On Thu, Feb 21, 2008 at 7:59 AM, Mike Frysinger [off-list ref] wrote:

On Thursday 21 February 2008, Lasse Kärkkäinen wrote:
 > The man page says that realloc(ptr, 0) is equivalent to free, even
 > though it isn't. The text on the man page says
 >
 > ---
 > realloc()  changes the size of the memory block pointed to by ptr to
 > size bytes.  The contents will be unchanged to the minimum of the old
 > and new sizes;  newly  allocated memory  will  be  uninitialized.   If
 > ptr  is  NULL,  the call is equivalent to malloc(size); if size is equal
 > to zero, the call is equivalent to free(ptr).  Unless ptr is NULL, it
 > must have been returned by an earlier call to malloc(), calloc() or
 > realloc().  If the area pointed to was moved, a free(ptr) is done.
 > [...]
 > realloc()  returns a pointer to the newly allocated memory, which is
 > suitably aligned for any kind of variable and may be different from ptr,
 > or NULL if the request fails.  If  size  was equal to 0, either NULL or
 > a pointer suitable to be passed to free() is returned.  If realloc()
 > fails the original block is left untouched; it is not freed or moved.

 i would just word it to say that when realloc() is given a size of 0, it is
 implementation defined as to the behavior, but it tends to match the behavior
 of malloc(0) (which too is implementation defined).  POSIX and C99 allow both
 cases to return either a NULL pointer or a "unique" pointer.  glibc will
 return a unique pointer (which cannot actually be used to store anything),
 but uClibc may return NULL.
 -mike

Lasse, thanks for raising this; Mike, thanks for your input.

For man-pages-2.79, I propose to amend the description of realloc() to be:

       realloc() changes the size of the memory block pointed to
       by  ptr to size bytes.  The contents will be unchanged to
       the minimum of the old and  new  sizes;  newly  allocated
       memory  will  be uninitialized.  If ptr is NULL, then the
       call is equivalent to malloc(size); if size is  equal  to
       zero, and ptr is not NULL, then the call is equivalent to
       free(ptr).   Unless  ptr  is  NULL,  it  must  have  been
       returned  by  an  earlier  call  to malloc(), calloc() or
       realloc().  If the area pointed to was moved, a free(ptr)
       is done.

Note that the page already says the following about malloc(0):

       If size is 0, then malloc() returns either NULL, or a unique
       pointer value that can later be successfully passed to free().

Does this seem okay to you folks?

Cheers,

Michael


-- 
Michael Kerrisk
Maintainer of the Linux man-pages project
http://www.kernel.org/doc/man-pages/
Want to report a man-pages bug?  Look here:
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