Re: [libvirt] [PATCH] build: make ATTRIBUTE_NONNULL() a NOP unless STATIC_ANALYSIS is on

On 04/26/2012 12:56 AM, Laine Stump wrote: > The ATTRIBUTE_NONNULL(m) macro normally resolves to the gcc builtin > __attribute__((__nonnull__(m))). The effect of this in gcc is > unfortunately only to make gcc believe that "m" can never possibly be > NULL, *not* to add in any checks to guarantee that it isn't ever NULL > (i.e. it is an optimization aid, *not* something to verify code > correctness.) - see the following gcc bug report for more details: > > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17308 > > Static source analyzers such as clang and coverity apparently can use > ATTRIBUTE_NONNULL(), though, to detect dead code (in the case that the > arg really is guaranteed non-NULL), as well as situations where an > obviously NULL arg is given to the function. > > https://bugzilla.redhat.com/show_bug.cgi?id=815270 is a good example > of a bug caused by erroneous application of ATTRIBUTE_NONNULL(). > Several people spent a long time staring at this code and not finding > the problem, because the problem wasn't in the function itself, but in > the prototype that specified ATTRIBUTE_NONNULL() for an arg that > actually *wasn't* always non-NULL, and caused a segv when dereferenced > (even though the code that dereferenced the pointer was inside an if() > that checked for a NULL pointer, that code was optimized out by gcc). > > There may be some very small gain to be had from the optimizations > that can be inferred from ATTRIBUTE_NONNULL(), but it seems safer to > err on the side of generating code that behaves as expected, while > turning on the attribute for static analyzers. > > (dissenting opinions welcome :-) None from me :)