modbus_send_raw_request - Man Page

send a raw request


int modbus_send_raw_request(modbus_t *ctx, const uint8_t *raw_req, int raw_req_length);


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.

Return Value

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_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;

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

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


See Also



The libmodbus documentation was written by Stéphane Raimbault <>

Referenced By

libmodbus(7), modbus_receive_confirmation(3).

07/20/2023 libmodbus v3.1.7 libmodbus Manual