On Mon, Oct 15, 2012 at 6:50 AM, Randi Botse <nightdecoder@xxxxxxxxx> wrote:
> Hi All
>
> When using TCP socket, I loop send() or recv() until ALL the data has
> been transmitted (or error, disconnect, etc.), because TCP socket
> packet is transmitted in stream nature, maybe a byte, bytes or all
> bytes in one transfer.
TCP is like a water tap (a character device) that you need to "format"
yourself with boundaries/etc.
> The UDP socket preserve message boundary which TCP socket doesn't.
> Does this means single call to sendto() will processed by single call
> recvfrom()?, and how about packet that exceeds UDP data MAX size?.
UDP is more like a block device, a letter/package sent. It's only the
single packet that is sent, and only that same single packet that will
be received (or go missing in the post)
I believe the error you might be refering to is this one from a Linux
2.6 based Ubuntu 6.0.5 man page for recvfrom:
<quote man recv(2)>
All three routines return the length of the message on
successful completion. If a message is too long to fit in the supâ
plied buffer, excess bytes may be discarded depending on the
type of socket the message is received from.
<snip>
(recv(2) flags orred together:)
MSG_TRUNC (since Linux 2.2)
For raw (AF_PACKET), Internet datagram (since Linux
2.4.27/2.6.8), and netlink (since Linux 2.6.22) sockets: return
the real length of the packet or datagram, even when it
was longer than the passed buffer. Not implemented for Unix
domain (unix(7)) sockets.
<snip>
(recvmsg(2) flags return status:)
MSG_TRUNC
indicates that the trailing portion of a datagram was
discarded because the datagram was larger than the buffer supâ
plied.
</quote>
In other words, you have been sent a 1024byte long packet with
sentto(2), but recvfrom(2) only had a 900 bytes buffer, then sorry,
you've lost 124 bytes.
Looking at the recv(2) manual page, I recall that recv(2)/recvfrom(2)
will return the size of the packter received, so if you've provided a
65535 byte buffer, and was only sent 1024bytes, then
recv(2)/recvfrom(2) will return the 1024bytes answer.
> So in code, do I need to loop sendto() or recvfrom() to transmit the data?.
It depends on the data being sent, but for every sendto(2), you will
need a single recv(2)/recvfrom(2) with a correctly sized buffer to
receive that single message.
> Example codes is:
>
> char packet[100];
> size_t nbytes = 0;
> int ret;
>
> while (nbytes < sizeof(packet)) {
> ret = recvfrom(socket, packet + nbytes, addr, 0, sizeof(packet) - nbytes);
> if (ret <= 0) {
> /* deal with recvfrom() error */
> }
> nbytes += ret
> }
That is TCP, for UDP you will have something like:
sender:
char buffer[1024];
int size_of_data=fill_buffer_withdata(buffer,1024);
sendto(socket,buffer,size_of_data,flags);
receiver:
chat buffer [1024];
int size_of_data=recv(socket,buffer,1024,flags);
if size_of_data>1024
then
throw_error
else
do_something_with_data(buffer,size_of_data);
end
there are another method, using MSG_PEEK in the flags before reading
the real data and remove that from the queue,
>
>
> Thanks
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