On Sun, 05 Oct 2014 20:23:42 +0100, mm0fmf wrote: > On 04/10/2014 02:02, Steven D'Aprano wrote: >> Way back in the mid 1980s, Apple Macintoshes used a memory manager >> which could move memory around. > > But the memory manager didn't return a pointer to memory the way malloc > does. It returned a pointer to the pointer and you had to double > dereference it to get the heap address (ISTR, 30 years ago now).
Correct. > The > advantage being the memory manager could shuffle the memory about and > update the pointers. Your pointer to a pointer would still point to the > same block after a shuffle. Of course you couldn't hold on to a partial > dereference across system calls... can you guess why? :-) Because system calls might trigger a memory compaction or move. Before the move, you have a managed "pointer to pointer" (handle) pointing to a managed point which points to a block of memory: handle -----> pointer -----> "Some stuff here" Grab a copy of the pointer with a partial deref: myPtr := handle^; (*I'm an old Pascal guy.*) So we have this: handle -----> pointer -----> "Some stuff here" myPtr -----------------------^ Then you call a system routine that moves memory, and the memory manager moves the block, updating the pointer, but leaving myPtr pointing at garbage: handle -----> pointer ----------------------------> "Some stuff here" myPtr -----------------------^ and as soon as you try using myPtr, you likely get the dreaded Bomb dialog box: http://www.macobserver.com/tmo/article/happy-birthday-mac-how-to-recover-from-the-dreaded-bomb-box-error-message I'm not suggesting that a 1984 memory manager is the solution to all our problems. I'm just pointing it out as proof that the concept works. If I knew more about Java and .Net, I could use them as examples instead :-) -- Steven -- https://mail.python.org/mailman/listinfo/python-list
