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Re: proper realloc(p,0) behavior?

From: Eric Blake
Subject: Re: proper realloc(p,0) behavior?
Date: Fri, 25 Mar 2011 07:53:15 -0600
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On 03/25/2011 05:59 AM, Paolo Bonzini wrote:
> On 03/24/2011 08:47 PM, Eric Blake wrote:
>> No argument by me if we enforce saner semantics to xrealloc.
> What's the saner semantic?  One results in double frees, the other in
> memory leaks.  I think I prefer the latter, but perhaps the best choice
> is just to crash...

Hmm, thinking about this some more:

xmalloc guarantees that the result will be non-NULL (otherwise we called

Prior to yesterday's patch, xrealloc(NULL, 0) was guaranteed to return a
unique zero-size pointer (assuming GNU semantics), but now it returns
NULL (which means that Paul's patch yesterday introduced a regression).

Furthermore, xrealloc(p, 0) returns NULL (both before and after Paul's
patch yesterday, although the behavior before depended on the underlying
realloc behavior).  This means that xrealloc(p,0) violates the x*
premise of guaranteeing a non-NULL result (and calling xalloc_die on

Would anyone object to this patch?

diff --git i/ChangeLog w/ChangeLog
index 8988e31..0c2e87e 100644
--- i/ChangeLog
+++ w/ChangeLog
@@ -1,3 +1,9 @@
+2011-03-25  Eric Blake  <address@hidden>
+       xmalloc: never return NULL for xrealloc
+       * lib/xmalloc.c (xrealloc): Revert yesterday's change that broke
+       xrealloc(NULL,0), and reject xrealloc(p,0).
 2010-11-30  Reuben Thomas  <address@hidden>

        posix-modules: say what it does.
diff --git i/lib/xmalloc.c w/lib/xmalloc.c
index 4589e7d..e7eeaff 100644
--- i/lib/xmalloc.c
+++ w/lib/xmalloc.c
@@ -46,18 +46,23 @@ xmalloc (size_t n)
   return p;

-/* Change the size of an allocated block of memory P to N bytes,
-   with error checking.  */
+/* Change the size of an allocated block of memory P to N bytes, with
+   error checking.  This version explicitly rejects non-NULL P
+   combined with N of 0, since the GNU realloc semantics of freeing P
+   and returning NULL interfere with the promise of x*alloc methods
+   never returning NULL.  */

 void *
 xrealloc (void *p, size_t n)
   if (!n)
-      /* The GNU and C99 realloc behaviors disagree here.  Act like
-         GNU, even if the underlying realloc is C99.  */
-      free (p);
-      return NULL;
+      /* The GNU and C99 realloc behaviors disagree here.  We prefer
+         GNU semantics, but unless p is also NULL, those semantics
+         would require returning NULL.  */
+      if (p)
+        xalloc_die ();
+      n = 1;

   p = realloc (p, n);

Eric Blake   address@hidden    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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