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# modbus_reply
## Name
modbus_reply - send a response to the received request
## Synopsis
```c
int modbus_reply(modbus_t *ctx, const uint8_t *req, int req_length, modbus_mapping_t *mb_mapping);
```
## Description
The *modbus_reply()* function shall send a response to received request. The
request `req` given in argument is analyzed, a response is then built and sent
by using the information of the modbus context `ctx`.
If the request indicates to read or write a value the operation will done in the
modbus mapping `mb_mapping` according to the type of the manipulated data.
If an error occurs, an exception response will be sent.
This function is designed for Modbus servers.
## Return value
The function shall return the length of the response sent if
successful. Otherwise it shall return -1 and set errno.
## Errors
- *EMBMDATA*, sending has failed
See also the errors returned by the syscall used to send the response (eg. send or write).
## See also
- [modbus_reply_exception](modbus_reply_exception.md)
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