Messages in this thread | | | From | Kyle Moffett <> | Subject | Re: kfree(0) - ok? | Date | Wed, 15 Aug 2007 09:58:29 -0400 |
| |
On Aug 15, 2007, at 06:20:27, Jan Engelhardt wrote: > On Aug 15 2007 11:58, Rene Herman wrote: >>>> NULL is not 0 though. >>> It is. Its representation isn't guaranteed to be all-bits-zero, >> >> He said the null _pointer_ isn't guaranteed to be all-bits zero. >> And it isn't. Read the standard or the faq. > > 0 is all-bits-zero. > NULL is 0. ("It is.", above) > > Transitively, this would make NULL all-bits-zero. I might have > missed something, though, perhaps that the cast to void* makes it > intransitive. But leave it at whatever the standard says. > >> but the constant value 0 when used in pointer context is always a >> null pointer (and in fact the standard requires that NULL be >> #defined as 0 or a cast thereof).
Irrespective of whatever the standard says, EVERY platform and compiler anybody makes nowadays has a NULL pointer value with all bits clear. Theoretically the standard allows otherwise, but such a decision would break so much code. Linux especially, we rely on the uninitialized data to have all bits clear and we depend on that producing NULL pointers; if a NULL pointer was not bitwise exactly 0 then the test "if (some_ptr != NULL)" would fail and we would start dereferencing garbage.
Cheers, Kyle Moffett
- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |