site_archiver@lists.apple.com Delivered-To: darwin-kernel@lists.apple.com On Oct 4, 2008, at 3:29 PM, Terry Lambert wrote: unsigned int a = 4294967295; // 2^32-1 int b = (int)a; unsigned int c = (unsigned int)b; printf("%s\n", a == c ? "equal" : "not equal"); So what's going on? It doesn't appear to lose any information here. No worries, this is not occurring. :-) - Greg _______________________________________________ Do not post admin requests to the list. They will be ignored. Darwin-kernel mailing list (Darwin-kernel@lists.apple.com) Help/Unsubscribe/Update your Subscription: http://lists.apple.com/mailman/options/darwin-kernel/site_archiver%40lists.a... Kernel virtual address space for a 32 bit kernel is limited to 32 bits. Pointers are NOT signed values, so do not store the returned pointer into a signed integer, or you may truncate it to 31 bits and overwrite/access something in lower memory than your allocation actually occurred when you go to use it. This would be Bad(tm). Thanks Terry, and I searched online and found that it appears that Apple will continue to use a 32-bit kernel (I hope). I'm aware that void* is unsigned, but I wasn't sure if converting that to a signed type and then back again would lose information. For example this program (with gcc -O0) displays "equal": If this value is intended to be communicated to user space, consider using a sized type capable of storing either a 32 or 64 bit pointer, such as user_addr_t or uint64_t. This is normally discouraged, since the likely reason for wanting to do it is because you intend to pass the pointer back into the kernel later, and that would ne a gaping security hole. This email sent to site_archiver@lists.apple.com