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Re: memcpy can't take null.

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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.

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memcpy can't take null. Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2020-04-05 06:47 -0700
  Re: memcpy can't take null. Öö Tiib <ootiib@hot.ee> - 2020-04-05 07:30 -0700
    Re: memcpy can't take null. Richard Damon <Richard@Damon-Family.org> - 2020-04-05 14:39 -0400
      Re: memcpy can't take null. Philipp Klaus Krause <pkk@spth.de> - 2020-04-06 18:46 +0200
        Re: memcpy can't take null. Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-04-06 12:45 -0700
          Re: memcpy can't take null. Philipp Klaus Krause <pkk@spth.de> - 2020-04-07 07:28 +0200
  Re: memcpy can't take null. Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-04-05 16:00 -0700
    Re: memcpy can't take null. Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2020-04-05 18:29 -0600
      Re: memcpy can't take null. scott@slp53.sl.home (Scott Lurndal) - 2020-04-06 18:08 +0000
        Re: memcpy can't take null. Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2020-04-06 15:44 -0600
    Re: memcpy can't take null. Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2020-04-06 13:54 -0700
      Re: memcpy can't take null. Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-04-06 14:30 -0700
  Re: memcpy can't take null. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-04-06 01:20 +0100
  Re: memcpy can't take null. Siri Cruise <chine.bleu@yahoo.com> - 2020-04-05 19:09 -0700
  Re: memcpy can't take null. Guillaume <message@bottle.org> - 2020-04-07 16:50 +0200
    Re: memcpy can't take null. Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2020-04-07 09:19 -0700
    Re: memcpy can't take null. Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-04-07 09:21 -0700
      Re: memcpy can't take null. Guillaume <message@bottle.org> - 2020-04-08 01:41 +0200
        Re: memcpy can't take null. scott@slp53.sl.home (Scott Lurndal) - 2020-04-08 00:30 +0000
          Re: memcpy can't take null. Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2020-04-08 02:06 -0700
            Re: memcpy can't take null. Richard Damon <Richard@Damon-Family.org> - 2020-04-08 09:17 -0400
              Re: memcpy can't take null. Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-04-09 07:16 -0700
            Re: memcpy can't take null. Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-04-08 13:33 -0700
              Re: memcpy can't take null. Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2020-04-09 09:05 -0700
                Re: memcpy can't take null. Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-04-09 13:28 -0700
                Re: memcpy can't take null. richard@cogsci.ed.ac.uk (Richard Tobin) - 2020-04-09 23:41 +0000
          Re: memcpy can't take null. Guillaume <message@bottle.org> - 2020-04-08 16:17 +0200
    Re: memcpy can't take null. Ike Naar <ike@sdf.lonestar.org> - 2020-04-07 22:01 +0000
      Re: memcpy can't take null. Guillaume <message@bottle.org> - 2020-04-08 01:32 +0200

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