things are like this:
I used AsyncSocket to connect with server. when the server send some bytes to client at a time, app will invoke (onSocket: didReadData: withTag:) method; Sometimes, because of the network or other reason, some bytes arrived and the rest bytes arrived later( which means some bytes are delayed).(timeOut = -1)
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Question: how much times did the method(onSocket: didReadData: withTag: ) invoke?
1)only one time - it will wait automatically until the bytes are complete？
2)2 or more times - some arrived(invoke), other arrived(invoke)、、、、、like so?
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which thoughts is right? plz geiv me some advice. Thanks very much.
OK， maybe it's just like this.
Transfer data on TCP: when you send 1000 bytes at a time, because of lag or other reasons, you may receive the 900 bytes first, but the TCP socket will wait the rest 100 bytes
automatically. So if I use asyncSocket to transfer datas and set the timeout, during these seconds, I don't need to deal with the lagged data.
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EDIT
maybe the title of ths question is not appropriate, no one cares the question.
I used the method -readDataToLength: instead, first read header.length of data, and parse it, then read the body.length of data;
Related
I have an RFID scanner attached to a RedPark serial cable connected to an iPad app. When people scan their RFID cards, I get a callback with -readBytesAvailable:. However, sometimes it doesn't give me the entire RFID in one call. Sometimes it send it in two calls.
How can I determine if I've received everything? When my code takes the first callback's data and tries to use it, I get an error, because let's say the RFID was "123456789" sometimes I'll get one call with #"12" and a second call with #"3456789". So I try to process #"12" and get a user not found error, then I try to process #"3456789" and get a user not found error.
How can I tell if I'm done reading data? The lengths of the RFIDs can vary from vendor to vendor, so I can't just assume I need to read a certain number of digits.
This is the method I use to receive the data from the scanner through the RedPark:
- (void) readBytesAvailable:(UInt32)length {
NSLog(#"readBytesAvailable: %lu", length);
UInt8 rxLoopBuff[LOOPBACK_TEST_LEN];
[self.rfidManager read:rxLoopBuff Length:length];
NSString *rfid = [[NSString alloc] initWithBytes:rxLoopBuff length:length encoding:NSUTF8StringEncoding];
NSLog(#"rfid=%#", rfid);
[self receivedScanOfRFID:rfid];
}
Serial port gives you no control over packetization. Data is just a stream of bytes with no way to predict which bytes appear in each read call. You have to parse the data stream itself to interpret the contents and understand start/end of your messages. You either need to look for a reliable terminating character or potentially use a timeout approach where you do multiple reads until you get no more data for some period of time. I don't recommend the timeout approach.
Could you please help me? I've a problem related with the gen_tcp send function. I've been trying to send few tuples, about 10-15 items, which were decoded to amf objects, from my erlang server to my flash client.
case get_tcp:send(Socket, Msg) of
ok -> io:format("sent~n");
{erorr, Err} -> io:format("~w~n", [Err])
end
No errors but the flash client doesn't recieve whole data just about 8-11 items. The socket options are [binary, {active, true}, {reuseaddr, true}]. I've checked my network where I use the client - sent packets were fragmented into two fragments, big and small ones. The big one is the first fragment of packet and small one is the next. Amount of the bigs is the same as amount of sent messages, but the smalls are much less and about number of received messages by the flash client.
It only reproduces if I send data fast, if I do it slowly it seems ok. Does anyone know why it happens? It will be very helpful.
My guess is that this is a framing issue.
TCP is a streaming protocol, so when you read in flash, you are not guaranteed to get all the messages right away. Rather you need some kind of framing setup, say {packet, 2} or {packet, 4} on the socket options. This effectively turns TCP from a streaming protocol into a messaging protocol. And I think you want the latter.
I think you can do like this:
1 Before send msg(your tuple), use term_to_binary(Msg) to get binary, then sizeof the binary, then, after your client receive the msg, use binary_to_term to get tuple.
2 Both the client and server you need to set the socket option{packet, 2 or 4}
I not tried amf, but I use erlang+flash and json instead of amf
What packet option of socket? (I use {packet, 2}, for example)
How did you read data on client side? Maybe you not read all data to end and no new events raised?
I have a device that sends data to my server via gprs . The problem is that it sends raw data and i don't know where i can stop the reading
Currently i am using something TIdHttpServer and something like this to read the strings :
var
s : string;
repeat
s:=s+acontext.Connection.Socket.ReadChar;
until acontext.Connection.Socket.InputBufferIsEmpty;
Is there a better solution to my problem ?
TCP is stream oriented. If the protocol is unknown, the server only can try to read into a byte array (if memory is not a problem) or a file stream. If the client disconnects normally, the data is 'complete'. Unfortunately, if the protocol is unknown, the server can not tell wether the client died or disconnected normally.
InputBufferIsEmpty does not help, as it only says if there is data in the (TCP) buffer - and depending on latency this can happen frequently, but it does not mean that there are no more in-flight bytes.
You could try to 'reverse engineer' the protocol, by sending known strings over the client devices. But if the sender is a black box, there can be many special cases - think of encoding or 'escape' characters etc.
You could make up you own protocol.
Some ideas are:
use a special character or characters combo to define the end of the
message.
append at the start of the message some fixed size field with the size of the message
I have been doing overlapped serial port communication in Delphi lately and there is one problem I'm not sure how to solve.
