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

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Robert Latest <boblatest@yahoo.com> writes:

> Richard Damon wrote:
>
>> My understanding is that, in effect, the call to realloc(p,0), if
>> it is using the second option of returning a suitable pointer, is
>> effectively do a free(p), and then allocating a "0 byte" block of
>> memory and returning the address of that.
>
> Or it just returns p, as it might do whenever the requested new size
> is smaller than the already-allocated size.  No reason for 0 to be a
> special case.

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.

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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) 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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