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The collector uses a large amount of conditional compilation in order to
deal with platform dependencies. This violates a number of known coding
standards. On the other hand, it seems to be the only practical way to
support this many platforms without excessive code duplication.
A few guidelines have mostly been followed in order to keep this manageable:
1) #if and #ifdef directives are properly indented whenever easily possible.
All known C compilers allow whitespace between the "#" and the "if" to make
this possible. ANSI C also allows white space before the "#", though we
avoid that. It has the known disadvantages that it differs from the normal
GNU conventions, and that it makes patches larger than otherwise necessary.
In my opinion, it's still well worth it, for the same reason that we indent
ordinary "if" statements.
2) Whenever possible, tests are performed on the macros defined in gcconfig.h
instead of directly testing patform-specific predefined macros. This makes it
relatively easy to adapt to new compilers with a different set of predefined
macros. Currently these macros generally identify platforms instead of
features. In many cases, this is a mistake.
3) The code currently avoids #elif, eventhough that would make it more
readable. This was done since #elif would need to be understood by ALL
compilers used to build the collector, and that hasn't always been the case.
It makes sense to reconsider this decision at some point, since #elif has been
standardized at least since 1989.
Many of the tested configuration macros are at least somewhat defined in
either include/private/gcconfig.h or in Makefile.direct. Here is an attempt
at defining some of the remainder: (Thanks to Walter Bright for suggesting
this. This is a work in progress)
MACRO EXPLANATION
----- -----------
__DMC__ Always #define'd by the Digital Mars compiler. Expands
to the compiler version number in hex, i.e. 0x810 is
version 8.1b0
_ENABLE_ARRAYNEW
#define'd by the Digital Mars C++ compiler when
operator new[] and delete[] are separately
overloadable. Used in gc_cpp.h.
_MSC_VER Expands to the Visual C++ compiler version. Assumed to
not be defined for other compilers (at least if they behave
appreciably differently).
_DLL Defined by Visual C++ if dynamic libraries are being built
or used. Used to test whether __declspec(dllimport) or
__declspec(dllexport) needs to be added to declarations
to support the case in which the collector is in a dll.
GC_DLL User-settable macro that forces the effect of _DLL. Set
by gc.h if _DLL is defined and GC_NOT_DLL is undefined.
This is the macro that is tested internally to determine
whether the GC is in its own dynamic library. May need
to be set by clients before including gc.h. Note that
inside the GC implementation it indicates that the
collector is in its own dynamic library, should export
its symbols, etc. But in clients it indicates that the
GC resides in a different DLL, its entry points should
be referenced accordingly, and precautions may need to
be taken to properly deal with statically allocated
variables in the main program. Used only for MS Windows.
GC_NOT_DLL User-settable macro that overrides _DLL, e.g. if dynamic
libraries are used, but the collector is in a static library.
__STDC__ Assumed to be defined only by compilers that understand
prototypes and other C89 features. Its value is generally
not used, since we are fine with most nonconforming extensions.
SUNOS5SIGS Solaris-like signal handling. This is probably misnamed,
since it really doesn't guarantee much more than Posix.
Currently set only for Solaris2.X, HPUX, and DRSNX. Should
probably be set for some other platforms.
PCR Set if the collector is being built as part of the Xerox
Portable Common Runtime.
SRC_M3 Set if the collector is being built as a replacement of the
one in the DEC/Compaq SRC Modula-3 runtime. I suspect this
was last used around 1994, and no doubt broke a long time ago.
It's there primarily incase someone wants to port to a similar
system.
USE_COMPILER_TLS Assume the existence of __thread-style thread-local
storage. Set automatically for thread-local allocation with
the HP/UX vendor compiler. Usable with gcc on sufficiently
up-to-date ELF platforms.
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