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Subject: RE: memset (0, 0, 0);
Date: Tue, 08 Apr 2003 07:52:00 -0000
Message-ID: <9FF3133289A7A84E81E2ED8F5E56B379604386@sh-uk-ex01.uk.w2k.superh.com>
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From: "Thomas,Stephen" <stephen.thomas@superh.com>
To: "Geoff Keating" <geoffk@geoffk.org>
Cc: <gdb@sources.redhat.com>, <newlib@sources.redhat.com>, <bug-glibc@gnu.org>,
   "McGoogan,Sean" <sean.mcgoogan@superh.com>
X-SW-Source: 2003-04/txt/msg00067.txt.bz2


Hi Geoff,

Which xmalloc are you referring to? The xmalloc in this case is a gdb inter=
nal 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 =3D=3D 0)
    {
      val =3D NULL;
    }
  else
    {
      val =3D mmalloc (md, size);
      if (val =3D=3D NULL)
	nomem (size);
    }
  return (val);
}

So size=3D0 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]=20
Sent: 07 April 2003 18:18
To: Thomas,Stephen
Cc: gdb@sources.redhat.com; newlib@sources.redhat.com; bug-glibc@gnu.org; M=
cGoogan,Sean
Subject: Re: memset (0, 0, 0);


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

> Hi,
>=20
> gdb appears to call memset(0,0,0) from build_regcache() in=20
> gdb/regcache.c. I can't really claim to understand how this works, but=20
> this function appears to get called 3 times during gdb initialization:
>=20
>   static void build_regcache (void)
>   {
>     ...
>     int sizeof_register_valid;
>     ...
>     sizeof_register_valid =3D ((NUM_REGS + NUM_PSEUDO_REGS) * sizeof (*re=
gister_valid));
>     register_valid =3D xmalloc (sizeof_register_valid);
>     memset (register_valid, 0, sizeof_register_valid);
>   }
>=20
> On the 1st time of calling, none of the gdbarch stuff is set up, so=20
> NUM_REGS =3D NUM_PSEUDO_REGS =3D 0. So xmalloc gets called with size=3D0.=
=20
> That returns 0 as the 'address', which gets passed to memset. I guess=20
> this just works OK on other architectures (it does on x86 anyway).
>=20
> 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 =3D=3D 0)
    size =3D 1;
  newmem =3D malloc (size);
  if (!newmem)
    xmalloc_failed (size);
  return (newmem);

xmalloc_failed finishes with

  xexit (1);

so xmalloc should never return NULL.

--=20
- Geoffrey Keating <geoffk@geoffk.org>