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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).

Return value

The function shall return the number of read registers if successful. Otherwise it shall return -1 and set errno.


  • EMBMDATA, too many registers requested, Too many registers to write

See also