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Message-ID: <bebe5f59-0943-0f5c-cd12-f6afdc327e6e@cs.ucla.edu>
Date: Thu, 25 Feb 2021 11:57:58 -0800
From: Paul Eggert <eggert@...ucla.edu>
To: David Goldblatt <davidtgoldblatt@...il.com>
Cc: libc-coord@...ts.openwall.com
Subject: Re: Sized deallocation for C
The draft text says:
"If ptr is the result obtained from a call to malloc(size),
realloc(old_ptr, size), or calloc(nmemb, memb_size), where nmemb *
memb_size is equal to size, this function behaves equivalently to
free(ptr)."
Suppose my code does the following but nmemb * memb_size overflows and
does not fit into size_t:
void
sample (size_t nmemb, size_t memb_size)
{
void *ptr = calloc (nmemb, memb_size);
if (ptr != NULL)
do_something_with (ptr);
free_sized (ptr, nmemb * memb_size);
}
It's not clear from the wording whether behavior would be defined here,
in the typical case where size_t is at least as width as int so the
overflow wraps around in a well-defined way. Presumably the intent is
that behavior should be defined. (Admittedly this is a weird corner
case, but corner cases are what standards are for....)
A simple fix for this problem would be to change the above-quoted text
to this:
"If ptr is a null pointer, or is the result obtained from a call to
malloc(size), realloc(old_ptr, size), or calloc(nmemb, memb_size) where
size is the mathematical product of nmemb and size, this function
behaves equivalently to free(ptr)."
and similarly for free_aligned_sized. Plus, we add a requirement to
calloc that it must return NULL if nmemb * memb_size does not fit into
size_t.
Alternatively there could be a different 'free' function for calloc, but
this proposal already has too many 'free' functions.
PS. Come to think of it, we could easily get by with one new 'free'
function instead of two, as free_aligned_sized (ptr, 0, size) could take
on the role of free_sized(ptr, size). I'd prefer this simplification.
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