1 ======================== 2 Symbol Visibility Macros 3 ======================== 4 5 .. contents:: 6 :local: 7 8 Overview 9 ======== 10 11 Libc++ uses various "visibility" macros in order to provide a stable ABI in 12 both the library and the headers. These macros work by changing the 13 visibility and inlining characteristics of the symbols they are applied to. 14 15 Visibility Macros 16 ================= 17 18 **_LIBCPP_HIDDEN** 19 Mark a symbol as hidden so it will not be exported from shared libraries. 20 21 **_LIBCPP_FUNC_VIS** 22 Mark a symbol as being exported by the libc++ library. This attribute must 23 be applied to the declaration of all functions exported by the libc++ dylib. 24 25 **_LIBCPP_EXTERN_VIS** 26 Mark a symbol as being exported by the libc++ library. This attribute may 27 only be applied to objects defined in the libc++ library. On Windows this 28 macro applies `dllimport`/`dllexport` to the symbol. On all other platforms 29 this macro has no effect. 30 31 **_LIBCPP_OVERRIDABLE_FUNC_VIS** 32 Mark a symbol as being exported by the libc++ library, but allow it to be 33 overridden locally. On non-Windows, this is equivalent to `_LIBCPP_FUNC_VIS`. 34 This macro is applied to all `operator new` and `operator delete` overloads. 35 36 **Windows Behavior**: Any symbol marked `dllimport` cannot be overridden 37 locally, since `dllimport` indicates the symbol should be bound to a separate 38 DLL. All `operator new` and `operator delete` overloads are required to be 39 locally overridable, and therefore must not be marked `dllimport`. On Windows, 40 this macro therefore expands to `__declspec(dllexport)` when building the 41 library and has an empty definition otherwise. 42 43 **_LIBCPP_INLINE_VISIBILITY** 44 Mark a function as hidden and force inlining whenever possible. 45 46 **_LIBCPP_ALWAYS_INLINE** 47 A synonym for `_LIBCPP_INLINE_VISIBILITY` 48 49 **_LIBCPP_TYPE_VIS** 50 Mark a type's typeinfo, vtable and members as having default visibility. 51 This attribute cannot be used on class templates. 52 53 **_LIBCPP_TEMPLATE_VIS** 54 Mark a type's typeinfo and vtable as having default visibility. 55 This macro has no effect on the visibility of the type's member functions. 56 57 **GCC Behavior**: GCC does not support Clang's `type_visibility(...)` 58 attribute. With GCC the `visibility(...)` attribute is used and member 59 functions are affected. 60 61 **Windows Behavior**: DLLs do not support dllimport/export on class templates. 62 The macro has an empty definition on this platform. 63 64 65 **_LIBCPP_ENUM_VIS** 66 Mark the typeinfo of an enum as having default visibility. This attribute 67 should be applied to all enum declarations. 68 69 **Windows Behavior**: DLLs do not support importing or exporting enumeration 70 typeinfo. The macro has an empty definition on this platform. 71 72 **GCC Behavior**: GCC un-hides the typeinfo for enumerations by default, even 73 if `-fvisibility=hidden` is specified. Additionally applying a visibility 74 attribute to an enum class results in a warning. The macro has an empty 75 definition with GCC. 76 77 **_LIBCPP_EXTERN_TEMPLATE_TYPE_VIS** 78 Mark the member functions, typeinfo, and vtable of the type named in 79 a `_LIBCPP_EXTERN_TEMPLATE` declaration as being exported by the libc++ library. 80 This attribute must be specified on all extern class template declarations. 81 82 This macro is used to override the `_LIBCPP_TEMPLATE_VIS` attribute 83 specified on the primary template and to export the member functions produced 84 by the explicit instantiation in the dylib. 85 86 **GCC Behavior**: GCC ignores visibility attributes applied the type in 87 extern template declarations and applying an attribute results in a warning. 88 However since `_LIBCPP_TEMPLATE_VIS` is the same as 89 `__attribute__((visibility("default"))` the visibility is already correct. 