# modbus_send_raw_request(3) Manual Page

## NAME

modbus_send_raw_request - send a raw request

## SYNOPSIS

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

## DESCRIPTION

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.

## RETURN VALUE

The modbus_send_raw_request() 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.

## EXAMPLE

    modbus_t *ctx;
    /* Read 5 holding registers from address 1 */
    uint8_t raw_req[] = { 0xFF, 0x03, 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);

## SEE ALSO

modbus_receive_confirmation(3)

## AUTHORS

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