Date: Sun, 19 Jan 2020 19:33:08 +0300 From: Alexander Cherepanov <ch3root@...nwall.com> To: musl@...ts.openwall.com Subject: Re: Minor style patch to exit.c On 19/01/2020 17.46, Alexander Monakov wrote: > On Sun, 19 Jan 2020, Alexander Cherepanov wrote: > >> Couldn't _start defined as an array? Then separate values could be accessed >> simply as elements of this array. And casts to integers could be limited to >> calculating the number of elements, the terminating value or something. > > Yeah, I think usually such linker-provided symbols are declared as > extern arrays. I'm surprised that isn't the case in musl. I don't think > declaring them as arrays helps with making casts pedantically suitable for > calculating number of elements though - as you said, any bijection between > intptr_t and pointers would be a valid implementation of a cast, you're not Well, we want use from C some outside info, there could be no pedantic way to do this. Let's see, we know that the _end array follows the _start array in memory. This means that &_start[i] == &_end for some i. But different provenance of the pointers means that we cannot do it just like that. Adding a cast should fix this. Summarizing, it should look like this: for (size_t i = 0; (uintptr_t)&_start[i] != (uintptr_t)&_end; i++) or for (type *p = _start; (uintptr_t)p != (uintptr_t)_end; p++) > guaranteed that (intptr_t)&a[i] == (intptr_t)a + i * sizeof *a. While you are inside one object, I think this should be safe in practice. For gcc, this is more or less guaranteed by . BTW there is an explicit restriction there: "When casting from pointer to integer and back again, the resulting pointer must reference the same object as the original pointer, otherwise the behavior is undefined. That is, one may not use integer arithmetic to avoid the undefined behavior of pointer arithmetic as proscribed in C99 and C11 6.5.6/8."  https://gcc.gnu.org/onlinedocs/gcc/Arrays-and-pointers-implementation.html -- Alexander Cherepanov
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