modbus_send_raw_request - send a raw request

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

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

       The modbus_send_raw_request() function shall send a request via the socket of the context ctx. This
       function must be used for debugging purposes because you have to take care to make a valid request by
       hand. The function only adds to the message, the header or CRC of the selected backend, so raw_req must
       start and contain at least a slave/unit identifier and a function code. This function can be used to send
       request not handled by the library.

       The public header of libmodbus provides a list of supported Modbus functions codes, prefixed by
       MODBUS_FC_ (eg. MODBUS_FC_READ_HOLDING_REGISTERS), to help build of raw requests.

           modbus_t *ctx;
           /* Read 5 holding registers from address 1 */
           uint8_t raw_req[] = { 0xFF, MODBUS_FC_READ_HOLDING_REGISTERS, 0x00, 0x01, 0x0, 0x05 };
           int req_length;
           uint8_t rsp[MODBUS_TCP_MAX_ADU_LENGTH];

           ctx = modbus_new_tcp("127.0.0.1", 1502);
           if (modbus_connect(ctx) == -1) {
               fprintf(stderr, "Connection failed: %s\n", modbus_strerror(errno));
               modbus_free(ctx);
               return -1;
           }

           req_length = modbus_send_raw_request(ctx, raw_req, 6 * sizeof(uint8_t));
           modbus_receive_confirmation(ctx, rsp);

           modbus_close(ctx);
           modbus_free(ctx);

       modbus_send_raw_request - send a raw request

       The function shall return the full message length, counting the extra data relating to the backend, if
       successful. Otherwise it shall return -1 and set errno.

modbus_receive_confirmation(3)

intmodbus_send_raw_request(modbus_t*ctx,constuint8_t*raw_req,intraw_req_length);