Re: mudflap problem

["Doug Graham" <dgraham at nortelnetworks dot com>]

On Wed, Sep 29, 2004 at 06:50:40PM -0400, Graham, Doug [CAR:QT96:EXCH] wrote:
> On Wed, Sep 29, 2004 at 06:05:56PM -0400, Frank Ch. Eigler wrote:
> > 
> > > I thought that since b+16 points to the base of a, mudflap would not
> > > complain about the dereference in foo().  But it did somehow know that
> > > the pointer passed in to foo() was derived from 'b' rather than 'a'.
> > 
> > I believe this is an optimization artifact.  With "-O2", the declaration of
> > "a" is marked somehow as unused (it is), and libmudflap does not get
> > a startup-time registration call for it.  Thus b[16] is checked and is found
> > to refer to an invalid address.
> 
> Aha!  You're right, only 'b' is passed to __mf_register.  This happened
> without -O2; my compile line was just: gcc -fmudflap -o foo foo.c.

Hey wait a minute.  Isn't this a bug?  a[16] has external linkage,
so mudflap can't know that it isn't going to be accessed from some
other compilation unit.  When I add a second source file that
does this:

  extern int a[];
  void baz() { a[0] = 42; }

mudflap complains during compilation of this new module that it can't
track the lifetime of `a', and then complains at runtime when legal
accesses to a[] are made.  I'm not sure what the compilation warning
is all about, as a[]'s lifetime is obviously the lifetime of the program,
but the runtime warning occurs because a[] wasn't registered.

--Doug.