[TUHS] Did realloc ever zero the new memory?

[jgevaryahu@hotmail.com (Jonathan Gevaryahu)]

On 9/13/2015 11:20 PM, Greg 'groggy' Lehey wrote:
> On Sunday, 13 September 2015 at 16:09:55 +1000, Dave Horsfall wrote:
>> On Sat, 12 Sep 2015, Larry McVoy wrote:
>>
>>> That's a really good point.  Anyone like these?
>>>
>>> void *alloc(size_t bytes);
>>> void *realloc(void *old, size_t want);
>>>
>>> void *zalloc(size_t bytes);
>>> void *zealloc(void *old, size_t want);
>>>
>>> So alloc is new, we can have that.  Realloc() is OK?  Or not?  Does the
>>> current one have weird semantics?
>>>
>>> zalloc/zealloc are new, we can have those.
>>>
>>> If you guys like these I can push on Linus and the glibc people.
>> I'll vote for those, although my dev days are pretty much over.  I've
>> never liked malloc() etc not clearing memory, although it has been, ahem,
>> useful, in the past...
> http://xkcd.com/927/
>
> Greg
Why not crealloc? pronounced 'cereal-oc'? it would behave the same as 
realloc, but explicitly zeroes the area upon allocating it (and maybe 
also upon reducing the size of the area, it zeroes the area freed, or 
zeroes the area the memory structure was previously located at, if the 
whole memory structure has to be moved.)

Borrowing from 
http://pubs.opengroup.org/onlinepubs/009695399/functions/realloc.html :

The /crealloc/() function shall change the size of the memory object 
pointed to by /ptr/ to the size specified by /size/. The contents of the 
object shall remain unchanged up to the lesser of the new and old sizes. 
If the new size of the memory object would require movement of the 
object, the space for the previous instantiation of the object is 
zeroed, then freed. If the new size is larger, the contents of the newly 
allocated portion of the object are explicitly set to the value of 0. If 
/size/ is 0 and /ptr/ is not a null pointer, the object pointed to is 
zeroed, then freed. If the space cannot be allocated, the object shall 
remain unchanged.

If /ptr/ is a null pointer, /crealloc/() shall be equivalent to 
/calloc/() 
<http://pubs.opengroup.org/onlinepubs/009695399/functions/malloc.html> 
for the specified size.

If /ptr/ does not match a pointer returned earlier by /calloc/() 
<http://pubs.opengroup.org/onlinepubs/009695399/functions/calloc.html>, 
/malloc/() 
<http://pubs.opengroup.org/onlinepubs/009695399/functions/malloc.html>, 
/realloc/() or /crealloc/() or if the space has previously been 
deallocated by a call to /free/() 
<http://pubs.opengroup.org/onlinepubs/009695399/functions/free.html>, 
/realloc/() or /crealloc/(), the behavior is undefined.

The order and contiguity of storage allocated by successive calls to 
/crealloc/() is unspecified. The pointer returned if the allocation 
succeeds shall be suitably aligned so that it may be assigned to a 
pointer to any type of object and then used to access such an object in 
the space allocated (until the space is explicitly freed or 
reallocated). Each such allocation shall yield a pointer to an object 
disjoint from any other object. The pointer returned shall point to the 
start (lowest byte address) of the allocated space. If the space cannot 
be allocated, a null pointer shall be returned.


-- 
Jonathan Gevaryahu AKA Lord Nightmare
jgevaryahu at gmail.com
jgevaryahu at hotmail.com

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