UART/Serial.write with communication status return

[trustmiao]

while using ESP-NOW

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esp_err_tesp_now_send(const uint8_t *peer_addr, const uint8_t *data, size_t len)

it will return ESP_OK while the message is successfully delivered.

However, while using UART,

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int uart_write_bytes(uart_port_tuart_num, const char *src, size_t size)

Return
(-1) Parameter error
OTHERS (>=0) The number of bytes pushed to the TX FIFO

How can we confirm the data is successfully delivered, without manually send a call back signal?
We are worried, when the usb is disconnected, the data transfering might be interrupted.

[FreddyVictor]

With standard UART, data can be (validly) sent, but, if nothing is waiting at the other end to receive it, will simply disappear...

I believe the only reliable way to know if the data was sent is if you get the receiver to reply back confirming it has been received