modbus_write_and_read_registers man page
modbus_write_and_read_registers — write and read many registers in a single transaction
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);
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).
The modbus_write_and_read_registers() function shall return the number of read registers if successful. Otherwise it shall return -1 and set errno.
Too many registers requested, Too many registers to write
modbus_read_registers(3) modbus_write_register(3) modbus_write_registers(3)
The libmodbus documentation was written by Stéphane Raimbault <email@example.com>