---
name: "modbus_send_raw_request"
section: "3"
platform: "linux"
package: "libmodbus-dev"
version: "3.1.6-2.1"
release: "Debian 12 (Bookworm)"
platforms: ["linux"]
url: "https://new.carta.tech/modbus_send_raw_request"
---

# modbus\_send\_raw\_request(3)

send a raw request

## 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

```
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

[**modbus\_receive\_confirmation**(3)](/modbus_receive_confirmation/3.md)

## AUTHORS

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