How to solve: sending UDP packet using Sendto() got “message too long?

You can not send messages (datagrams) larger than 2^16 65536 octets with UDP. The length field of a UDP packet is 16 bits. The buffer sizes you're requesting are not about the size for a packet, but how many octets the OS does buffer incoming and outgoing in total (spread over multiple packets).

But a single packet can not get larger.

Per @datenwolf's answer, you simply can't send more than 64k in a single UDP datagram, as that limit is implicit in the two-byte length field in the protocol.

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