Le 31 juil. 2011 à 09:27, Ron Hunsinger a écrit : > When a process forks, the child and parent get different virtual memory maps > that, for the time being, happen to point to the same physical memory pages. > The sharing is an implementation detail, for optimization. Conceptually, the > parent's memory is copied into the child's memory space, so that if one of > the processes modifies the "shared" memory, the other process does not see > the change. The implementation, under the hood, does copy-on-write so that > the cost of that copying can be deferred until needed. Since fork is usually > followed by exec in the child, the copying usually never takes place, so it's > a particularly good optimization. Don't be misled into thinking that just > because the copying is deferred that the parent and child are actually > sharing a single virtual memory space.
Thanks for these precisions. Actually, you’re right with fork (2), but vfork (2), AFAIK, does not copy anything; note that I’m aware that vfork usually is meant to be immediately followed by exec (3). > The other methods you mention of sharing memory are by special arrangement, > and rarely used (although extremely useful when they are). I agree also. My point was to mitigate the statement that processes cannot share common memory. Of course, as you mentioned, in normal operation they don’t: That’s, after all, the whole point of the MMU, applications live in a memory sandbox. As far as I remember, wasn’t it D. Ritchie who related that, when developing on old PDP11/70, it was customary to yell "a.out !" when you were about to launch a program, because it gave others time to save their work before a possible general crash or data corruption? Vincent _______________________________________________ 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: http://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
