Trigger function, C, lookup of attribute type

I'm writing a trigger procedure in C to catch updates to a
table and write them to a log file.

The function must walk along trigdata->tg_trigtuple
pulling out the attributes, comparing them with those in
trigdata->tg_newtuple and writing the diffs to a flat ASCII
file.

I've got a loop over the attributes, and I can get the values
if I know the type, eg

  Datum d = heap_getattr( trigdata->tg_trigtuple, ...
  int32_t val = DatumGetInt32( d);

but I'm struggling to find how the function can determine the
attribute types.   I'm only using the built-in scalar types.

Do I have to query the system catalogs?  Or maybe there's a
function that works from a TupleDesc?

--
Paul Nicholson
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