Date: Sun, 19 Jan 2020 18:53:49 +0300 From: Alexander Cherepanov <ch3root@...nwall.com> To: musl@...ts.openwall.com Subject: Re: Minor style patch to exit.c On 19/01/2020 17.24, Markus Wichmann wrote: > On Sun, Jan 19, 2020 at 04:33:47PM +0300, 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. > > That reminds me of something I read in the C standard: Two pointers must > compare equal if, among other possibilities, one is a pointer to > one-past its underlying array, and the other is a pointer to the start > of its array, and the arrays happen to lie behind one another in address > space. One of the gcc bug reports I mentioned is exactly about this issue. DR 260 allows to take the provenance of the pointers into account when comparing them and gcc really does this.  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61502  http://open-std.org/jtc1/sc22/wg14/www/docs/dr_260.htm As a side note, I thinks this is the wildest gcc bug report, it contains really mind-blowing comments (like comment 3). I don't mean it in a bad way at all and if you want to turn your understanding of C language inside-out you can try to read it. OTOH I think it's all wrong after all and I have some hope for it to be settled after my recent comments there. But I don't hold my breath. > Therefore, if _start and _end were arrays, even the GCC devs must agree > that there might be an integer i such that _start + i == _end. For the C > language, _start and _end would be arrays that happen to lie adjacent in > address space. > > And if we have guarantees from the outside attesting to that, then > _end - _start is no longer an undefined expression, right? Even if we know that _start + k == _end it doesn't mean that we allowed to subtract them. -- Alexander Cherepanov
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