Re: The return value of realloc(p,0)

From	Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups	comp.lang.c
Subject	Re: The return value of realloc(p,0)
Date	2023-09-01 00:14 -0700
Organization	A noiseless patient Spider
Message-ID	<86wmxas6ty.fsf@linuxsc.com> (permalink)
References	<ua63if$2u50c$2@news.xmission.com> <DeyxM.134678$ens9.28488@fx45.iad> <kl972jF24chU3@mid.individual.net> <86edjju668.fsf@linuxsc.com> <ucqrn0$3dvle$1@dont-email.me>

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Vir Campestris <vir.campestris@invalid.invalid> writes:

> On 31/08/2023 06:33, Tim Rentsch wrote:
>
>> There are reasons for realloc() to zize 0 to be a special case.
>> Specifically, because the pointer value returned doesn't have to
>> be dereferencable, it can point to parts of the address space
>> that are otherwise unusable.  The overhead for such "zero-byte
>> blocks" is very low, just a smidge over one bit.  The usuable
>> memory previously allocated can be reclaimed and used in its
>> entirely for subsequent allocations.  If running in a 64-bit
>> address space, vast amounts of otherwise unusable areas can
>> be used for these "special objects", having very little impact
>> on memory usable for actual objects.
>
> Tim, I'm confused by a little bit of that:
>
> "just a smidge over one bit"
>
> I can see how you might point them all to a single place that can't be
> read or written, and have safety in case anyone tries to use them.
>
> I can see that you might have a section of memory with a series of
> bytes, each one reserved for these empty allocations.  It might be
> inaccessible, but will still use address space.
>
> But how do you get anything smaller than a byte, but non-zero?  I've
> only ever used one CPU with addressing smaller than a byte.

I was tempted to make a joke here, but I couldn't see how to turn
out something funny.

What I meant was just over one bit /per zero-byte block/.  So if
there were 20,000 zero-byte blocks, they could be managed with
just a little more than 2500 bytes of allocated memory.  Sorry
for the confusion.

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Re: The return value of realloc(p,0) Robert Latest <boblatest@yahoo.com> - 2023-08-30 15:54 +0000
  Re: The return value of realloc(p,0) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-30 22:33 -0700
    Re: The return value of realloc(p,0) Vir Campestris <vir.campestris@invalid.invalid> - 2023-08-31 21:03 +0100
      Re: The return value of realloc(p,0) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-01 00:14 -0700
        Re: The return value of realloc(p,0) Vir Campestris <vir.campestris@invalid.invalid> - 2023-09-01 11:32 +0100
          Re: The return value of realloc(p,0) Kaz Kylheku <864-117-4973@kylheku.com> - 2023-09-01 17:08 +0000
          Re: The return value of realloc(p,0) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-01 11:42 -0700
            Re: The return value of realloc(p,0) scott@slp53.sl.home (Scott Lurndal) - 2023-09-01 19:06 +0000
              Re: The return value of realloc(p,0) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-03 01:45 -0700
                Re: The return value of realloc(p,0) scott@slp53.sl.home (Scott Lurndal) - 2023-09-03 16:12 +0000
                Re: The return value of realloc(p,0) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-03 12:01 -0700
                Re: The return value of realloc(p,0) scott@slp53.sl.home (Scott Lurndal) - 2023-09-03 21:40 +0000
                Re: The return value of realloc(p,0) Spiros Bousbouras <spibou@gmail.com> - 2023-09-03 23:08 +0000
                Re: The return value of realloc(p,0) Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2023-09-03 16:23 -0700
                Re: The return value of realloc(p,0) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-03 17:01 -0700
                Re: The return value of realloc(p,0) scott@slp53.sl.home (Scott Lurndal) - 2023-09-04 14:22 +0000
                Re: The return value of realloc(p,0) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-05 05:23 -0700
                Re: The return value of realloc(p,0) Vir Campestris <vir.campestris@invalid.invalid> - 2023-09-04 16:40 +0100
            Re: The return value of realloc(p,0) Vir Campestris <vir.campestris@invalid.invalid> - 2023-09-02 21:40 +0100
        Re: The return value of realloc(p,0) Kaz Kylheku <864-117-4973@kylheku.com> - 2023-09-01 17:03 +0000
          Re: The return value of realloc(p,0) Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2023-09-01 17:57 +0000
            Re: The return value of realloc(p,0) Kaz Kylheku <864-117-4973@kylheku.com> - 2023-09-01 18:44 +0000
              Re: The return value of realloc(p,0) Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2023-09-01 20:24 +0000
            Re: The return value of realloc(p,0) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-01 11:58 -0700
              Re: The return value of realloc(p,0) Kaz Kylheku <864-117-4973@kylheku.com> - 2023-09-01 19:14 +0000
                Re: The return value of realloc(p,0) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-03 12:03 -0700
          Re: The return value of realloc(p,0) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-01 11:55 -0700

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