1. Is it possible to receive multiple messages in one receive call?
Sender pseudo-code:
target = ("xxx.xxx.xxx.xxx", 1234)
sender = new_udp_socket()
sender.send("Hello", target)
sender.send("World", target)
Receiver pseudo-code:
receiver = new_udp_socket()
receiver.bind("", 1234)
while true
data = receiver.recvfrom(512)
print(data)
Is it possible that the receiver will receive "HelloWorld" in one receive call instead of "Hello" and "World" separately?
I have been told that it is possible, but I'd like to make sure.
2. If it is possible to receive multiple messages in one receive call, how do I ensure that my code processes both messages separately?
I've been thinking about this but couldn't come up with any solution.
My first idea was that I would add a byte at the beginning of the send call stating the length of the message.
I don't believe this would be reliable either because if too much data is in the receivers buffer then the beginning (the message length) may be cut off and therefore my program would fail.
Thanks for any help!
After much research I have found an answer to my question.
One recvfrom call will only ever receive one sendto call.
Sources:
https://stackoverflow.com/a/8748884/1541397
https://stackoverflow.com/a/26185032/1541397
Related
I'm writing a very specific application protocol to enable communication between 2 nodes. Node 1 is an embedded platform (a microcontroller), while node 2 is a common computer.
Such protocol defines messages of variable length. This means that sometimes node 1 sends a message of 100 bytes to node 2, while another time it sends a message of 452 bytes.
Such protocol shall be independent on how the messages are transmitted. For instance, the same message can be sent over USB, Bluetooth, etc.
Let's assume that a protocol message is defined as:
| Length (4 bytes) | ...Payload (variable length)... |
I'm struggling about how the receiver can recognise how long is the incoming message. So far, I have thought about 2 approaches.
1st approach
The sender sends the length first (4 bytes, always fixed size), and the message afterwards.
For instance, the sender does something like this:
// assuming that the parameters of send() are: data, length of data
send(msg_length, 4)
send(msg, msg_length - 4)
While the receiver side does:
msg_length = receive(4)
msg = receive(msg_length)
This may be ok with some "physical protocols" (e.g. UART), but with more complex ones (e.g. USB) transmitting the length with a separate packet may introduce some overhead. The reason being that an additional USB packet (with control data, ACK packets as well) is required to be transmitted for only 4 bytes.
However, with this approach the receiver side is pretty simple.
2nd approach
The alternative would be that the receiver keeps receiving data into a buffer, and at some point tries to find a valid message. Valid means: finding the length of the message first, and then its payload.
Most likely this approach requires adding some "start message" byte(s) at the beginning of the message, such that the receiver can use them to identify where a message is starting.
I have a GSM module hooked up to PIC18F87J11 and they communicate just fine . I can send an AT command from the Microcontroller and read the response back. However, I have to know how many characters are in the response so I can have the PIC wait for that many characters. But if an error occurs, the response length might change. What is the best way to handle such scenario?
For Example:
AT+CMGF=1
Will result in the following response.
\r\nOK\r\n
So I have to tell the PIC to wait for 6 characters. However, if there response was an error message. It would be something like this.
\r\nERROR\r\n
And if I already told the PIC to wait for only 6 characters then it will mess out the rest of characters, as a result they might appear on the next time I tell the PIC to read the response of a new AT command.
What is the best way to find the end of the line automatically and handle any error messages?
Thanks!
In a single line
There is no single best way, only trade-offs.
In detail
The problem can be divided in two related subproblems.
1. Receiving messages of arbitrary finite length
The trade-offs:
available memory vs implementation complexity;
bandwidth overhead vs implementation complexity.
In the simplest case, the amount of available RAM is not restricted. We just use a buffer wide enough to hold the longest possible message and keep receiving the messages bytewise. Then, we have to determine somehow that a complete message has been received and can be passed to further processing. That essentially means analyzing the received data.
