Date: Sun, 19 Jan 2020 11:22:33 -0500 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: Minor style patch to exit.c On Sun, Jan 19, 2020 at 06:53:49PM +0300, Alexander Cherepanov wrote: > 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. Consider a function that takes a pointer p, an array a, and a length l, and does: for (i=0; i<l; i++) if (a+i == p) return p-a; Can f(_end,_start,k) and f(_start+k,_start,k) legitimately differ, despite _end==_start+k? I think the answer is no, in the existing C language, in that the result of an expression is a pure function of the *values* put into it. But compiler folks do not want to interpret it this way and are pushing through hidden "provenance" state, so... Rich
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