modbus_reply_exception - send an exception response

       The libmodbus documentation was written by Stéphane Raimbault <stephane.raimbault@gmail.com>

libmodbus v3.1.6                                   12/09/2019                            MODBUS_REPLY_EXCEPTI(3)

       The modbus_reply_exception() function shall send an exception response based on the exception_code in
       argument.

       The libmodbus provides the following exception codes:

       •   MODBUS_EXCEPTION_ILLEGAL_FUNCTION (1)

       •   MODBUS_EXCEPTION_ILLEGAL_DATA_ADDRESS (2)

       •   MODBUS_EXCEPTION_ILLEGAL_DATA_VALUE (3)

       •   MODBUS_EXCEPTION_SLAVE_OR_SERVER_FAILURE (4)

       •   MODBUS_EXCEPTION_ACKNOWLEDGE (5)

       •   MODBUS_EXCEPTION_SLAVE_OR_SERVER_BUSY (6)

       •   MODBUS_EXCEPTION_NEGATIVE_ACKNOWLEDGE (7)

       •   MODBUS_EXCEPTION_MEMORY_PARITY (8)

       •   MODBUS_EXCEPTION_NOT_DEFINED (9)

       •   MODBUS_EXCEPTION_GATEWAY_PATH (10)

       •   MODBUS_EXCEPTION_GATEWAY_TARGET (11)

       The initial request req is required to build a valid response.

EINVAL
           The exception code is invalid

       modbus_reply_exception - send an exception response

       The function shall return the length of the response sent if successful. Otherwise it shall return -1 and
       set errno.

modbus_reply(3) libmodbus(7)

       *int modbus_reply_exception(modbus_t *ctx, const uint8_t *req, unsigned int exception_code);