On Wed, Aug 01, 2018 at 03:28:51PM +0300, Yury Norov wrote: > Hi Oleg, > > On Wed, Aug 01, 2018 at 12:58:01PM +0300, Oleg wrote: > > Hello. > > > > I have a code where i need more than one "wrong" pointer value(this one is > > NULL pointer). Something like a signal handler SIG_IGN that equal to 1. > > > > So, my question is, what is the guarantied range of addresses (from 0 to X), > > which can be safely used for this purpose on any hardware architecture? > > > > Thanks. > > > > -- > > Олег Неманов (Oleg Nemanov) > > -- > > Assuming your code is userspace code, right? Yes. > There is no generic rule that NULL is invalid pointer. One can mmap() > memory at NULL with MAP_FIXED and it will make NULL dereferencable. You broke my world :-). > Though, usually the whole page containing NULL is not mapped to > userspace. So, i can assume that address from 0 to 4k can't be returned from malloc (or be in code in form of a function pointer or pointer to a global variable) and i can use it to specify a value distinguished from any pointer? > You can check valid mappings with 'cat /proc/<pid>/maps'. > Dereferencing of any address out of that list will cause segfault. > > If you need the range of addresses that cause segfaults if > dereferenced, the most portable way would be mapping sufficient > amount of memory with PROT_NONE. No-no. E.g. i have some function that set an action on some event: int set_event_act(enum event e, void (*act)(void)); User of set_event_act() can supply a custom act() for event e OR use one of predefined actions. E.g.: #define ACT_NONE 0 #define ACT_ACT1 1 #define ACT_ACT2 2 and later in a code: set_event_act(e, ACT_NONE); or set_event_act(e, ACT_ACT1); or set_event_act(e, ACT_ACT2); or set_event_act(e, custom); If i understand you correctly, i can do this safely, can't i? -- Олег Неманов (Oleg Nemanov) -- To unsubscribe from this list: send the line "unsubscribe linux-c-programming" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
