Can realloc(p,0) return NULL when p is non-NULL and memory aplenty?

Can realloc(p,0) return NULL when p is non-NULL and memory aplenty?

Post by Francois G » Sat, 08 Mar 2008 02:54:23


When running the following code under MinGW, I get
realloc(p,0) returned NULL
Is that a non-conformance?

TIA,
Francois Grieu

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
void *p;
p = malloc(0);
if (p==NULL)
puts("malloc(0) returned NULL");
else
{
p = realloc(p,0);
if (p==NULL)
puts("realloc(p,0) returned NULL");
else
puts("realloc(p,0) did not return NULL");
}
return 0;
}
 
 
 

Can realloc(p,0) return NULL when p is non-NULL and memory aplenty?

Post by Joachim Sc » Sat, 08 Mar 2008 03:01:31


Whether malloc(0) returns NULL or a pointerr to 0 bytes is implementation
defined
I think the same is true for realloc(p, 0)

Bye, Jojo

 
 
 

Can realloc(p,0) return NULL when p is non-NULL and memory aplenty?

Post by roberso » Sat, 08 Mar 2008 03:03:02

In article < XXXX@XXXXX.COM >,


No, it is conformance, and returning non-NULL would be non-conformance.

C89 4.10.3.4 The realloc Funciton

If size is zero and ptr is not a null pointer, the object
it points to is freed.

Returns

The realloc function returns either a null pointer or a
pointer to the possibly moved allocated space.


In the case of zero as the size, there is no more allocated space
(because zero size requires freeing), so the other branch of the
'either' kicks in, requiring realloc to return a null pointer
for this case.
--
amazon.com's top 8 books about "walter" are Kotzwinkle/ Gundy/ Colman's
"Walter the Farting Dog"
 
 
 

Can realloc(p,0) return NULL when p is non-NULL and memory aplenty?

Post by Joachim Sc » Sat, 08 Mar 2008 03:18:40


From n1256:
7.20.3 Memory management functions

1 The order and contiguity of storage allocated by successive calls to the
calloc,
malloc, and realloc functions is unspecified. The pointer returned if the
allocation
succeeds is suitably aligned so that it may be assigned to a pointer to any
type of object
and then used to access such an object or an array of such objects in the
space allocated
(until the space is explicitly deallocated). The lifetime of an allocated
object extends
from the allocation until the deallocation. Each such allocation shall yield
a pointer to an
object disjoint from any other object. The pointer returned points to the
start (lowest byte
address) of the allocated space. If the space cannot be allocated, a null
pointer is
returned. If the size of the space requested is zero, the behavior is
implementationdefined:
either a null pointer is returned, or the behavior is as if the size were
some
nonzero value, except that the returned pointer shall not be used to access
an object.

...

7.20.3.4 The realloc function
...
3 If ptr is a null pointer, the realloc function behaves like the malloc
function for the
specified size



So it surely seems implementation defined to me.



Bye, Jojo
 
 
 

Can realloc(p,0) return NULL when p is non-NULL and memory aplenty?

Post by Joachim Sc » Sat, 08 Mar 2008 03:20:52


Oops, that section is irrelevant...

But from the 1st section I think this still holds true

Bye, Jojo
 
 
 

Can realloc(p,0) return NULL when p is non-NULL and memory aplenty?

Post by Joachim Sc » Sat, 08 Mar 2008 03:23:25


Esp. as the section Walter quoted isn't in n1256 (anymore?)

Bye, Jojo
 
 
 

Can realloc(p,0) return NULL when p is non-NULL and memory aplenty?

Post by Harald van » Sat, 08 Mar 2008 03:23:28


While you're not at all wrong, please keep in mind that this is one of the
areas in which C99 differs from the previous standard. In C99, it's
unspecified whether realloc(p, 0) returns a null pointer, but if it
returns a null pointer, then p is *not* freed.
 
 
 

Can realloc(p,0) return NULL when p is non-NULL and memory aplenty?

Post by roberso » Sat, 08 Mar 2008 03:32:18

In article <da591$47d036a0$541dfcd3$ XXXX@XXXXX.COM >,







Good point -- but as best I -recall- MinGW is not C99.

--
"Beware of bugs in the above code; I have only proved it correct,
not tried it." -- Donald Knuth
 
 
 

Can realloc(p,0) return NULL when p is non-NULL and memory aplenty?

