threads
created without the GC-provided pthread_create() do not have the
necessary data structures in the GC to store this data. 


Implementation Information
==========================
Darwin/MacOSX support is nearly complete. Thread support is reliable on 
Darwin 6.x (MacOSX 10.2) and there have been reports of success on older
Darwin versions (MacOSX 10.1). Shared library support had also been
added and the gc can be run from a shared library. There is currently only
support for Darwin/PPC although adding x86 support should be trivial.

Thread support is implemented in terms of mach thread_suspend and
thread_resume calls. These provide a very clean interface to thread
suspension. This implementation doesn't rely on pthread_kill so the
code works on Darwin < 6.0 (MacOSX 10.1). All the code to stop and
start the world is located in darwin_stop_world.c.

Since not all uses of the GC enable clients to override pthread_create()
before threads have been created, the code for stopping the world has
been rewritten to look for threads using Mach kernel calls. Each
thread identified in this way is suspended and resumed as above. In
addition, since Mach kernel threads do not contain pointers to their
stacks, a stack-walking function has been written to find the stack
limits. Given an initial stack pointer (for the current thread, a
pointer to a stack-allocated local variable will do; for a non-active
thread, we grab the value of register 1 (on PowerPC)), it
will walk the PPC Mach-O-ABI compliant stack chain until it reaches the
top of the stack. This appears to work correctly for GCC-compiled C,
C++, Objective-C, and Objective-C++ code, as well as for Java
programs that use JNI. If you run code that does not follow the stack
layout or stack pointer conventions laid out in the PPC Mach-O ABI,
then this will likely crash the garbage collector. 

The original incremental collector support unfortunatelly no longer works
on recent Darwin versions. It also relied on some undocumented kernel
structures. Mach, however, does have a very clean interface to exception
handing. The current implementation uses Mach's exception handling. 

Much thanks goes to Andrew Stone, Dietmar Planitzer, Andrew Begel, 
Jeff Sturm, and Jesse Rosenstock for all their work on the 
Darwin/OS X port.

-Brian Alliet
brian@brianweb.net


Older Information (Most of this no longer applies to the current code)
======================================================================

While the GC should work on MacOS X Server, MacOS X and Darwin, I only tested
it on MacOS X Server.
I've added a PPC assembly version of GC_push_regs(), thus the setjmp() hack is
no longer necessary. Incremental collection is supported via mprotect/signal.
The current solution isn't really optimal because the signal handler must decode
the faulting PPC machine instruction in order to find the correct heap address.
Further, it must poke around in the register state which the kernel saved away
in some obscure register state structure before it calls the signal handler -
needless to say the layout of this structure is no where documented.
Threads and dynamic libraries are not yet supported (adding dynamic library
support via the low-level dyld API shouldn't be that hard).

The original MacOS X port was brought to you by Andrew Stone.


June, 1 2000

Dietmar Planitzer
dave.pl@ping.at

Note from Andrew Begel:

One more fix to enable gc.a to link successfully into a shared library for
MacOS X. You have to add -fno-common to the CFLAGS in the Makefile. MacOSX
disallows common symbols in anything that eventually finds its way into a
shared library. (I don't completely understand why, but -fno-common seems to
work and doesn't mess up the garbage collector's functionality).

Feb 26, 2003

Jeff Sturm and Jesse Rosenstock provided a patch that adds thread support.
GC_MACOSX_THREADS should be defined in the build and in clients.  Real
dynamic library support is still missing, i.e. dynamic library data segments
are still not scanned.  Code that stores pointers to the garbage collected
heap in statically allocated variables should not reside in a dynamic
library.  This still doesn't appear to be 100% reliable.  

Mar 10, 2003
Brian Alliet contributed dynamic library support for MacOSX.  It could also
use more testing.