allow user provided address checking

If user provided address checking is enabled, the user function takes
complete responsibility for validating.  An example function would be:

```
BOOL isMBUserAddressValid( UCHAR ucMBAddress, UCHAR ucRcvAddress )
{
        if (
                /* Accept our originally configured address */
                (ucMBAddress == ucRcvAddress) ||
                /* Accept all even, non-broadcast addresses less than 50 */
                (ucRcvAddress > 0 && ucRcvAddress < 50 && ((ucRcvAddress & 0x1) == 0))
        ) {
                return true;
        }
        return false;
}

```

This allows fixing issues such as: cwalter-at#27
in a more general fashion.

Note that this does not affect the STRICT_ADDRESSING or
QUIRK_REPLY_BROADCAST_READ options, which are _after_ we have decided to
look at the frame.

Signed-off-by: Karl Palsson <[email protected]>

modbus/include/mb.h

@@ -243,6 +243,19 @@ eMBErrorCode    eMBSetSlaveID( UCHAR ucSlaveID, BOOL xIsRunning,
                                UCHAR const *pucAdditional,
                                USHORT usAdditionalLen );
 
+/*! \ingroup modbus
+ * \brief Require user validation of the addresses.
+ * You can use this to "allow" a custom range of modbus addresses,
+ * instead of just the single address provided in \ref eMBInit
+ * This function is only called if MB_USER_ADDRESS_VALIDATION > 0
+ * in your mbconfig.h
+ *
+ * \param ucMBAddress Value provided at eMBInit time
+ * \param  ucRcvAddress Value currently being evaluated
+ * \return true if the address should be accepted
+ */
+BOOL isMBUserAddressValid( UCHAR ucMBAddress, UCHAR ucRcvAddress );
+
 /*! \ingroup modbus
  * \brief Registers a callback handler for a given function code.
  *

modbus/include/mbconfig.h

@@ -175,6 +175,11 @@ PR_BEGIN_EXTERN_C
 #endif
 
 
+#ifndef MB_USER_ADDRESS_VALIDATION
+/*! \brief require  user code to validate addresses */
+#define MB_USER_ADDRESS_VALIDATION		( 0 )
+#endif
+
 /*! @} */
 #ifdef __cplusplus
     PR_END_EXTERN_C

modbus/mb.c

@@ -361,7 +361,11 @@ eMBPoll( void )
             if( eStatus == MB_ENOERR )
             {
                 /* Check if the frame is for us. If not ignore the frame. */
-                if( ( ucRcvAddress == ucMBAddress ) || ( ucRcvAddress == MB_ADDRESS_BROADCAST ) )
+#if MB_USER_ADDRESS_VALIDATION > 0
+                if( isMBUserAddressValid( ucMBAddress, ucRcvAddress ) )
+#else
+                if( ( ucRcvAddress == ucMBAddress ) || ( ucRcvAddress == MB_ADDRESS_BROADCAST )  )
+#endif
                 {
                     ( void )xMBPortEventPost( EV_EXECUTE );
                 }