libsanitizer/sanitizer_common/sanitizer_win_defs.h

   1 //===-- sanitizer_win_defs.h ------------------------------------*- C++ -*-===//
   2 //
   3 // This file is distributed under the University of Illinois Open Source
   4 // License. See LICENSE.TXT for details.
   5 //
   6 //===----------------------------------------------------------------------===//
   7 //
   8 // Common definitions for Windows-specific code.
   9 //
  10 //===----------------------------------------------------------------------===//
  11 #ifndef SANITIZER_WIN_DEFS_H
  12 #define SANITIZER_WIN_DEFS_H
  13
  14 #include "sanitizer_platform.h"
  15 #if SANITIZER_WINDOWS
  16
  17 #ifndef WINAPI
  18 #if defined(_M_IX86) || defined(__i386__)
  19 #define WINAPI __stdcall
  20 #else
  21 #define WINAPI
  22 #endif
  23 #endif
  24
  25 #if defined(_M_IX86) || defined(__i386__)
  26 #define WIN_SYM_PREFIX "_"
  27 #else
  28 #define WIN_SYM_PREFIX
  29 #endif
  30
  31 // For MinGW, the /export: directives contain undecorated symbols, contrary to
  32 // link/lld-link. The GNU linker doesn't support /alternatename and /include
  33 // though, thus lld-link in MinGW mode interprets them in the same way as
  34 // in the default mode.
  35 #ifdef __MINGW32__
  36 #define WIN_EXPORT_PREFIX
  37 #else
  38 #define WIN_EXPORT_PREFIX WIN_SYM_PREFIX
  39 #endif
  40
  41 // Intermediate macro to ensure the parameter is expanded before stringified.
  42 #define STRINGIFY_(A) #A
  43 #define STRINGIFY(A) STRINGIFY_(A)
  44
  45 // ----------------- A workaround for the absence of weak symbols --------------
  46 // We don't have a direct equivalent of weak symbols when using MSVC, but we can
  47 // use the /alternatename directive to tell the linker to default a specific
  48 // symbol to a specific value.
  49 // Take into account that this is a pragma directive for the linker, so it will
  50 // be ignored by the compiler and the function will be marked as UNDEF in the
  51 // symbol table of the resulting object file. The linker won't find the default
  52 // implementation until it links with that object file.
  53 // So, suppose we provide a default implementation "fundef" for "fun", and this
  54 // is compiled into the object file "test.obj" including the pragma directive.
  55 // If we have some code with references to "fun" and we link that code with
  56 // "test.obj", it will work because the linker always link object files.
  57 // But, if "test.obj" is included in a static library, like "test.lib", then the
  58 // liker will only link to "test.obj" if necessary. If we only included the
  59 // definition of "fun", it won't link to "test.obj" (from test.lib) because
  60 // "fun" appears as UNDEF, so it doesn't resolve the symbol "fun", and will
  61 // result in a link error (the linker doesn't find the pragma directive).
  62 // So, a workaround is to force linkage with the modules that include weak
  63 // definitions, with the following macro: WIN_FORCE_LINK()
  64
  65 #define WIN_WEAK_ALIAS(Name, Default)                                          \
  66   __pragma(comment(linker, "/alternatename:" WIN_SYM_PREFIX STRINGIFY(Name) "="\
  67                                              WIN_SYM_PREFIX STRINGIFY(Default)))
  68
  69 #define WIN_FORCE_LINK(Name)                                                   \
  70   __pragma(comment(linker, "/include:" WIN_SYM_PREFIX STRINGIFY(Name)))
  71
  72 #define WIN_EXPORT(ExportedName, Name)                                         \
  73   __pragma(comment(linker, "/export:" WIN_EXPORT_PREFIX STRINGIFY(ExportedName)\
  74                                   "=" WIN_EXPORT_PREFIX STRINGIFY(Name)))
  75
  76 // We cannot define weak functions on Windows, but we can use WIN_WEAK_ALIAS()
  77 // which defines an alias to a default implementation, and only works when
  78 // linking statically.
  79 // So, to define a weak function "fun", we define a default implementation with
  80 // a different name "fun__def" and we create a "weak alias" fun = fun__def.