2. Parsing the received messages
Analyzing the data in search of its syntactic structure is parsing by definition. And that is where the subtasks are related. Parsing in general is a very complex topic, dealing with it is expensive, both in computational and laboriousness senses. It's often possible to reduce the costs if we limit the genericity of the data: the simpler the data structure, the easier to parse it. And that limitation is called "transport layer protocol".
Thus, we have to read the data to parse it, and parse the data to read it. This kind of interlocked problems is generally solved with coroutines.
In your case we have to deal with the AT protocol. It is old and it is human-oriented by design. That's bad news, because parsing it correctly can be challenging despite how simple it can look sometimes. It has some terribly inconvenient features, such as '+++' escape timing!
Things become worse when you're short of memory. In such situation we can't defer parsing until the end of the message, because it very well might not even fit in the available RAM -- we have to parse it chunkwise.
...And we are not even close to opening the TCP connections or making calls! And you'll meet some unexpected troubles there as well, such as these dreaded "unsolicited result codes". The matter is wide enough for a whole book. Please have a look at least here:
http://en.wikibooks.org/wiki/Serial_Programming/Modems_and_AT_Commands. The wikibook discloses many more problems with the Hayes protocol, and describes some approaches to solve them.
Let's break the problem down into some layers of abstraction.
At the top layer is your application. The application layer deals with the response message as a whole and understands the meaning of a message. It shouldn't be mired down with details such as how many characters it should expect to receive.
The next layer is responsible from framing a message from a stream of characters. Framing is extracting the message from a stream by identifying the beginning and end of a message.
The bottom layer is responsible for reading individual characters from the port.
Your application could call a function such as GetResponse(), which implements the framing layer. And GetResponse() could call GetChar(), which implements the bottom layer. It sounds like you've got the bottom layer under control and your question is about the framing layer.
A good pattern for framing a stream of characters into a message is to use a state machine. In your case the state machine includes states such as BEGIN_DELIM, MESSAGE_BODY, and END_DELIM. For more complex serial protocols other states might include MESSAGE_HEADER and MESSAGE_CHECKSUM, for example.
Here is some very basic code to give you an idea of how to implement the state machine in GetResponse(). You should add various types of error checking to prevent a buffer overflow and to handle dropped characters and such.
void GetResponse(char *message_buffer)
{
unsigned int state = BEGIN_DELIM1;
bool is_message_complete = false;
while(!is_message_complete)
{
char c = GetChar();
switch(state)
{
case BEGIN_DELIM1:
if (c = '\r')
state = BEGIN_DELIM2;
break;
case BEGIN_DELIM2:
if (c = '\n')
state = MESSAGE_BODY:
break;
case MESSAGE_BODY:
if (c = '\r')
state = END_DELIM;
else
*message_buffer++ = c;
break;
case END_DELIM:
if (c = '\n')
is_message_complete = true;
break;
}
}
}
I have a sending application that uses TCP to send files. Sometimes these files contain one message, and other times the file may contain multiple messages. Unfortunately, I do not have access to the Sending application's code.
I am working on editing legacy code to receive these messages. I have managed to get the legacy application to accept a file when there is a single message sent. However, since I disconnect the socket after receiving a single message, the Sender gives a disconnect error.
I wrote a small process to help determine whether there was another message. If it worked, I was going to incorporate it into the code, but I had mixed results:
Dim check(1) As Byte
If (handler.Receive(check, SocketFlags.Peek) > 0) Then
Dim bytesRec As Integer
ReDim bytes(1024)
bytesRec = handler.Receive(bytes)
End If
If there is another message being sent, this will detect it. However, if the file only has a single message, it locks up on Receive until I send another file, and then it is accepted.
Is there a way to tell if there is another message pending that will not lock up if the stream is empty?
I won't post all of the code for accepting the message, as it is a legacy rat's nest, but the general idea is below:
s2 = CType(ar.AsyncState, Socket)
handler = s2.EndAccept(ar)
bytes = New Byte(1024) {}
Dim bytesRec As Integer = handler.Receive(bytes)
' Send Ack/Nak.
numAckBytesSent = handler.Send(myByte)
Thank you in advance for any assistance.
