On Wed, Mar 15, 2017 at 10:44 PM, Ngie Cooper (yaneurabeya) <yaneurab...@gmail.com> wrote: > >> On Mar 15, 2017, at 21:32, Warner Losh <i...@bsdimp.com> wrote: >> >> On Wed, Mar 15, 2017 at 8:31 PM, Ngie Cooper <n...@freebsd.org> wrote: >>> Author: ngie >>> Date: Thu Mar 16 02:31:42 2017 >>> New Revision: 315360 >>> URL: https://svnweb.freebsd.org/changeset/base/315360 >>> >>> Log: >>> Return NULL instead of 0 on failure in _kvm_open, kvm_open{,2,files} >>> >>> This is being done for the following reasons: >>> - kvm_open(3), etc says they will return NULL. >>> - NULL by definition is (void*)0 per POSIX, but can be redefined, >>> depending on the compiler, etc. >> >> No, it can't. The C language requires all integral expressions that >> evaluate to zero to convert to the NULL pointer. This is independent >> of the internal representation of the NULL pointer. >> >> So this change is an NOP for all compilers. It's a good STYLE change. > > Someone made an argument a few weeks ago about NULL being definable as a > non-zero value on some esoteric architectures or OSes.
No. That's confused. NULL must always be 0. A conversion between 0 and a pointer always must give a null-pointer. Always. You can't defined NULL to -1 ever. Even if that happens to be the binary representation of a NULL pointer, it must be 0. > I agree though, this is largely stylistic/pedantic for a good cause. If > someone set NULL to something non-zero in value, they would be looking for > pain :). You can never set NULL to non-zero integral value (possibly with a cast). You can have the internal representation have non-zero bits, but the compiler must hide that. This does mean that M_ZERO and calloc() won't set pointers to null pointers on such architectures, but this 0 that you replaced is completely safe. I can provide references to the appropriate standards. I made the same point when someone made that (incorrect) argument. _______________________________________________ svn-src-head@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
