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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c
Subject: Re: memcpy can't take null.
Date: Thu, 09 Apr 2020 07:16:02 -0700
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Richard Damon <Richard@Damon-Family.org> writes:

> On 4/8/20 5:06 AM, Malcolm McLean wrote:
>
>> If Nemployees is non-zero while the pointer employees points to invalid
>> memory, it's likely that the bug will be discovered soon enough.
>> But if Nemployees is zero and employees is NULL, then the loops through
>> the employees array will all be no-ops and have defined, correct
>> behaviour.  It's only if you call a library function like memcpy that
>> the code becomes incorrect.
>
> The issue is that since memcpy is a very basic function, you want it as
> efficient as possible.  On some machines, it may be quicker to load the
> pointers into registers that might object to having a null pointer
> before processing the loop, and thus the machine might trap on a null
> pointer.
>
> Also, if you really want this safe version, you can make a memcpy_safe0
> function defined as:
>
> void* memcpy_safe0(void* dest, const void* src, size_t num) {
>   if(num) return memcpy(dest, src, num);
>   else return dest;
> }
>
> Note, that if the library function includes the check, there is no way
> to wrap it to remove that check, thus it make sense to make the standard
> version the quick one.
>
> Note that in the above, with a decent optimizing compiler, the call to
> memcpy above is tail-recursive,

It is a tail call, but not a recursive tail call.