Socket.Select can be used as a quick way of polling a socket for readability. Pass in a timeout of 0 seconds, and the socket in question in the readability list, and it will simply check and report back immediately.
Two other options might be to set Socket.ReceiveTimeout on your socket, or make the socket non-blocking using Socket.Blocking, so that you can find out (as part of the Receive call) whether there is incoming data. These look a bit inconvenient to do in .NET, though, as they throw exceptions rather than simply returning a value, which might make the code a little longer.
Just keep reading. If there is nothing left you will get an end-of-stream indication of some kind, depending on your API.
I'm trying to use ActiveMQPrefetchPolicy but cannot quite understand how to use it.
I'm using queue, there are 3 params that I can define for PrefetchPolicy:
queuePrefetch, queueBrowserPrefetch, inputStreamPrefetch
Actually I don't get the meaning of queueBrowserPrefetch and inputStreamPrefetch so I do not know how to use it.
I assume that you have seen the ActiveMQ page on prefetch limits.
queueBrowserPrefetch sets the maximum number of messages sent to a
ActiveMQQueueBrowser until acks are received.
inputStreamPrefetch sets the maximum number of messages sent
through a jms-stream until acks are received
Both queue-browser and jms-stream are specialized consumers. You can read more about each one of them but if you are not using them it won't matter what you assign to their prefetch limits.
I'm executing 4 startup commands and also expecting to receive 4 responses. The server is already implemented and another dev who is developing android, is able to receive those 4 separate responses, however, I'm getting 2 good responses (separate) and then 3rd and 4th responses come as one response. I'v placed NSLog of NSData result in completeCurrentRead, and it outputs me merged packet "0106000000000b0600000000" instead of separate packets "010600000000" and "0b0600000000". I'v also tested those 3rd and 4th commands separatedly (only one at a time) and everything is OK with the server, it sends them separately, however there occurs merge (with 3rd and 4th) if all four commands are executed in a line. Any ideas?
UPDATE: I think I'v traced to the problem roots. There's a call that reads packet data from a stream in doBytesAvailable method:
CFIndex result = [self readIntoBuffer:subBuffer maxLength:bytesToRead];
And in readIntoBuffer:maxLength, there's a call (length == 256) :
return CFReadStreamRead(theReadStream, (UInt8 *)buffer, length);
So, CFReadStreamRead returns incorrect length of packet - it return length of 12 (instead of 6), and also grabs merged data. Hm, what might causing CFReadStreamRead to read two packets into one, instead of reading them separately...
UPDATE2: I'm using onSocket:didReadData:withTag: delegate method and expecting to receive response data with the tag of request I performed. I have realized recently, streams are streams, not packets but how I can solve that? Server responses does not have terminating chars at start and end of response, just response size, that comes as 2 - 5 bytes. I can cut the first part of response (first packet) and ignore the second part but how AsyncSocket will make another callback with the second part of the response (second packet)? If I will cut only the first parts and ignore the second then IMHO the second "packet" will be lost...
How to cut the first part of response and tell AsyncSocket to make another callback with tag and the second part of response as separate callback?
UPDATE3: In onSocket:didReadData:withTag:, I manually cut merged response, handle the first part (first packet) and then at the end, throwing a call to onSocket:didReadData:withTag: again:
if (isMergedPacket) {
...
[self onSocket:sock didReadData:restPartOfTheResponse withTag:myCommandTag];
}
However, it looks like AsyncSocket itself pairs every request packet with its response packet (via AsyncReadPacket class) using tags. So, my manual cutting works, but AsyncSocket does not know that I already handled both packets, and it still tries to read the second packet. So, I'm getting sock:shouldTimeoutReadWithTag:... callback which is called when a read operation has reached its timeout without completing.
Found solution. It's not necessary to change and dig into AsyncSocket. You just need to define the length of each response - how much bytes are you interested in reading and getting your callback. More info you can on other post here