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 exercise 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/.

Tests can be marked as XFAIL based on multiple features made available by lit:

• if –param=platform=macosx10.8 is passed, 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 or a method marked as unavailable and that is expected to fail if deployed on an older system.

• if use_system_cxx_lib and –param=platform=macosx10.8 are 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 or a method marked as unavailable but that is expected to fail if deployed on an older system. For example, if the test exhibits a bug in the libc on a particular system version, or if the test uses a symbol that is not available on an older version of the dylib (but for which there is no availability markup, otherwise the XFAIL should use availability above).