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| -rw-r--r-- | gc/doc/README.macros | 78 |
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diff --git a/gc/doc/README.macros b/gc/doc/README.macros new file mode 100644 index 0000000..d9df8dd --- /dev/null +++ b/gc/doc/README.macros @@ -0,0 +1,78 @@ +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. + +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. + + + |