Availability Markup¶
Overview¶
Libc++ is used as a system library on macOS and iOS (amongst others). In order for users to be able to compile a binary that is intended to be deployed to an older version of the platform, clang provides the availability attribute that can be placed on declarations to describe the lifecycle of a symbol in the library.
Design¶
When a new feature is introduced that requires dylib support, a macro should be created in include/__config to mark this feature as unavailable for all the systems. For example:
// Define availability macros.
#if defined(_LIBCPP_USE_AVAILABILITY_APPLE)
#define _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS __attribute__((unavailable))
#else if defined(_LIBCPP_USE_AVAILABILITY_SOME_OTHER_VENDOR)
#define _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS __attribute__((unavailable))
#else
#define _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS
#endif
When the library is updated by the platform vendor, the markup can be updated. For example:
#define _LIBCPP_AVAILABILITY_SHARED_MUTEX \
__attribute__((availability(macosx,strict,introduced=10.12))) \
__attribute__((availability(ios,strict,introduced=10.0))) \
__attribute__((availability(tvos,strict,introduced=10.0))) \
__attribute__((availability(watchos,strict,introduced=3.0)))
In the source code, the macro can be added on a class if the full class requires type info from the library for example:
_LIBCPP_BEGIN_NAMESPACE_EXPERIMENTAL
class _LIBCPP_EXCEPTION_ABI _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS bad_optional_access
: public std::logic_error {
or on a particular symbol:
_LIBCPP_OVERRIDABLE_FUNC_VIS _LIBCPP_AVAILABILITY_SIZED_NEW_DELETE void operator delete(void* __p, std::size_t __sz) _NOEXCEPT;
Testing¶
Some parameters can be passed to lit to run the test-suite and exercising the availability.
- The platform parameter controls the deployment target. For example lit can be invoked with –param=platform=macosx10.8. Default is the current host.
- The use_system_cxx_lib parameter indicates to use another library than the just built one. Invoking lit with –param=use_system_cxx_lib=true will run the test-suite against the host system library. Alternatively a path to the directory containing a specific prebuilt libc++ can be used, for example: –param=use_system_cxx_lib=/path/to/macOS/10.8/.
- The with_availability boolean parameter enables the availability markup.
Tests can be marked as XFAIL based on multiple features made available by lit:
if either use_system_cxx_lib or with_availability is passed to lit, assuming –param=platform=macosx10.8 is passed as well the following features will be available:
- availability
- availability=x86_64
- availability=macosx
- availability=x86_64-macosx
- availability=x86_64-apple-macosx10.8
- availability=macosx10.8
This feature is used to XFAIL a test that is using a class of a method marked as unavailable and that is expected to fail if deployed on an older system.
if use_system_cxx_lib is passed to lit, the following features will also be available:
- with_system_cxx_lib
- with_system_cxx_lib=x86_64
- with_system_cxx_lib=macosx
- with_system_cxx_lib=x86_64-macosx
- with_system_cxx_lib=x86_64-apple-macosx10.8
- with_system_cxx_lib=macosx10.8
This feature is used to XFAIL a test that is not using a class of a method marked as unavailable but that is expected to fail if deployed on an older system. For example if we know that it exhibits a but in the libc on a particular system version.
if with_availability is passed to lit, the following features will also be available:
- availability_markup
- availability_markup=x86_64
- availability_markup=macosx
- availability_markup=x86_64-macosx
- availability_markup=x86_64-apple-macosx10.8
- availability_markup=macosx10.8
This feature is used to XFAIL a test that is using a class of a method marked as unavailable but that is expected to pass if deployed on an older system. For example if it is using a symbol in a statically evaluated context.