modbus_send_raw_request(3)
==========================


NAME
----
modbus_send_raw_request - send a raw request


SYNOPSIS
--------
*int modbus_send_raw_request(modbus_t *'ctx', const 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.

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.


EXAMPLE
-------
[source,c]
-------------------
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);
-------------------

SEE ALSO
--------
linkmb:modbus_receive_confirmation[3]


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