Re: C compiler warning messages?

John Roberts <johnr@ims.com> wrote:
...
>The situation was a function like this:
>
> char *fun()
> {
> char rstr[20];
> return (char *)&rstr[0];
> }
...


>However, in general, it seems to me that the C compiler should of
>given me a warning about returning a pointer to a string about to go
>out of scope.


Certainly, a compiler could generate a warning for your particular
example by treating the return statement as a special case.
Unfortunately, there's no general way to solve the problem without
generating many spurious warning messages.


Consider:
...
if( some_test() ) {
pstr = &stack_array[0];
} else {
pstr = malloc(42);
}
At the end of the conditional, all the compiler can determine is that
pstr *might* point to stack-allocated memory. Even more difficult:
pstr = foo(&stack_array[0]);
To detect the *potential* error, a compiler has to be able to solve
the interprocederal points-to problem, which is highly nontrivial.


In general, the kind of defect detection you're interested in is very
difficult to do at compile time. You're much better off using a C
interpreter like Saber C (are they still around?). The interpreter
can much more easily check such conditions at run time.




Morris
--
Morris Bernstein morris@cam.org
[It's certainly useful to warn about returning a direct reference to an auto
variable, which is an error 99+% of the time. Saber C is indeed still around,
now it's called Code Center and the vendor is called Centerline. -John]
--

Return to the comp.compilers page.
Search the comp.compilers archives again.