Re: Trivial fix in value_sub

[Jim Blandy <jimb@redhat.com>]

(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?