1======================== 2Symbol Visibility Macros 3======================== 4 5.. contents:: 6 :local: 7 8.. _visibility-macros: 9 10Overview 11======== 12 13Libc++ uses various "visibility" macros in order to provide a stable ABI in 14both the library and the headers. These macros work by changing the 15visibility and inlining characteristics of the symbols they are applied to. 16 17The std namespace also has visibility attributes applied to avoid having to 18add visibility macros in as many places. Namespace std has default 19type_visibility to export RTTI and other type-specific information. Note that 20type_visibility is only supported by Clang, so this doesn't replace 21type-specific attributes. The only exception are enums, which GCC always gives 22default visibility, thus removing the need for any annotations. 23 24Visibility Macros 25================= 26 27**_LIBCPP_HIDDEN** 28 Mark a symbol as hidden so it will not be exported from shared libraries. 29 30**_LIBCPP_EXPORTED_FROM_ABI** 31 Mark a symbol as being part of our ABI. This includes functions that are part 32 of the libc++ library, type information and other symbols. On Windows, 33 this macro applies `dllimport`/`dllexport` to the symbol, and on other 34 platforms it gives the symbol default visibility. 35 36**_LIBCPP_OVERRIDABLE_FUNC_VIS** 37 Mark a symbol as being exported by the libc++ library, but allow it to be 38 overridden locally. On non-Windows, this is equivalent to `_LIBCPP_FUNC_VIS`. 39 This macro is applied to all `operator new` and `operator delete` overloads. 40 41 **Windows Behavior**: Any symbol marked `dllimport` cannot be overridden 42 locally, since `dllimport` indicates the symbol should be bound to a separate 43 DLL. All `operator new` and `operator delete` overloads are required to be 44 locally overridable, and therefore must not be marked `dllimport`. On Windows, 45 this macro therefore expands to `__declspec(dllexport)` when building the 46 library and has an empty definition otherwise. 47 48**_LIBCPP_HIDE_FROM_ABI** 49 Mark a function as not being part of the ABI of any final linked image that 50 uses it. 51 52**_LIBCPP_HIDE_FROM_ABI_AFTER_V1** 53 Mark a function as being hidden from the ABI (per `_LIBCPP_HIDE_FROM_ABI`) 54 when libc++ is built with an ABI version after ABI v1. This macro is used to 55 maintain ABI compatibility for symbols that have been historically exported 56 by libc++ in v1 of the ABI, but that we don't want to export in the future. 57 58 This macro works as follows. When we build libc++, we either hide the symbol 59 from the ABI (if the symbol is not part of the ABI in the version we're 60 building), or we leave it included. From user code (i.e. when we're not 61 building libc++), the macro always marks symbols as internal so that programs 62 built using new libc++ headers stop relying on symbols that are removed from 63 the ABI in a future version. Each time we release a new stable version of the 64 ABI, we should create a new _LIBCPP_HIDE_FROM_ABI_AFTER_XXX macro, and we can 65 use it to start removing symbols from the ABI after that stable version. 66 67**_LIBCPP_TEMPLATE_VIS** 68 Mark a type's typeinfo and vtable as having default visibility. 69 This macro has no effect on the visibility of the type's member functions. 70 71 **GCC Behavior**: GCC does not support Clang's `type_visibility(...)` 72 attribute. With GCC the `visibility(...)` attribute is used and member 73 functions are affected. 74 75 **Windows Behavior**: DLLs do not support dllimport/export on class templates. 76 The macro has an empty definition on this platform. 77 78**_LIBCPP_EXTERN_TEMPLATE_TYPE_VIS** 79 Mark the member functions, typeinfo, and vtable of the type named in 80 an extern template declaration as being exported by the libc++ library. 81 This attribute must be specified on all extern class template declarations. 82 83 This macro is used to override the `_LIBCPP_TEMPLATE_VIS` attribute 84 specified on the primary template and to export the member functions produced 85 by the explicit instantiation in the dylib. 86 87 **Windows Behavior**: `extern template` and `dllexport` are fundamentally 88 incompatible *on a class template* on Windows; the former suppresses 89 instantiation, while the latter forces it. Specifying both on the same 90 declaration makes the class template be instantiated, which is not desirable 91 inside headers. This macro therefore expands to `dllimport` outside of libc++ 92 but nothing inside of it (rather than expanding to `dllexport`); instead, the 93 explicit instantiations themselves are marked as exported. Note that this 94 applies *only* to extern *class* templates. Extern *function* templates obey 95 regular import/export semantics, and applying `dllexport` directly to the 96 extern template declaration (i.e. using `_LIBCPP_FUNC_VIS`) is the correct 97 thing to do for them. 98 99**_LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS** 100 Mark the member functions, typeinfo, and vtable of an explicit instantiation 101 of a class template as being exported by the libc++ library. This attribute 102 must be specified on all class template explicit instantiations. 103 104 It is only necessary to mark the explicit instantiation itself (as opposed to 105 the extern template declaration) as exported on Windows, as discussed above. 106 On all other platforms, this macro has an empty definition. 107 108**_LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS** 109 Mark a symbol as hidden so it will not be exported from shared libraries. This 110 is intended specifically for method templates of either classes marked with 111 `_LIBCPP_TYPE_VIS` or classes with an extern template instantiation 112 declaration marked with `_LIBCPP_EXTERN_TEMPLATE_TYPE_VIS`. 113 114 When building libc++ with hidden visibility, we want explicit template 115 instantiations to export members, which is consistent with existing Windows 116 behavior. We also want classes annotated with `_LIBCPP_TYPE_VIS` to export 117 their members, which is again consistent with existing Windows behavior. 118 Both these changes are necessary for clients to be able to link against a 119 libc++ DSO built with hidden visibility without encountering missing symbols. 120 121 An unfortunate side effect, however, is that method templates of classes 122 either marked `_LIBCPP_TYPE_VIS` or with extern template instantiation 123 declarations marked with `_LIBCPP_EXTERN_TEMPLATE_TYPE_VIS` also get default 124 visibility when instantiated. These methods are often implicitly instantiated 125 inside other libraries which use the libc++ headers, and will therefore end up 126 being exported from those libraries, since those implicit instantiations will 127 receive default visibility. This is not acceptable for libraries that wish to 128 control their visibility, and led to PR30642. 129 130 Consequently, all such problematic method templates are explicitly marked 131 either hidden (via this macro) or inline, so that they don't leak into client 132 libraries. The problematic methods were found by running 133 `bad-visibility-finder <https://github.com/smeenai/bad-visibility-finder>`_ 134 against the libc++ headers after making `_LIBCPP_TYPE_VIS` and 135 `_LIBCPP_EXTERN_TEMPLATE_TYPE_VIS` expand to default visibility. 136 137Links 138===== 139 140* `[cfe-dev] Visibility in libc++ - 1 <http://lists.llvm.org/pipermail/cfe-dev/2013-July/030610.html>`_ 141* `[cfe-dev] Visibility in libc++ - 2 <http://lists.llvm.org/pipermail/cfe-dev/2013-August/031195.html>`_ 142* `[libcxx] Visibility fixes for Windows <http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20130805/085461.html>`_ 143