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Date: Sun, 23 Sep 2018 03:10:14 +0000
From: Pascal Cuoq <>
To: "" <>
Subject: Re: un-UBify-strings

On 23 Sep 2018, at 04:45, Rich Felker <<>> wrote:
I'm also trying to fix the UB in
address range checks for implementing memmove as memcpy, etc. Is this

if ((uintptr_t)s-(uintptr_t)d-n <= -2*n) return memcpy(d, s, n);


It looks okay to me. You want to test whether (uintptr_t)s-(uintptr_t)d, computed as a mathematical integer, is between -n and n, and since uintptr_t is unsigned, you are using the well-known trick of aligning one of the bounds with 0 so that both inequalities can be tested in one instruction.

It would seen more natural to me to work on the right-hand side of zero, that it, to compute (uintptr_t)s-(uintptr_t)d+n and to check whether that is <= 2*n (overlap) or > 2*n (no overlap). The generated code may even be one instruction shorter. Apart from that, as long as we have the hypothesis that n <= UINTPTR_MAX/2, I cannot immediately see any reason why it would not work.


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