Help with parameter passing

Hi everybody,

We're trying to integrate a vendor supplied service program, written in C,
into our RPGIV applications.  We've attempted to translate the .H include
into RPG prototypes, and have encountered a usage problem.

One of the functions is passing tax_amt and tax_rate as 

double *pTransTotalTax,
double *pTransCombRate,

which we've coded as

D  pTransExtdAmt                 8f         
D  pTransTotalTx                 8f         

So far so good.  Now the problem.  The use of this api states:

"Total tax amount for the transaction. This parameter is used when
processing special (tax-only debit and tax-only credit) transactions. This
parameter is ignored if NULL is passed. Passing 0 results in zero tax being
calculated."

Entended amount is fine, since we will have a calculated value for that
every time we call the api.  But for the call to work properly, we must send
the other parm as NULL.  How in the he!! am I supposed to do that?  Does C
consider a parm to be null if the address is null? 

Can anyone give me some insight? 

Thanks in advance....


Eric DeLong
Sally Beauty Company
MIS-Project Manager (BSG)
940-898-7863 or ext. 1863