When does a UDP sendto() block?

This can happen if you filled up your socket buffer, but it is highly operating system dependent . Since UDP does not provide any guarantee your operating system can decide to do whatever it wants when your socket buffer is full: block or drop. You can try to increase SO_SNDBUF for temporary relief.

This may be because your operating system is attempting to perform an ARP request in order to get the hardware address of the remote host.

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