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Availability Markup
.. contents::
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
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.
# define _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS __attribute__((unavailable))
# define _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS __attribute__((unavailable))
When the library is updated by the platform vendor, the markup can be updated.
For example::
__attribute__((availability(macosx,strict,introduced=10.12))) \
__attribute__((availability(ios,strict,introduced=10.0))) \
__attribute__((availability(tvos,strict,introduced=10.0))) \
In the source code, the macro can be added on a class if the full class requires
type info from the library for example::
: public std::logic_error {
or on a particular symbol:
Some parameters can be passed to lit to run the test-suite and exercise the
* 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:
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).