Question

This is my first message here.
I’m working with a non-blocking UDP socket on a real-time OS (OnTime and VxWorks).
I have read the documentation and some forums but I have some doubts about ‘atomicity’ of sendto() and recvfrom() functions:

• sendto() returns the number of bytes enqueued or error. Is it possible that it’s less then the input buffer length? Maybe the output buffer has not enough free space and just few bytes are enqueued…

• recvfrom() returns the number of byte received or error. Is it possible that it’s less then the size of message the source has sent? I mean partial message reading…

I hopes reading and writing functions are atomic (full message or no message read/write).

Thanks.
Emanuele.

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I asked to OnTime support and they told me that it’s possible that sendto() will enqueue a partial message if the output buffer has not enough free space. I don’t know if also recvfrom() could return a partial message in some cases. I suppose that there’s no standard behavior on socket implementations among different OS.

Answer 1

I’m not really familiar with these systems, but I would be very surprised if they break normal UDP socket semantics, which is always to enqueue a full datagram on “send” and to dequeue a full single datagram on a “receive”.