news.ycombinator.com• May 17, 2022• 1 min read
Ask HN: How does Rosetta 2 actually work?Apple's docs say that Rosetta 2 works by translating x86 code into ARM64 code when an application starts up and then running the resulting ARM code natively. But that can't actually be true because if I use dlopen to load a dynamic library at runtime, the architecture of the library has to match the architecture of the executable that called dlopen or it results in an error (Error opening shared library foo.dylib : dlopen(foo.dylib, 0x000A): tried: 'foo.dylib' (mach-o file, but is an incompatible architecture (have 'arm64', need 'x86_64')So at a minimum, dlopen can invoke Rosetta again in this case. But why thrown an error? Why not allow an x86 application to open an ARM library if it is actually ARM code at run-time?The situation is actually even weirder than that because there is an application (Clozure Common Lisp) that produces x86 code at run time but does not go through dlopen, it just writes x86 opcodes directly into memory (AFAIK). And yet, it works when run under Rosetta. How
