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 #ifdef _M_IX86
  19 #define WINAPI __stdcall
  20 #else
  21 #define WINAPI
  22 #endif
  23 #endif
  24
  25 #if defined(_WIN64)
  26 #define WIN_SYM_PREFIX
  27 #else
  28 #define WIN_SYM_PREFIX "_"
  29 #endif
  30
  31 // Intermediate macro to ensure the parameter is expanded before stringified.
  32 #define STRINGIFY_(A) #A
  33 #define STRINGIFY(A) STRINGIFY_(A)
  34
  35 // ----------------- A workaround for the absence of weak symbols --------------
  36 // We don't have a direct equivalent of weak symbols when using MSVC, but we can
  37 // use the /alternatename directive to tell the linker to default a specific
  38 // symbol to a specific value.
  39 // Take into account that this is a pragma directive for the linker, so it will
  40 // be ignored by the compiler and the function will be marked as UNDEF in the
  41 // symbol table of the resulting object file. The linker won't find the default
  42 // implementation until it links with that object file.
  43 // So, suppose we provide a default implementation "fundef" for "fun", and this
  44 // is compiled into the object file "test.obj" including the pragma directive.
  45 // If we have some code with references to "fun" and we link that code with
  46 // "test.obj", it will work because the linker always link object files.
  47 // But, if "test.obj" is included in a static library, like "test.lib", then the
  48 // liker will only link to "test.obj" if necessary. If we only included the
  49 // definition of "fun", it won't link to "test.obj" (from test.lib) because
  50 // "fun" appears as UNDEF, so it doesn't resolve the symbol "fun", and will
  51 // result in a link error (the linker doesn't find the pragma directive).
  52 // So, a workaround is to force linkage with the modules that include weak
  53 // definitions, with the following macro: WIN_FORCE_LINK()
  54
  55 #define WIN_WEAK_ALIAS(Name, Default)                                          \
  56   __pragma(comment(linker, "/alternatename:" WIN_SYM_PREFIX STRINGIFY(Name) "="\
  57                                              WIN_SYM_PREFIX STRINGIFY(Default)))
  58
  59 #define WIN_FORCE_LINK(Name)                                                   \
  60   __pragma(comment(linker, "/include:" WIN_SYM_PREFIX STRINGIFY(Name)))
  61
  62 #define WIN_EXPORT(ExportedName, Name)                                         \
  63   __pragma(comment(linker, "/export:" WIN_SYM_PREFIX STRINGIFY(ExportedName)   \
  64                                   "=" WIN_SYM_PREFIX STRINGIFY(Name)))
  65
  66 // We cannot define weak functions on Windows, but we can use WIN_WEAK_ALIAS()
  67 // which defines an alias to a default implementation, and only works when
  68 // linking statically.
  69 // So, to define a weak function "fun", we define a default implementation with
  70 // a different name "fun__def" and we create a "weak alias" fun = fun__def.
  71 // Then, users can override it just defining "fun".
  72 // We impose "extern "C"" because otherwise WIN_WEAK_ALIAS() will fail because
  73 // of name mangling.
  74
  75 // Dummy name for default implementation of weak function.
  76 # define WEAK_DEFAULT_NAME(Name) Name##__def
  77 // Name for exported implementation of weak function.
  78 # define WEAK_EXPORT_NAME(Name) Name##__dll
  79
  80 // Use this macro when you need to define and export a weak function from a
  81 // library. For example:
  82 //   WIN_WEAK_EXPORT_DEF(bool, compare, int a, int b) { return a > b; }
  83 # define WIN_WEAK_EXPORT_DEF(ReturnType, Name, ...)                            \
  84   WIN_WEAK_ALIAS(Name, WEAK_DEFAULT_NAME(Name))                                \
  85   WIN_EXPORT(WEAK_EXPORT_NAME(Name), Name)                                     \
  86   extern "C" ReturnType Name(__VA_ARGS__);                                     \
  87   extern "C" ReturnType WEAK_DEFAULT_NAME(Name)(__VA_ARGS__)
  88
  89 // Use this macro when you need to import a weak function from a library. It
  90 // defines a weak alias to the imported function from the dll. For example:
  91 //   WIN_WEAK_IMPORT_DEF(compare)
  92 # define WIN_WEAK_IMPORT_DEF(Name)                                             \
  93   WIN_WEAK_ALIAS(Name, WEAK_EXPORT_NAME(Name))
  94
  95 // So, for Windows we provide something similar to weak symbols in Linux, with
  96 // some differences:
  97 // + A default implementation must always be provided.
  98 //
  99 // + When linking statically it works quite similarly. For example:
 100 //
 101 //   // libExample.cc
 102 //   WIN_WEAK_EXPORT_DEF(bool, compare, int a, int b) { return a > b; }
 103 //
 104 //   // client.cc
 105 //   // We can use the default implementation from the library:
 106 //   compare(1, 2);
 107 //   // Or we can override it:
 108 //   extern "C" bool compare (int a, int b) { return a >= b; }
 109 //
 110 //  And it will work fine. If we don't override the function, we need to ensure
 111 //  that the linker includes the object file with the default implementation.
 112 //  We can do so with the linker option "-wholearchive:".
 113 //
 114 // + When linking dynamically with a library (dll), weak functions are exported
 115 //  with "__dll" suffix. Clients can use the macro WIN_WEAK_IMPORT_DEF(fun)
 116 //  which defines a "weak alias" fun = fun__dll.
 117 //
 118 //   // libExample.cc
 119 //   WIN_WEAK_EXPORT_DEF(bool, compare, int a, int b) { return a > b; }
 120 //
 121 //   // client.cc
 122 //   WIN_WEAK_IMPORT_DEF(compare)
 123 //   // We can use the default implementation from the library:
 124 //   compare(1, 2);
 125 //   // Or we can override it:
 126 //   extern "C" bool compare (int a, int b) { return a >= b; }
 127 //
 128 //  But if we override the function, the dlls don't have access to it (which
 129 //  is different in linux). If that is desired, the strong definition must be
 130 //  exported and interception can be used from the rest of the dlls.
 131 //
 132 //   // libExample.cc
 133 //   WIN_WEAK_EXPORT_DEF(bool, compare, int a, int b) { return a > b; }
 134 //   // When initialized, check if the main executable defined "compare".
 135 //   int libExample_init() {
 136 //     uptr fnptr = __interception::InternalGetProcAddress(
 137 //         (void *)GetModuleHandleA(0), "compare");
 138 //     if (fnptr && !__interception::OverrideFunction((uptr)compare, fnptr, 0))
 139 //       abort();
 140 //     return 0;
 141 //   }
 142 //
 143 //   // client.cc
 144 //   WIN_WEAK_IMPORT_DEF(compare)
 145 //   // We override and export compare:
 146 //   extern "C" __declspec(dllexport) bool compare (int a, int b) {
 147 //     return a >= b;
 148 //   }
 149 //
 150 #endif // SANITIZER_WINDOWS
 151 #endif // SANITIZER_WIN_DEFS_H