Error handling

Liblightmodbus v3.0 introduces a new type for error handling - ModbusErrorInfo - returned by majority of the library functions. This new type allows to store both error type and its source - whether it was caused by an invalid request/response frame or by an actual library/user error.

In older versions, error checking would usually look like this:

ModbusError err = modbusDoSomething();
if (err != MODBUS_OK)
    doSomething();

In the new version, it's a bit more sophisticated:

ModbusErrorInfo err = modbusDoSomething();
 
// We can check if any error happened at all.
// modbusIsOk() returning true indicates that request/response
// frame can be safely accessed and is ready to be sent.
if (modbusIsOk(err))
    sendData();
else
    doSomething();
 
// We can check if the library reported any general errors.
// Those usually mean that the user provided an invalid argument
// or that something failed internally. These are the serious ones.
if (modbusGetGeneralError(err) != MODBUS_OK)
    handleSeriousError();
 
// We can check if the request frame contains an error.
// This includes parsing a frame meant for other slave, invalid CRC value, etc.
// Usually, you want to ignore these, because they are not application errors
// and only provide additional information about what prevented the library
// from processing request/response.
if (modbusGetRequestError(err) != MODBUS_OK)
    handleRequestError();
 
// We can also check if the response frame contained an error
// Interpretation is analogous to the one for modbusGetRequestError()
if (modbusGetResponseError(err) != MODBUS_OK)
    handleResponseError();

modbusGetGeneralError(), modbusGetRequestError() and modbusGetResponseError() return a ModbusError enum value which can be easily handled with a switch statement.

You can find more information about errors returned by certain functions in their documentation. For example, please take a look at modbusParseResponseRTU().

See also

MODBUS_NO_ERROR()

MODBUS_GENERAL_ERROR()

MODBUS_REQUEST_ERROR()

MODBUS_RESPONSE_ERROR()

modbusGetErrorSource()

modbusGetErrorCode()

ModbusErrorInfo

Warning

You should not be accessing the fields of ModbusErrorInfo directly. They are not considered a part of the library interface, and hence are subject to change in future versions of the library. Instead, please use functions described above.