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The GNUstep Objective-C runtime was designed as a drop-in replacement for
the GCC runtime. It supports three ABIs:
* The old GCC ABI, which provides support for Objective-C 1.0 features.
This can be selected via the -fobjc-runtime=gcc flag in Clang or by
compiling with GCC.
* The initial GNUstep non-fragile ABI, which was intended to be
compatible with the GCC ABI, but provide support for modern Objective-C
features. This can be selected with the -fobjc-runtime=gnustep-1.9 flag in
Clang.
* The modern (v2) ABI, which provides richer reflection metadata, smaller
binaries and reduced memory usage. This is selected with the
-fobjc-runtime=gnustep-2.0 flag in Clang 7.0 or later.
All ABIs support the following feature above and beyond the GCC runtime:
* The modern Objective-C runtime APIs, initially introduced with OS X
10.5.
* Blocks (closures).
* Synthesised property accessors.
* Efficient support for @synchronized()
* Type-dependent dispatch, eliminating stack corruption from mismatched
selectors.
* Support for the associated reference APIs introduced with Mac OS X 10.6.
* Support for the automatic reference counting APIs introduced with Mac
OS X 10.7
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