On Wed, Mar 05, 2025 at 02:17:32PM +0000, David Laight wrote: > On Wed, 5 Mar 2025 11:51:59 +0300 > Dan Carpenter <dan.carpenter@xxxxxxxxxx> wrote: > > > On Wed, Mar 05, 2025 at 09:40:43AM +0100, Markus Elfring wrote: > > > >>> The address of a data structure member was determined before > > > >>> a corresponding null pointer check in the implementation of > > > >>> the functions “dpu_hw_pp_enable_te” and “dpu_hw_pp_get_vsync_info”. > > > >>> > > > >>> Thus avoid the risk for undefined behaviour by removing extra > > > >>> initialisations for the variable “c” (also because it was already > > > >>> reassigned with the same value behind this pointer check). > > > > There is no undefined behavior here. > > > > > > Is there a need to improve the wording precision? > > > > > > There are words which denote a special meaning according to aspects of > > > the programming language “C”. > > > https://en.cppreference.com/w/c/language/behavior > > > > > > Dereferences of null pointers are treated in special ways. > > > > This not a dereference. It's just pointer math. > > And the 'fun' starts because NULL isn't required to use the all-zero > bit pattern. > Regardless of the bit-pattern, things like (void *)(1 - 1) are valid > NULL pointers. > > Of course, while C allows this, I doubt NULL has ever been other than 0. > (It was 0 on a system I used many years ago where the O/S invalid pointer > was ~0.) Kernel style guidelines don't even allow if (p == NULL) so we would be screwed. :P > > I know Clang has started warning about arithmetic on NULL. Huh. You're right. $ clang -Weverything test.c test.c:13:22: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic] 13 | printf("%p\n", NULL + 1); | ~~~~ ^ test.c:13:22: warning: arithmetic on a pointer to void is a GNU extension [-Wgnu-pointer-arith] 13 | printf("%p\n", NULL + 1); | ~~~~ ^ test.c:11:14: warning: unused parameter 'argc' [-Wunused-parameter] 11 | int main(int argc, char *argv[]) | ^ test.c:11:26: warning: unused parameter 'argv' [-Wunused-parameter] 11 | int main(int argc, char *argv[]) | ^ test.c:13:17: warning: unsafe pointer arithmetic [-Wunsafe-buffer-usage] 13 | printf("%p\n", NULL + 1); | ^~~~ /usr/lib/llvm-19/lib/clang/19/include/__stddef_null.h:26:14: note: expanded from macro 'NULL' 26 | #define NULL ((void*)0) | ^~~~~~~~~~ 5 warnings generated. Well, that's stupid. regards, dan carpenter
