RE: memset (0, 0, 0);

["Thomas,Stephen" <stephen.thomas@superh.com>]

Hi Geoff,

Which xmalloc are you referring to? The xmalloc in this case is a gdb internal function, defined in gdb/utils.c:

PTR xmalloc (size_t size)
{
  return xmmalloc (NULL, size);
}

And xmmalloc is:

void * xmmalloc (void *md, size_t size)
{
  void *val;

  if (size == 0)
    {
      val = NULL;
    }
  else
    {
      val = mmalloc (md, size);
      if (val == NULL)
	nomem (size);
    }
  return (val);
}

So size=0 does indeed return NULL. Also, I have single stepped this code to verify that this is actually what happens.

Steve Thomas
SuperH (UK) Ltd.

-----Original Message-----
From: Geoff Keating [mailto:geoffk@geoffk.org] 
Sent: 07 April 2003 18:18
To: Thomas,Stephen
Cc: gdb@sources.redhat.com; newlib@sources.redhat.com; bug-glibc@gnu.org; McGoogan,Sean
Subject: Re: memset (0, 0, 0);


"Thomas,Stephen" <stephen.thomas@superh.com> writes:

> Hi,
> 
> gdb appears to call memset(0,0,0) from build_regcache() in 
> gdb/regcache.c. I can't really claim to understand how this works, but 
> this function appears to get called 3 times during gdb initialization:
> 
>   static void build_regcache (void)
>   {
>     ...
>     int sizeof_register_valid;
>     ...
>     sizeof_register_valid = ((NUM_REGS + NUM_PSEUDO_REGS) * sizeof (*register_valid));
>     register_valid = xmalloc (sizeof_register_valid);
>     memset (register_valid, 0, sizeof_register_valid);
>   }
> 
> On the 1st time of calling, none of the gdbarch stuff is set up, so 
> NUM_REGS = NUM_PSEUDO_REGS = 0. So xmalloc gets called with size=0. 
> That returns 0 as the 'address', which gets passed to memset. I guess 
> this just works OK on other architectures (it does on x86 anyway).
> 
> Easy enough to fix I suppose, but is that really the point?

xmalloc is never supposed to return 0, and in fact, there's code to prevent it:

  if (size == 0)
    size = 1;
  newmem = malloc (size);
  if (!newmem)
    xmalloc_failed (size);
  return (newmem);

xmalloc_failed finishes with

  xexit (1);

so xmalloc should never return NULL.

-- 
- Geoffrey Keating <geoffk@geoffk.org>