recvfrom() returns 0 after sendto() call on the blocking udp socket

When I call recvfrom() on the blocking udp socket after sendto() call it returns 0.

But when I call Sleep() function between them, recvfrom() returns the correct lenghth of the received udp packet.

Th开发者_如何学Ce same correct behaviour is present when I use the debugger in step over mode.

Can some clarify this behavior of the udp socket?