boehm-gc/doc/README.macros

   1 The collector uses a large amount of conditional compilation in order to
   2 deal with platform dependencies.  This violates a number of known coding
   3 standards.  On the other hand, it seems to be the only practical way to
   4 support this many platforms without excessive code duplication.
   5
   6 A few guidelines have mostly been followed in order to keep this manageable:
   7
   8 1) #if and #ifdef directives are properly indented whenever easily possible.
   9 All known C compilers allow whitespace between the "#" and the "if" to make
  10 this possible.  ANSI C also allows white space before the "#", though we
  11 avoid that.  It has the known disadvantages that it differs from the normal
  12 GNU conventions, and that it makes patches larger than otherwise necessary.
  13 In my opinion, it's still well worth it, for the same reason that we indent
  14 ordinary "if" statements.
  15
  16 2) Whenever possible, tests are performed on the macros defined in gcconfig.h
  17 instead of directly testing patform-specific predefined macros.  This makes it
  18 relatively easy to adapt to new compilers with a different set of predefined
  19 macros.  Currently these macros generally identify platforms instead of
  20 features.  In many cases, this is a mistake.
  21
  22 3) The code currently avoids #elif, eventhough that would make it more
  23 readable.  This was done since #elif would need to be understood by ALL
  24 compilers used to build the collector, and that hasn't always been the case.
  25 It makes sense to reconsider this decision at some point, since #elif has been
  26 standardized at least since 1989.
  27
  28 Many of the tested configuration macros are at least somewhat defined in
  29 either include/private/gcconfig.h or in Makefile.direct.  Here is an attempt
  30 at defining some of the remainder:  (Thanks to Walter Bright for suggesting
  31 this.  This is a work in progress)
  32
  33 MACRO           EXPLANATION
  34 -----           -----------
  35
  36 __DMC__ Always #define'd by the Digital Mars compiler. Expands
  37                 to the compiler version number in hex, i.e. 0x810 is
  38                 version 8.1b0
  39
  40 _ENABLE_ARRAYNEW
  41                 #define'd by the Digital Mars C++ compiler when
  42                 operator new[] and delete[] are separately
  43                 overloadable. Used in gc_cpp.h.
  44
  45 _MSC_VER        Expands to the Visual C++ compiler version.  Assumed to
  46                 not be defined for other compilers (at least if they behave
  47                 appreciably differently).
  48
  49 _DLL            Defined by Visual C++ if dynamic libraries are being built
  50                 or used.  Used to test whether __declspec(dllimport) or
  51                 __declspec(dllexport) needs to be added to declarations
  52                 to support the case in which the collector is in a dll.
  53
  54 GC_DLL          User-settable macro that forces the effect of _DLL.  Set
  55                 by gc.h if _DLL is defined and GC_NOT_DLL is undefined.
  56                 This is the macro that is tested internally to determine
  57                 whether the GC is in its own dynamic library.  May need
  58                 to be set by clients before including gc.h.  Note that
  59                 inside the GC implementation it indicates that the
  60                 collector is in its own dynamic library, should export
  61                 its symbols, etc.  But in clients it indicates that the
  62                 GC resides in a different DLL, its entry points should
  63                 be referenced accordingly, and precautions may need to
  64                 be taken to properly deal with statically allocated
  65                 variables in the main program.  Used only for MS Windows.
  66
  67 GC_NOT_DLL      User-settable macro that overrides _DLL, e.g. if dynamic
  68                 libraries are used, but the collector is in a static library.
  69
  70 __STDC__        Assumed to be defined only by compilers that understand
  71                 prototypes and other C89 features.  Its value is generally
  72                 not used, since we are fine with most nonconforming extensions.
  73
  74 SUNOS5SIGS      Solaris-like signal handling.  This is probably misnamed,
  75                 since it really doesn't guarantee much more than Posix.
  76                 Currently set only for Solaris2.X, HPUX, and DRSNX.  Should
  77                 probably be set for some other platforms.
  78
  79 PCR             Set if the collector is being built as part of the Xerox
  80                 Portable Common Runtime.
  81
  82 SRC_M3          Set if the collector is being built as a replacement of the
  83                 one in the DEC/Compaq SRC Modula-3 runtime.  I suspect this
  84                 was last used around 1994, and no doubt broke a long time ago.
  85                 It's there primarily incase someone wants to port to a similar
  86                 system.
  87
  88 USE_COMPILER_TLS  Assume the existence of __thread-style thread-local
  89                 storage.  Set automatically for thread-local allocation with
  90                 the HP/UX vendor compiler.  Usable with gcc on sufficiently
  91                 up-to-date ELF platforms.
  92
  93
  94