On Mon, Mar 12, 2012 at 1:03 AM, Vijay Chauhan <kernel.vijay@xxxxxxxxx> wrote: > On Sat, Mar 10, 2012 at 12:10 AM, Manohar Vanga <manohar.vanga@xxxxxxxxx> wrote: >> Also, from the >> archives: http://www.mail-archive.com/kernelnewbies@xxxxxxxxxxxx/msg12320.html > > Thank you for links which gives good explanation. > But I am not able to understand the part accessing a member through > NULL pointer like ((size_t) &((TYPE *)0)->MEMBER) > How it is handled? This macro is interpreted at 'compile-time', i.e during the preprocessor stage...the NULL pointer exception would be generated if the code was 'run' at runtime. The typeof GCC builtin is also interpreted at compile-time. CMIIW. Hope that explains the seeming NULL pointer dereference. -mandeep > > In C code if we access a member through NULL pointer like this will > cause the code to crash. > Am I missing something? > > >> >> On Fri, Mar 9, 2012 at 7:21 PM, Vijay Chauhan <kernel.vijay@xxxxxxxxx> >> wrote: >>> >>> Hi, >>> >>> I'm checking the container_of and offsetof macro >>> >>> #define container_of(ptr, type, member) ({ \ >>> const typeof( ((type *)0)->member ) *__mptr = (ptr); >>> (type *)( (char *)__mptr - offsetof(type,member) );}) >>> >>> #define offsetof(TYPE, MEMBER) \ >>> ((size_t) &((TYPE *)0)->MEMBER) >>> >>> >>> I did not understand the first line of both macro. Will it not create >>> the NULL pointer dereference problem? I know it works and read some >>> article but it did not explain this part, so can anyone explain why >>> the NULL pointer error is not coming. Am I missing something? >>> Any C language specification of such example. >>> >>> _______________________________________________ >>> Kernelnewbies mailing list >>> Kernelnewbies@xxxxxxxxxxxxxxxxx >>> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies >> >> >> >> >> -- >> /manohar > > _______________________________________________ > Kernelnewbies mailing list > Kernelnewbies@xxxxxxxxxxxxxxxxx > http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
