Function esp_idf_sys::uart_get_buffered_data_len

pub unsafe extern "C" fn uart_get_buffered_data_len(
    uart_num: uart_port_t, 
    size: *mut usize
) -> esp_err_t

@brief UART get RX ring buffer cached data length

@param uart_num UART port number. @param size Pointer of size_t to accept cached data length

@return - ESP_OK Success - ESP_FAIL Parameter error