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# modbus_reply_exception
## Name
modbus_reply_exception - send an exception response
## Synopsis
```c
int modbus_reply_exception(modbus_t *ctx, const uint8_t *req, unsigned int exception_code);
```
## Description
The *modbus_reply_exception()* function shall send an exception response based
on the 'exception_code' in argument.
The libmodbus provides the following exception codes:
- MODBUS_EXCEPTION_ILLEGAL_FUNCTION (1)
- MODBUS_EXCEPTION_ILLEGAL_DATA_ADDRESS (2)
- MODBUS_EXCEPTION_ILLEGAL_DATA_VALUE (3)
- MODBUS_EXCEPTION_SLAVE_OR_SERVER_FAILURE (4)
- MODBUS_EXCEPTION_ACKNOWLEDGE (5)
- MODBUS_EXCEPTION_SLAVE_OR_SERVER_BUSY (6)
- MODBUS_EXCEPTION_NEGATIVE_ACKNOWLEDGE (7)
- MODBUS_EXCEPTION_MEMORY_PARITY (8)
- MODBUS_EXCEPTION_NOT_DEFINED (9)
- MODBUS_EXCEPTION_GATEWAY_PATH (10)
- MODBUS_EXCEPTION_GATEWAY_TARGET (11)
The initial request `req` is required to build a valid response.
## Return value
The function shall return the length of the response sent if
successful. Otherwise it shall return -1 and set errno.
## Errors
- *EINVAL*, the exception code is invalid
## See also
- [modbus_reply](modbus_reply.md)
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