90 The macro has an empty definition with GCC. 91 92 **Windows Behavior**: `extern template` and `dllexport` are fundamentally 93 incompatible *on a class template* on Windows; the former suppresses 94 instantiation, while the latter forces it. Specifying both on the same 95 declaration makes the class template be instantiated, which is not desirable 96 inside headers. This macro therefore expands to `dllimport` outside of libc++ 97 but nothing inside of it (rather than expanding to `dllexport`); instead, the 98 explicit instantiations themselves are marked as exported. Note that this 99 applies *only* to extern *class* templates. Extern *function* templates obey 100 regular import/export semantics, and applying `dllexport` directly to the 101 extern template declaration (i.e. using `_LIBCPP_FUNC_VIS`) is the correct 102 thing to do for them. 103 104 **_LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS** 105 Mark the member functions, typeinfo, and vtable of an explicit instantiation 106 of a class template as being exported by the libc++ library. This attribute 107 must be specified on all class template explicit instantiations. 108 109 It is only necessary to mark the explicit instantiation itself (as opposed to 110 the extern template declaration) as exported on Windows, as discussed above. 111 On all other platforms, this macro has an empty definition. 112 113 **_LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS** 114 Mark a symbol as hidden so it will not be exported from shared libraries. This 115 is intended specifically for method templates of either classes marked with 116 `_LIBCPP_TYPE_VIS` or classes with an extern template instantiation 117 declaration marked with `_LIBCPP_EXTERN_TEMPLATE_TYPE_VIS`. 118 119 When building libc++ with hidden visibility, we want explicit template 120 instantiations to export members, which is consistent with existing Windows 121 behavior. We also want classes annotated with `_LIBCPP_TYPE_VIS` to export 122 their members, which is again consistent with existing Windows behavior. 123 Both these changes are necessary for clients to be able to link against a 124 libc++ DSO built with hidden visibility without encountering missing symbols. 125 126 An unfortunate side effect, however, is that method templates of classes 127 either marked `_LIBCPP_TYPE_VIS` or with extern template instantiation 128 declarations marked with `_LIBCPP_EXTERN_TEMPLATE_TYPE_VIS` also get default 129 visibility when instantiated. These methods are often implicitly instantiated 130 inside other libraries which use the libc++ headers, and will therefore end up 131 being exported from those libraries, since those implicit instantiations will 132 receive default visibility. This is not acceptable for libraries that wish to 133 control their visibility, and led to PR30642. 134 135 Consequently, all such problematic method templates are explicitly marked 136 either hidden (via this macro) or inline, so that they don't leak into client 137 libraries. The problematic methods were found by running 138 `bad-visibility-finder <https://github.com/smeenai/bad-visibility-finder>`_ 139 against the libc++ headers after making `_LIBCPP_TYPE_VIS` and 140 `_LIBCPP_EXTERN_TEMPLATE_TYPE_VIS` expand to default visibility. 141 142 **_LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY** 143 Mark a member function of a class template as visible and always inline. This 144 macro should only be applied to member functions of class templates that are 145 externally instantiated. It is important that these symbols are not marked 146 as hidden as that will prevent the dylib definition from being found. 147 148 This macro is used to maintain ABI compatibility for symbols that have been 149 historically exported by the libc++ library but are now marked inline. 150 151 **_LIBCPP_EXCEPTION_ABI** 152 Mark the member functions, typeinfo, and vtable of the type as being exported 153 by the libc++ library. This macro must be applied to all *exception types*. 154 Exception types should be defined directly in namespace `std` and not the 155 versioning namespace. This allows throwing and catching some exception types 156 between libc++ and libstdc++. 157 158 Links 159 ===== 160 161 * `[cfe-dev] Visibility in libc++ - 1 <http://lists.llvm.org/pipermail/cfe-dev/2013-July/030610.html>`_ 162 * `[cfe-dev] Visibility in libc++ - 2 <http://lists.llvm.org/pipermail/cfe-dev/2013-August/031195.html>`_ 163 * `[libcxx] Visibility fixes for Windows <http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20130805/085461.html>`_ 164
