======================== Symbol Visibility Macros ======================== .. contents:: :local: Overview ======== Libc++ uses various "visibility" macros in order to provide a stable ABI in both the library and the headers. These macros work by changing the visibility and inlining characteristics of the symbols they are applied to. Visibility Macros ================= **_LIBCPP_HIDDEN** Mark a symbol as hidden so it will not be exported from shared libraries. **_LIBCPP_FUNC_VIS** Mark a symbol as being exported by the libc++ library. This attribute must be applied to the declaration of all functions exported by the libc++ dylib. **_LIBCPP_EXTERN_VIS** Mark a symbol as being exported by the libc++ library. This attribute may only be applied to objects defined in the libc++ library. On Windows this macro applies `dllimport`/`dllexport` to the symbol. On all other platforms this macro has no effect. **_LIBCPP_OVERRIDABLE_FUNC_VIS** Mark a symbol as being exported by the libc++ library, but allow it to be overridden locally. On non-Windows, this is equivalent to `_LIBCPP_FUNC_VIS`. This macro is applied to all `operator new` and `operator delete` overloads. **Windows Behavior**: Any symbol marked `dllimport` cannot be overridden locally, since `dllimport` indicates the symbol should be bound to a separate DLL. All `operator new` and `operator delete` overloads are required to be locally overridable, and therefore must not be marked `dllimport`. On Windows, this macro therefore expands to `__declspec(dllexport)` when building the library and has an empty definition otherwise. **_LIBCPP_HIDE_FROM_ABI** Mark a function as not being part of the ABI of any final linked image that uses it, and also as being internal to each TU that uses that function. In other words, the address of a function marked with this attribute is not guaranteed to be the same across translation units. **_LIBCPP_HIDE_FROM_ABI_AFTER_V1** Mark a function as being hidden from the ABI (per `_LIBCPP_HIDE_FROM_ABI`) when libc++ is built with an ABI version after ABI v1. This macro is used to maintain ABI compatibility for symbols that have been historically exported by libc++ in v1 of the ABI, but that we don't want to export in the future. This macro works as follows. When we build libc++, we either hide the symbol from the ABI (if the symbol is not part of the ABI in the version we're building), or we leave it included. From user code (i.e. when we're not building libc++), the macro always marks symbols as internal so that programs built using new libc++ headers stop relying on symbols that are removed from the ABI in a future version. Each time we release a new stable version of the ABI, we should create a new _LIBCPP_HIDE_FROM_ABI_AFTER_XXX macro, and we can use it to start removing symbols from the ABI after that stable version. **_LIBCPP_TYPE_VIS** Mark a type's typeinfo, vtable and members as having default visibility. This attribute cannot be used on class templates. **_LIBCPP_TEMPLATE_VIS** Mark a type's typeinfo and vtable as having default visibility. This macro has no effect on the visibility of the type's member functions. **GCC Behavior**: GCC does not support Clang's `type_visibility(...)` attribute. With GCC the `visibility(...)` attribute is used and member functions are affected. **Windows Behavior**: DLLs do not support dllimport/export on class templates. The macro has an empty definition on this platform. **_LIBCPP_ENUM_VIS** Mark the typeinfo of an enum as having default visibility. This attribute should be applied to all enum declarations. **Windows Behavior**: DLLs do not support importing or exporting enumeration typeinfo. The macro has an empty definition on this platform. **GCC Behavior**: GCC un-hides the typeinfo for enumerations by default, even if `-fvisibility=hidden` is specified. Additionally applying a visibility attribute to an enum class results in a warning. The macro has an empty definition with GCC. **_LIBCPP_EXTERN_TEMPLATE_TYPE_VIS** Mark the member functions, typeinfo, and vtable of the type named in a `_LIBCPP_EXTERN_TEMPLATE` declaration as being exported by the libc++ library. This attribute must be specified on all extern class template declarations. This macro is used to override the `_LIBCPP_TEMPLATE_VIS` attribute specified on the primary template and to export the member functions produced by the explicit instantiation in the dylib. **GCC Behavior**: GCC ignores visibility attributes applied the type in extern template declarations and applying an attribute