Changes to C interface from 7.0 to 7.1

I have a function like this:

VARSIZE(result) = result_size - 1;
sha1_init(C);
sha1_write(C, VARDATA(arg), VARSIZE(arg)-VARHDRSZ);

anyway, the problem is that VARSIZE is no longer defined in a way that
I can make assignments to in 7.1. Does anyone have any sugestions?

Also, if there is an SHA1 function built in, I might as well use that,
but I still need to have my own user-defined C for other things. For
instance, I have a challenge-response function that works with the
CryptoCard token, linked directly into PG, which I need to recompile
for 7.1.

Thanks