Re: [PATCH] gdb: Remove LANG_MAGIC

[Pedro Alves <palves@redhat.com>]

On 4/12/19 12:23 AM, Andrew Burgess wrote:
> The language_defn structure has an la_magic field, this used to be
> used as a basic check that the language_defn structure had the
> expected layout - at least the end of the structure was where we
> expected it to be.
> 
> This feature only really makes sense if we imagine GDB dynamically
> loading language support from dynamic libraries, where a version
> mismatch might cause problems.

We used to have more magic fields like these in other structures,
like target_ops, for example.  IIRC, their intended purpose was to
catch miscompilation, before we had Makefile auto-dependencies.  Before
that, it was common to change the structure's definition, rebuild, and
then find out that gdb hit a "failed internal consistency check" assertion,
because some of the .o files hadn't been rebuilt to reflect the new layout.

For example, the target_ops magic said:

 /* Magic number for checking ops size.  If a struct doesn't end with this
    number, somebody changed the declaration but didn't change all the
    places that initialize one.  */

 #define        OPS_MAGIC       3840

With auto-dependencies, the problem this "solved" is largely a
thing of the past.

Thanks,
Pedro Alves