multiple packets received between recvfrom calls

This is a discussion on multiple packets received between recvfrom calls within the Networking/Device Communication forums, part of the General Programming Boards category; hi, (using udp) if i receive multiple packets on a given socket before i make a call to recvfrom, will ...

  1. #1
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    multiple packets received between recvfrom calls

    hi, (using udp)
    if i receive multiple packets on a given socket before i make a call to recvfrom, will recvfrom return all the data it has received or just the contents of a single udp packet it received, in a queue like fashion? in the case it is not the latter, is there a way to make it as such, or must i use a large buffer and parse it on my own? thanks.

  2. #2
    Carnivore ('-'v) Hunter2's Avatar
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    I *believe* the latter is the case, although I haven't used UDP myself. I read about it though [snip: Bad information; see posts below.]
    Last edited by Hunter2; 06-30-2006 at 08:05 AM.
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    and the hat of wrongness Salem's Avatar
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    http://en.wikipedia.org/wiki/User_Datagram_Protocol
    Sounds like you're making way too many assumptions about the link, namely order and delivery (udp guarantees neither)
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    Carnivore ('-'v) Hunter2's Avatar
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    Which of us, or both?

    >>assumptions about the link, namely order and delivery
    As I understood it previously, while whole packets may arrive out-of-order, not arrive at all, or arrive several times, if a packet arrives at all then it will have arrived in its entirety. Is this incorrect, or was I just ambiguous?.. I remember puzzling over the wording in Winsock API for a long time.
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  5. #5
    Carnivore ('-'v) Hunter2's Avatar
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    My apologies. I found this on MSDN, in the docs for recvfrom():
    For message-oriented sockets, data is extracted from the first enqueued message, up to the size of the buffer specified. If the datagram or message is larger than the buffer specified, the buffer is filled with the first part of the datagram, and recvfrom generates the error WSAEMSGSIZE. For unreliable protocols (for example, UDP) the excess data is lost.
    But AFAIK, as long as you use a large enough buffer, you'll get the entire packet in one recv(), and not get the next packet. Of course, this *may* mean that packets will just be rejected until the first one is read, but I hope not.
    Last edited by Hunter2; 06-30-2006 at 08:08 AM.
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