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# modbus_write_and_read_registers
## Name
modbus_write_and_read_registers - write and read many registers in a single transaction
## Synopsis
```c
int modbus_write_and_read_registers(
modbus_t *ctx,
int write_addr, int write_nb, const uint16_t *src,
int read_addr, int read_nb, const uint16_t *dest
);
```
## Description
The *modbus_write_and_read_registers()* function shall write the content of the
`write_nb` holding registers from the array 'src' to the address `write_addr` of
the remote device then shall read the content of the `read_nb` holding registers
to the address `read_addr` of the remote device. The result of reading is stored
in `dest` array as word values (16 bits).
You must take care to allocate enough memory to store the results in `dest`
(at least `nb * sizeof(uint16_t)`).
The function uses the Modbus function code 0x17 (write/read registers).
## Return value
The function shall return the number of read registers if successful. Otherwise
it shall return -1 and set errno.
## Errors
- *EMBMDATA*, too many registers requested, Too many registers to write
## See also
- [modbus_read_registers](modbus_read_registers.md)
- [modbus_write_register](modbus_write_register.md)
- [modbus_write_registers](modbus_write_registers.md)
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