Re: GDB frontends, MI-speak and object notation

Dmitry Dzhus
2009-05-30T16:44:33Z

I've managed to map MI output to structured data using JSON parser from
Emacs. I needed to wrap the whole GDB/MI answer in curly braces, wrap
Â«field namesÂ» in double quotes, and change equal signs in the
`VARIABLE=VALUE` pairs to semicolons. Using this approach I wrote rather
good-looking and good-working code to show information on threads in
Emacs.

That was a bit of a misplaced status report from me :) so I'll proceed to
some thoughts I had from my experience with GDB.

There is a dark corner in GDB/MI Output Syntax (section 26.2.2 of the
GDB manual) which doesn't fit to simple object model (for example, the
one JSON presents). This is the following case:

`LIST ==>'
` "[" RESULT ( "," RESULT )* "]" '

It looks badly wicked and all broken to me to have things like this in
MI output (this is from `break-info`):

body=[bkpt={number="1", … },bkpt={number="2", …}]

I feel that this should produce the following output instead:

body=[{number="1", … },{number="2", …}]

`-stack-list-frames` is another command which uses this evil notation.

It is evil because it disallows thinking of tuples as objects and of
lists as, well, plain lists. This abuses the notion of a list! Moreover,
this makes TUPLEs *redundant* parts of GDB/MI, as they turn out to be a
subset of LISTs with curly braces instead of square brackets.

I wonder why was GDB/MI syntax designed this way. I believe it ought to
be changed (that would be backwards-incompatible, though, and a lot of
front-ends would get broken).

The other question is, why not use JSON in GDB/MI at all? Look like
there are no such cases where GDB/MI information cannot be successfully
expressed with JSON. Utilizing that would lower costs of production for
various GDB front-ends because there would be no need to maintain extra
parser for MI instead of using JSON parser which can also be used in a
number of other applications.

Comments from MI developers are welcome!
--
Happy Hacking.

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