On 12/22/2024 4:17 AM, Corinna Vinschen wrote:
The point was that a pagesize of 4K is wrong from the POSIX point of view. I was pretty stubborn in understanding this myself, and it took quite some time until I caved in and produced the aforementioned patch back in 2005.The reason (or rather, one example) is that you can't do this: fd = open ("file1", O_RDONLY); addr1 = mmap (NULL, 4096, PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); addr2 = mmap (addr1 + 4096, 4096, PROT_READ, MAP_PRIVATE, fd, 0); This fails on Windows because the allocation granularity is 64K. So even though the mapping at addr1 is (supposedly) only one pagesize of 4K in size... - the entire region [addr1,addr1+64K[ is now blocked for other mappings, and - a mapping can only start at an address % 64K, so the "addr1+4096" expression doesn't work on Windows, because the mapping can only start at the next (or any other) 64K boundary. Multiple assumptions around file mappings differ between Windows and POSIX, and the most sane way to emulate POSIX on Windows in this respect is to define the POSIX pagesize as the Windows allocation granularity.
Thanks for explaining. It makes sense now. Ken