I communicate with a modem. I write a request frame (an AT command) to the modem's COM port and then wait for the modem to respond. The event mask of the port is set to EV_RXCHAR, so when I write a request, I call WaitCommEvent() and start waiting for data to appear in the input queue. When overlapped waiting for event finishes, I immediately start reading data from the queue and read all that the device sends at once:
1) write a request
2) call WaitCommEvent() and wait until waiting finishes
3) read all the data that the device sends (not only the data being in the input queue at that moment)
4) do something and then goto 1
Waiting for event finishes after first byte appears in the input queue. During my read operation, however, more bytes appear in the queue and each of them causes an internal event flag to be set. This means that when I read all the data from the queue and then call WaitCommEvent() for the second time, it will immediately return with EV_RXCHAR mask, even though there is no data to be read.
How should I handle reading and waiting for event to be sure that the event mask returned by WaitCommEvent() is always valid? Is it possible to reset the flags of the serial port so that when I read all data from the queue and call WaitCommEvent() after then, it will not return immediately with a mask that was valid before I read the data?
The only solution that comes to my mind is this:
1) write a request
2) call WaitCommEvent() and wait until waiting finishes
3) read all the data that the device sends (not only the data being in the input queue at that moment)
4) call WaitCommEvent() which should return true immediately at the same time resetting the event flag set internally
5) do something and goto 1
Is it a good idea or is it stupid? Of course I know that the modem almost always finishes its answers with CRLF characters so I could set the comm mask to EV_RXFLAG and wait for the #10 character to appear, but there are many other devices with which I communicate and they do not always send frame end characters.
Your help will be appreciated. Thanks in advance!
Mariusz.
Your solution does sound workable. I just use a state machine to handle the transitions.
(psuedocode)
ioState := ioIdle;
while (ioState <> ioFinished) and (not aborted) do
Case ioState of
ioIdle : if there is data to read then set state to ioMidFrame
ioMidframe : if data to read then read, if end of frame set to ioEndFrame
ioEndFrame : process the data and set to ioFinished
ioFinished : // don't do anything, for doc purposes only.
end;
I am sending data to the server twice. First, I send "Hello world" and then I send "Server".
But the server received the data at 1 read. But the server have to read the data in a two-read operation.
Also, I write the data. Then read data from server and then I write the data.
In this case, the server can read the first data. But server can not read the second data.
The server uses read, write, read.
So how to overcome this issue? How do I write data to socket in BlackBerry?
What you describe is how TCP is supposed to work by default. What you are seeing is the Nagle algorithm (RFC 896) at work, reducing the number of outbound packets being sent so they are processed as efficiently as possible. You may be sending 2 packets in your code, but they are being transmitted together as 1 packet. Since TCP is a byte stream, the receiver should not be making any assumptions about how many packets it gets. You have to delimit your packet data in a higher-level protocol, and the receiver has to process data according to that protocol. It has to handle cases where multiple packets arrive in a single read, a single pakcet arriving in multiple reads, and everything in between, only processing packet data when they have been received in full, caching whatever is left over for subsequent reads to process when needed.
Hard to say without a little more detail, but it sounds like you're using 1-directional communication in the first case - i.e. the client writes, then writes again. There are any number of reasons that the server would receive the 2 writes as 1 read. Buffering on the client, somewhere in the wireless stack (or in the BES), buffering on the server side. All of those are legal with TCP/IP.
Without knowing anything more about your solution, have you thought about defining a small protocol - i.e. the client writes a known byte or bytes (like a 0 byte?) before sending the second write? Then the server can read, then recognize the delimiting byte, and say 'aha, this is now a different write from the client'?
As previously said this is an expected TCP behavior to save bandwidth. Note that to deliver your package TCP adds lot of data (e.g. destination port,sequence number, checksums...).
Instead of flushing the data I´ll recommend you to put more work in your protocol. For example you can define a header that contains the number of bytes to read and then the payload (the actual data).
The following code is a protocol encoded in an string with the structure [length];[data]
StringBuffer headerStr = new StringBuffer();
StringBuffer data = new StringBuffer();
//read header
char headerByte = dataInputStream.readChar();
while (headerByte != ';') {
headerStr.append(headerByte);
headerByte = dataInputStream.readChar();
}
//header has the number of character to read
int header= Integer.parseInt(headerStr.toString());
int bytesReaded = 1;
char dataByte = dataInputStream.readChar();
//we should read the number of characters indicated in the header
while (bytesReaded < header) {
data.append(dataByte);
dataByte = dataInputStream.readChar();
bytesReaded++;
}
For the first query, I guess you are using TCP. If you use UDP, then the server will read the packets in the order you want.
Can you be more clear/elaborative on the second query ?
I would try explicitly telling Connector.open to open up the stream as read_write. Then I would ensure that I flush my connections after each time I talked to the server.
SocketConnection connection = (SocketConnection) Connector.open(url, Connector.READ_WRITE);
OutputStream out = connection.openOutputStream();
// ... write to server
out.flush()
I got a solution to overcome to extract both the string
On sender device
Create a header which contains details of that data eg the data
length, datatype etc.
Add this header to the actual data and send it
On recipient device
read the header
retrieve the actual data length from the header
read the next data upto the data length as specified by the header