results in a warning. However since `_LIBCPP_TEMPLATE_VIS` is the same as `__attribute__((visibility("default"))` the visibility is already correct. The macro has an empty definition with GCC. **Windows Behavior**: `extern template` and `dllexport` are fundamentally incompatible *on a class template* on Windows; the former suppresses instantiation, while the latter forces it. Specifying both on the same declaration makes the class template be instantiated, which is not desirable inside headers. This macro therefore expands to `dllimport` outside of libc++ but nothing inside of it (rather than expanding to `dllexport`); instead, the explicit instantiations themselves are marked as exported. Note that this applies *only* to extern *class* templates. Extern *function* templates obey regular import/export semantics, and applying `dllexport` directly to the extern template declaration (i.e. using `_LIBCPP_FUNC_VIS`) is the correct thing to do for them. **_LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS** Mark the member functions, typeinfo, and vtable of an explicit instantiation of a class template as being exported by the libc++ library. This attribute must be specified on all class template explicit instantiations. It is only necessary to mark the explicit instantiation itself (as opposed to the extern template declaration) as exported on Windows, as discussed above. On all other platforms, this macro has an empty definition. **_LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS** Mark a symbol as hidden so it will not be exported from shared libraries. This is intended specifically for method templates of either classes marked with `_LIBCPP_TYPE_VIS` or classes with an extern template instantiation declaration marked with `_LIBCPP_EXTERN_TEMPLATE_TYPE_VIS`. When building libc++ with hidden visibility, we want explicit template instantiations to export members, which is consistent with existing Windows behavior. We also want classes annotated with `_LIBCPP_TYPE_VIS` to export their members, which is again consistent with existing Windows behavior. Both these changes are necessary for clients to be able to link against a libc++ DSO built with hidden visibility without encountering missing symbols. An unfortunate side effect, however, is that method templates of classes either marked `_LIBCPP_TYPE_VIS` or with extern template instantiation declarations marked with `_LIBCPP_EXTERN_TEMPLATE_TYPE_VIS` also get default visibility when instantiated. These methods are often implicitly instantiated inside other libraries which use the libc++ headers, and will therefore end up being exported from those libraries, since those implicit instantiations will receive default visibility. This is not acceptable for libraries that wish to control their visibility, and led to PR30642. Consequently, all such problematic method templates are explicitly marked either hidden (via this macro) or inline, so that they don't leak into client libraries. The problematic methods were found by running `bad-visibility-finder `_ against the libc++ headers after making `_LIBCPP_TYPE_VIS` and `_LIBCPP_EXTERN_TEMPLATE_TYPE_VIS` expand to default visibility. **_LIBCPP_EXCEPTION_ABI** Mark the member functions, typeinfo, and vtable of the type as being exported by the libc++ library. This macro must be applied to all *exception types*. Exception types should be defined directly in namespace `std` and not the versioning namespace. This allows throwing and catching some exception types between libc++ and libstdc++. **_LIBCPP_INTERNAL_LINKAGE** Mark the affected entity as having internal linkage (i.e. the `static` keyword in C). This is only a best effort: when the `internal_linkage` attribute is not available, we fall back to forcing the function to be inlined, which approximates internal linkage since an externally visible symbol is never generated for that function. This is an internal macro used as an implementation detail by other visibility macros. Never mark a function or a class with this macro directly. **_LIBCPP_ALWAYS_INLINE** Forces inlining of the function it is applied to. For visibility purposes, this macro is used to make sure that an externally visible symbol is never generated in an object file when the `internal_linkage` attribute is not available. This is an internal macro used by other visibility macros, and it should not be used directly. Links ===== * `[cfe-dev] Visibility in libc++ - 1 `_ * `[cfe-dev] Visibility in libc++ - 2 `_ * `[libcxx] Visibility fixes for Windows `_