Re: malloc(0) returns an invalid address

On Thu, 2004-12-02 at 00:41, JINMEI Tatuya / 神明達哉 wrote:
> > pointer, once you cast it to a 'char *', you cannot dereference it because
> > it does not point to a character. This same problem would occur with
> > 'malloc(1)' and 'int *'.
> 
> BTW: the "same problem" (of segfault) does actually NOT occur with
> malloc(1) and int * on FreeBSD 5.3 (i386).  I suspect malloc(3) takes
> a special action with the size of zero.

I believe he misspoke; the result is undefined in that case, since it's
not generally possible to enforce a writable size of 1 in hardware(*)
and malloc() is required to return memory aligned for any fundamental C
type regardless of the amount of memory allocated (i.e. malloc(1) isn't
permitted to return an odd address on hardware where types larger than
(char) must be aligned).

With a size of 0 it's easy to cheat:  return a "magic" minimal-sized
pointer into an unmapped page (or a read-only page, getting you a trap
if something tries to assign to it; but I think the low pages in the
address space are not mapped for standard demand-paged executables on
FreeBSD), and when it's realloc()ed to a non-zero size recognize the
"magic" value and return a real chunk of allocated memory.  

(*) it can be done on some processors/MMUs, on others it can be done for
a limited number of addresses (Intel debug registers?), on still others
(e.g. Intel 486) the best granularity you can get is 16-byte and it's
going to be expensive...

-- 
brandon s. allbery    [linux,solaris,freebsd,perl]     allbery_at_kf8nh.com
system administrator      [WAY too many hats]        allbery_at_ece.cmu.edu
electrical and computer engineering, carnegie mellon univ.         KF8NH