  81 // Then, users can override it just defining "fun".
  82 // We impose "extern "C"" because otherwise WIN_WEAK_ALIAS() will fail because
  83 // of name mangling.
  84
  85 // Dummy name for default implementation of weak function.
  86 # define WEAK_DEFAULT_NAME(Name) Name##__def
  87 // Name for exported implementation of weak function.
  88 # define WEAK_EXPORT_NAME(Name) Name##__dll
  89
  90 // Use this macro when you need to define and export a weak function from a
  91 // library. For example:
  92 //   WIN_WEAK_EXPORT_DEF(bool, compare, int a, int b) { return a > b; }
  93 # define WIN_WEAK_EXPORT_DEF(ReturnType, Name, ...)                            \
  94   WIN_WEAK_ALIAS(Name, WEAK_DEFAULT_NAME(Name))                                \
  95   WIN_EXPORT(WEAK_EXPORT_NAME(Name), Name)                                     \
  96   extern "C" ReturnType Name(__VA_ARGS__);                                     \
  97   extern "C" ReturnType WEAK_DEFAULT_NAME(Name)(__VA_ARGS__)
  98
  99 // Use this macro when you need to import a weak function from a library. It
 100 // defines a weak alias to the imported function from the dll. For example:
 101 //   WIN_WEAK_IMPORT_DEF(compare)
 102 # define WIN_WEAK_IMPORT_DEF(Name)                                             \
 103   WIN_WEAK_ALIAS(Name, WEAK_EXPORT_NAME(Name))
 104
 105 // So, for Windows we provide something similar to weak symbols in Linux, with
 106 // some differences:
 107 // + A default implementation must always be provided.
 108 //
 109 // + When linking statically it works quite similarly. For example:
 110 //
 111 //   // libExample.cc
 112 //   WIN_WEAK_EXPORT_DEF(bool, compare, int a, int b) { return a > b; }
 113 //
 114 //   // client.cc
 115 //   // We can use the default implementation from the library:
 116 //   compare(1, 2);
 117 //   // Or we can override it:
 118 //   extern "C" bool compare (int a, int b) { return a >= b; }
 119 //
 120 //  And it will work fine. If we don't override the function, we need to ensure
 121 //  that the linker includes the object file with the default implementation.
 122 //  We can do so with the linker option "-wholearchive:".
 123 //
 124 // + When linking dynamically with a library (dll), weak functions are exported
 125 //  with "__dll" suffix. Clients can use the macro WIN_WEAK_IMPORT_DEF(fun)
 126 //  which defines a "weak alias" fun = fun__dll.
 127 //
 128 //   // libExample.cc
 129 //   WIN_WEAK_EXPORT_DEF(bool, compare, int a, int b) { return a > b; }
 130 //
 131 //   // client.cc
 132 //   WIN_WEAK_IMPORT_DEF(compare)
 133 //   // We can use the default implementation from the library:
 134 //   compare(1, 2);
 135 //   // Or we can override it:
 136 //   extern "C" bool compare (int a, int b) { return a >= b; }
 137 //
 138 //  But if we override the function, the dlls don't have access to it (which
 139 //  is different in linux). If that is desired, the strong definition must be
 140 //  exported and interception can be used from the rest of the dlls.
 141 //
 142 //   // libExample.cc
 143 //   WIN_WEAK_EXPORT_DEF(bool, compare, int a, int b) { return a > b; }
 144 //   // When initialized, check if the main executable defined "compare".
 145 //   int libExample_init() {
 146 //     uptr fnptr = __interception::InternalGetProcAddress(
 147 //         (void *)GetModuleHandleA(0), "compare");
 148 //     if (fnptr && !__interception::OverrideFunction((uptr)compare, fnptr, 0))
 149 //       abort();
 150 //     return 0;
 151 //   }
 152 //
 153 //   // client.cc
 154 //   WIN_WEAK_IMPORT_DEF(compare)
 155 //   // We override and export compare:
 156 //   extern "C" __declspec(dllexport) bool compare (int a, int b) {
 157 //     return a >= b;
 158 //   }
 159 //
 160 #endif // SANITIZER_WINDOWS
 161 #endif // SANITIZER_WIN_DEFS_H