Thanks Jens, that makes sense. This brings up a related question that I haven't seen a definitive answer to - on iOS, does NSDataReadingMappedIfSafe always map the file? After all, the iOS apps can only reference local files on the device.
On Thu, Feb 11, 2016 at 12:03 PM, Jens Alfke <j...@mooseyard.com> wrote: > > On Feb 11, 2016, at 11:44 AM, Dan Lau <d...@twitter.com> wrote: > > If a file has its contents mapped using NSData's > initWithContentsOfFile + NSDataReadingMappedIfSafe, > deleting it doesn't appear to affect reading it's mapped contents. Does > anyone know if NSData uses mmap(2) under the hood to ensure that this sort > of operation is valid? > > > Yes. (mmap() is the only way to memory-map a file, that I know of.) > > You can delete a memory-mapped file without problems, for the same reason > you can delete an open file and keep reading/writing it. Files are > reference-counted, and the filesystem doesn’t delete a file until its > number of links goes to zero. An open file handle counts as a link. > > Also note that NSDataReadingMappedIfSafe won’t always map the file. I > think the rule is that files on external filesystems won’t be mapped, just > copied into memory. The reason is that if the filesystem is unexpectedly > disconnected, accessing the mapped memory will crash with a segfault. > > —Jens > _______________________________________________ Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) Please do not post admin requests or moderator comments to the list. Contact the moderators at cocoa-dev-admins(at)lists.apple.com Help/Unsubscribe/Update your Subscription: https://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
