I expect that this is more a problem with how memory management works on macOS, the freeze and crash is likely an indication of eager allocation of memory by the system.

It is far from sure if implementing a workaround is feasible, the upper limit for safe calls to ftruncate likely depends on the amount of RAM in the system.

BTW. The sample code tries to allocate well over a petabyte of memory, how realistic is that code?  It might be good enough to document that using very large amounts of memory with multiprocessing.shared_memory causes problems on macOS.