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The NeXT/Apple Objective-C runtime has been Open Source since the late 1990s, the API and ABI have been documented since the late 1980s so anyone could create a
by eschaton 10y ago
The NeXT/Apple Objective-C runtime has been Open Source since the late 1990s, the API and ABI have been documented since the late 1980s so anyone could create a compatible runtime, and GCC has had a portable runtime almost since Objective-C was taken into its mainline.