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Date: Thu, 06 Jul 2017 21:10:31 +0200
From: Leah Neukirchen <>
To: Bartosz Brachaczek <>
Subject: Re: Documentation of memcpy and undefined behavior in memset

Bartosz Brachaczek <> writes:

> On 7/6/2017 6:23 PM, Rich Felker wrote:
>> I think you're correct, at least under a pessimistic interpretation of
>> the standard. I can't find where they actually define "modifies", and
>> you could argue that assignment of the same value twice "modifies" the
>> object at most once, but I don't like relying on that kind of
>> ambiguity and it's easy enough to fix just by adding a sequence point.
> I don't have a copy of C11, but N1570 reads in a note to 3.1:
>> ‘‘Modify’’ includes the case where the new value being stored is the
>> same as the previous value.

C11 also specifies a sequence for assignment (

> The side effect of updating the stored value of the left operand is
> sequenced after the value computations of the left and right
> operands.

Leah Neukirchen  <>

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