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
162