blob: f076dfecdaaa61b6b72326c7712327b7d26678f6 [file] [log] [blame]
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:
Furthermore, a lit feature should be added to match that availability macro,
so that tests depending on that feature can be marked to XFAIL if the feature
is not supported. This way, the test suite will work on platforms that have
not shipped the feature yet. This can be done by adding the appropriate lit
feature in test/
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).