Post by Francois G » Sat, 08 Mar 2008 03:47:58

On Mar 6, 7:03 pm, XXXX@XXXXX.COM (Walter Roberson)



I disagree (at least) about the second member of that answer under
C99. IMHO realloc(p,0) is allowed to return the same thing as
malloc(0), which is not required to return NULL.

I reason that when p is a valid non-NULL pointer to a pointer returned
by malloc() and memory is aplenty, realloc(p,0) must return a pointer
to a new object that has the size 0, which is the same thing a what
malloc(0) should return, which is not required to be NULL, and is not
NULL in many implementations, including that one I'm strugling with.



C99 says {
void *realloc(void *ptr, size_t size);

The realloc function deallocates the old object pointed to by ptr and
returns a pointer to a new object that has the size specified by size.
The contents of the new object shall be the same as that of the old
object prior to deallocation, up to the lesser of the new and old
sizes. Any bytes in the new object beyond the size of the old object
have indeterminate values.

If ptr is a null pointer, the realloc function behaves like the malloc
function for the specified size. Otherwise, if ptr does not match a
pointer earlier returned by the calloc, malloc, or realloc function,
or if the space has been deallocated by a call to the free or realloc
function, the behavior is undefined. If memory for the new object
cannot be allocated, the old object is not deallocated and its value
is unchanged.

The realloc function returns a pointer to the new object (which may
have the same value as a pointer to the old object), or a null pointer
if the new object could not be allocated.
}




I get the same result "realloc(p,0) returned NULL" with the source
changed to the deeper test:

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
void *p = malloc(1);
if (p==NULL)
puts("Memory is very low"); // does not occur
if (malloc(0)==NULL)
puts("malloc(0) returned NULL"); // does not occur
if (realloc(NULL,0)==NULL)
puts("realloc(NULL,0) returned NULL"); // does not occur
if (realloc(p,0)==NULL)
puts("realloc(p,0) returned NULL"); // occurs ?!
if (malloc(10000)==NULL)
puts("Memory was rather low"); // does not occur
}


So I get an implementation of realloc(ptr,0) that turns a NULL ptr to
a non-NULL result, and non-NULL ptr to NULL result; which is at the
very least a surprise, non-orthogonal, and error-prone. In my case a
perfectly sound-looking "read the whole file in memory, in several
chuncks as necessary" function failed for an empty file, claiming
there is no memory.

Francois Grieu
 
 
 

Can realloc(p,0) return NULL when p is non-NULL and memory aplenty?

Post by Harald van » Sat, 08 Mar 2008 03:48:53


7.20.3p1 applies to all allocation functions.
"If the size of the space requested is zero, the behavior is
implementation-defined: either a null pointer is returned, or the
behavior is as if the size were some nonzero value, except that the
returned pointer shall not be used to access an object."

This allows realloc(p, 0) to unconditionally fail (since the text you
quoted specifies that a null pointer return value signifies failure for
realloc).


<nit>realloc(p, 0), if it succeeded, hasn't allocated 0 bytes. It has
allocated one or more bytes, plus whatever information free needs to give
it back later.</nit>

Anyway, even if it weren't specifically allowed to always fail, it could
still for example fail on implementations where if the size is small,
*alloc returns pointers to pre-allocated buckets depending on the
requested size.
 
 
 

Can realloc(p,0) return NULL when p is non-NULL and memory aplenty?

Post by Joachim Sc » Sat, 08 Mar 2008 17:04:50


<snip>
Probably not with standrd C, but AFAIK POSIX reqires malloc to set errno on
failure.
So if malloc() return NULL, lookup errno, if that is 0, no failure occured.

Bye, Jojo
 
 
 

Can realloc(p,0) return NULL when p is non-NULL and memory aplenty?

Post by Micah Cowa » Sun, 09 Mar 2008 02:40:55

CBFalconer < XXXX@XXXXX.COM > writes:


While that's true, I'm not sure I see what practical use it would make
to be able to determine the difference between NULL returned because
of a system error, and NULL returned because you asked for either that
or an unusable pointer.

That being said, I agree about reallocing 1 or more bytes, since doing
0 without knowing that your pointer will be freed is pretty useless.

--
Micah J. Cowan
Programmer, musician, typesetting enthusiast, gamer...
http://www.yqcomputer.com/