Status question

I'm confused about something and need some help.  I wrote a c routine 
that uses libpq.    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 
successful 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;
                        
 
}                                                                              

}

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