The collector has at various times been compiled under Windows 95 & NT,
with the original Microsoft SDK, with Visual C++ 2.0, 4.0, and 6, with
the GNU win32 environment, with Borland 4.5, with Watcom C, and recently
with the Digital Mars compiler. It is likely that some of these have been
broken in the meantime. Patches are appreciated.
It runs under both win32s and win32, but with different semantics.
Under win32, all writable pages outside of the heaps and stack are
scanned for roots. Thus the collector sees pointers in DLL data
segments. Under win32s, only the main data segment is scanned.
(The main data segment should always be scanned. Under some
versions of win32s, other regions may also be scanned.)
Thus all accessible objects should be accessible from local variables
or variables in the main data segment. Alternatively, other data
segments (e.g. in DLLs) may be registered with the collector by
calling GC_init() and then GC_register_root_section(a), where
a is the address of some variable inside the data segment. (Duplicate
registrations are ignored, but not terribly quickly.)
(There are two reasons for this. We didn't want to see many 16:16
pointers. And the VirtualQuery call has different semantics under
the two systems, and under different versions of win32s.)
Win32 applications compiled with some flavor of gcc currently behave
like win32s applications, in that dynamic library data segments are
not scanned. (Gcc does not directly support Microsoft's "structured
exception handling". It turns out that use of this feature is
unavoidable if you scan arbitrary memory segments obtained from
The collector test program "gctest" is linked as a GUI application,
but does not open any windows. Its output appears in the file
"gc.log". It may be started from the file manager. The hour glass
cursor may appear as long as it's running. If it is started from the
command line, it will usually run in the background. Wait a few
minutes (a few seconds on a modern machine) before you check the output.
You should see either a failure indication or a "Collector appears to
The cord test program has not been ported (but should port
easily). A toy editor (cord/de.exe) based on cords (heavyweight
strings represented as trees) has been ported and is included.
It runs fine under either win32 or win32S. It serves as an example
of a true Windows application, except that it was written by a
nonexpert Windows programmer. (There are some peculiarities
in the way files are displayed. The <cr> is displayed explicitly
for standard DOS text files. As in the UNIX version, control
characters are displayed explicitly, but in this case as red text.
This may be suboptimal for some tastes and/or sets of default
In general -DREDIRECT_MALLOC is unlikely to work unless the
application is completely statically linked.
The collector normally allocates memory from the OS with VirtualAlloc.
This appears to cause problems under Windows NT and Windows 2000 (but
not Windows 95/98) if the memory is later passed to CreateDIBitmap.
To work around this problem, build the collector with -DUSE_GLOBAL_ALLOC.
This is currently incompatible with -DUSE_MUNMAP. (Thanks to Jonathan
Clark for tracking this down. There's some chance this may be fixed
in 6.1alpha4, since we now separate heap sections with an unused page.)
For Microsoft development tools, rename NT_MAKEFILE as
MAKEFILE. (Make sure that the CPU environment variable is defined
to be i386.) In order to use the gc_cpp.h C++ interface, all
client code should include gc_cpp.h.
If you would prefer a VC++.NET project file, ask firstname.lastname@example.org. One has
been contributed, but it seems to contain some absolute paths etc., so
it can presumably only be a starting point, and is not in the standard
distribution. It is unclear (to me, Hans Boehm) whether it is feasible to
Clients may need to define GC_NOT_DLL before including gc.h, if the
collector was built as a static library (as it normally is in the
absence of thread support).
For GNU-win32, use the regular makefile, possibly after uncommenting
the line "include Makefile.DLLs". The latter should be necessary only
if you want to package the collector as a DLL. The GNU-win32 port is
believed to work only for b18, not b19, probably due to linker changes
in b19. This is probably fixable with a different definition of
DATASTART and DATAEND in gcconfig.h.
For Borland tools, use BCC_MAKEFILE. Note that
Borland's compiler defaults to 1 byte alignment in structures (-a1),
whereas Visual C++ appears to default to 8 byte alignment (/Zp8).
The garbage collector in its default configuration EXPECTS AT
LEAST 4 BYTE ALIGNMENT. Thus the BORLAND DEFAULT MUST
BE OVERRIDDEN. (In my opinion, it should usually be anyway.
I expect that -a1 introduces major performance penalties on a
486 or Pentium.) Note that this changes structure layouts. (As a last
resort, gcconfig.h can be changed to allow 1 byte alignment. But
this has significant negative performance implications.)
The Makefile is set up to assume Borland 4.5. If you have another
version, change the line near the top. By default, it does not
require the assembler. If you do have the assembler, I recommend
removing the -DUSE_GENERIC.
There is some support for incremental collection. This is
currently pretty simple-minded. Pages are protected. Protection
faults are caught by a handler installed at the bottom of the handler
stack. This is both slow and interacts poorly with a debugger.
Whenever possible, I recommend adding a call to
GC_enable_incremental at the last possible moment, after most
debugging is complete. Unlike the UNIX versions, no system
calls are wrapped by the collector itself. It may be necessary
to wrap ReadFile calls that use a buffer in the heap, so that the
call does not encounter a protection fault while it's running.
(As usual, none of this is an issue unless GC_enable_incremental
Note that incremental collection is disabled with -DSMALL_CONFIG.
James Clark has contributed the necessary code to support win32 threads.
Use NT_THREADS_MAKEFILE (a.k.a gc.mak) instead of NT_MAKEFILE
to build this version. Note that this requires some files whose names
are more than 8 + 3 characters long. Thus you should unpack the tar file
so that long file names are preserved. To build the garbage collector
test with VC++ from the command line, use
nmake /F ".\gc.mak" CFG="gctest - Win32 Release"
This requires that the subdirectory gctest\Release exist.
The test program and DLL will reside in the Release directory.
This version relies on the collector residing in a dll.
This version currently supports incremental collection only if it is
enabled before any additional threads are created.
Version 4.13 attempts to fix some of the earlier problems, but there
may be other issues. If you need solid support for win32 threads, you
might check with Geodesic Systems. Their collector must be licensed,
but they have invested far more time in win32-specific issues.
Ivan V. Demakov's README for the Watcom port:
The collector has been compiled with Watcom C 10.6 and 11.0.
It runs under win32, win32s, and even under msdos with dos4gw
dos-extender. It should also run under OS/2, though this isn't
tested. Under win32 the collector can be built either as dll
or as static library.
Note that all compilations were done under Windows 95 or NT.
For unknown reason compiling under Windows 3.11 for NT (one
attempt has been made) leads to broken executables.
Incremental collection is not supported.
cord is not ported.
Before compiling you may need to edit WCC_MAKEFILE to set target
platform, library type (dynamic or static), calling conventions, and
To compile the collector and testing programs use the command:
wmake -f WCC_MAKEFILE
All programs using gc should be compiled with 4-byte alignment.
For further explanations on this see comments about Borland.
If the gc is compiled as dll, the macro ``GC_DLL'' should be defined before
including "gc.h" (for example, with -DGC_DLL compiler option). It's
important, otherwise resulting programs will not run.
Ivan Demakov (email: email@example.com)