Re: Trivial fix in value_sub

[Jim Ingham <jingham at apple dot com>]

Jim,

For incomplete types, TYPE_LENGTH does indeed return 0.  If it is an 
incomplete type, I don't think you should treat the size as 0, since 
that is CERTAINLY not what the user expected.  If you are going to 
handle it specially, then you should return an error in this case, 
saying something like "Can't do pointer arithmetic on incomplete types, 
try casting it as (void *)."

The way the problem came up is that the Toolbox folks here use lots of 
intentionally incomplete structures to mark tokens that are hiding real 
structures behind the curtains.  But they know the secret handshakes, so 
they know where things are around the pointers to fake structs, and use 
this kind of pointer arithmetic to poke around.  So treating the 
addition as void * is what they expect, and an error would probably 
mildly tick them off.

OTOH, you can always get around this by casting the pointer, either to 
void * or to what it is...  I am pretty sure their use is just shorthand.

So... I don't think you should keep the size at 0.  This seems like gdb 
is just silently ignoring the " -  x" part of what they typed, and you 
should always be explicit about what you have done.  But if you think an 
error is more appropriate, I am fine with that...

Jim

On Wednesday, April 3, 2002, at 02:52  PM, Jim Blandy wrote:

>
> (Thanks for making value_add and value_sub consistent!)
>
> If I use an incomplete type in my program --- say, by making a
> definition like this:
>
>         struct foo *x;
>
> where there is no definition for `struct foo' in scope --- does GDB
> set TYPE_LENGTH (TYPE_TARGET_TYPE (p)) to zero, where `p' is the type
> of x?
>
> See, that code in value_add (and now in value_sub) is supposed to
> handle void *; as an extension, GCC allows arithmetic on void *
> values, treating sizeof (void) as one.  This makes sense for void *
> values, since they're often used as pointers to raw memory.
>
> However, for things like incomplete struct types, treating the size as
> one is completely bogus.  That's surely not the behavior the user
> would expect; they may not even realize that the type is incomplete.
>
> If GDB really does set the length of an incomplete struct type to
> zero, then that code should really read something like:
>
>   if (sz == 0 && TYPE_CODE (TYPE_TARGET_TYPE (type1)) == TYPE_CODE_VOID)
>     sz = 1;
>
> Can you tell me more about the context in which you noticed this
> problem?
>
--
Jim Ingham                                   jingham@apple.com
Developer Tools - gdb
Apple Computer