Export MODBUS_MAX_ADU_LENGTH and documentation (ref #241)

doc/modbus_receive_confirmation.txt

@@ -14,11 +14,18 @@ SYNOPSIS
 
 DESCRIPTION
 -----------
-The *modbus_receive_confirmation(*_ function shall receive a request via the
+The *modbus_receive_confirmation()* function shall receive a request via the
 socket of the context _ctx_. This function must be used for debugging purposes
 because the received response isn't checked against the initial request. This
 function can be used to receive request not handled by the library.
 
+The maximum size of the response depends on the used backend, in RTU the _rsp_
+array must be _MODBUS_RTU_MAX_ADU_LENGTH_ bytes and in TCP it must be
+_MODBUS_TCP_MAX_ADU_LENGTH_ bytes. If you want to write code compatible with
+both, you can use the constant _MODBUS_MAX_ADU_LENGTH_ (maximum value of all
+libmodbus backends). Take care to allocate enough memory to store responses to
+avoid crashes of your server.
+
 
 RETURN VALUE
 ------------
@@ -27,6 +34,13 @@ response length if sucessful. The returned request length can be zero if the
 indication request is ignored (eg. a query for another slave in RTU
 mode). Otherwise it shall return -1 and set errno.
 
+EXAMPLE
+-------
+[source,c]
+-------------------
+uint8_t rsp[MODBUS_MAX_ADU_LENGTH];
+rc = modbus_receive_confirmation(ctx, rsp);
+-------------------
 
 SEE ALSO
 --------

src/modbus.h

@@ -97,14 +97,19 @@ MODBUS_BEGIN_DECLS
  * the first MODBUS implementation on Serial Line network (max. RS485 ADU = 256
  * bytes). Therefore, MODBUS PDU for serial line communication = 256 - Server
  * address (1 byte) - CRC (2 bytes) = 253 bytes.
- *
- * Consequently:
- * - RS232 / RS485 ADU = 253 bytes + Server address (1 byte) + CRC (2 bytes) =
- *   256 bytes.
- * - TCP MODBUS ADU = 253 bytes + MBAP (7 bytes) = 260 bytes.
  */
 #define MODBUS_MAX_PDU_LENGTH              253
 
+/* Consequently:
+ * - RTU MODBUS ADU = 253 bytes + Server address (1 byte) + CRC (2 bytes) = 256
+ *   bytes.
+ * - TCP MODBUS ADU = 253 bytes + MBAP (7 bytes) = 260 bytes.
+ * so the maximum of both backend in 260 bytes. This size can used to allocate
+ * an array of bytes to store responses and it will be compatible with the two
+ * backends.
+ */
+#define MODBUS_MAX_ADU_LENGTH              260
+
 /* Random number to avoid errno conflicts */
 #define MODBUS_ENOBASE 112345678