libpq status question

I'm confused about something and need some help. I wrote a c routine
that uses libpq. The result codes are good when I use a select or
insert. if I use an update sql command string like "update
largetable set t01='update1', t02='update2' where mainkey='12'" and the
mainkey of 12 does not exist the result code is a 0 if the mainkey
exists I get a result of 0. How can I tell if the update was
successfully or not. Here is the code.

void

pgsql_exec_query( int *dbhandle, char *query, int *qryhandle, int
*status ) {
char
*r,*n=query;
int nfields, i,
j;
PGconn *conn = (PGconn
*)*dbhandle;
PGresult
*res;
while ( *n != ';' || *(n+1) != ';' )
n++;

*n=0;
res =
0;
res = PQexec(conn,
query);
printf("Result = %d ", (int)
res);
*status =
PQresultStatus(res);
printf("status = %f", (int)
status);
*qryhandle = (int)
res;
}