I am trying to set a timeout on write operations when using GCDAsyncSocket. The code is pretty simple and is the following.
[iAsyncSocket writeData:bytesToSend withTimeout:3.0 tag:0];
Then I disable the Internet connection on my Mac and wait for write timeout to occur, but nothing happens. I don't get a disconnection with a GCDAsyncSocketWriteTimeoutError error as I should.
I have also validated that my server stops, as expected, receiving the messages after I turn off the Internet connection.
I have looked inside the source code and I have found out that the writeTimer, that is responsible for firing a write timeout event, is always cancelled (function endCurrentWrite is called). Tracing back to where the timer is cancelled, I ended up at the following line of code.
ssize_t result = write(socketFD, buffer, (size_t)bytesToWrite);
The write system call always returns the total number of bytes that I am sending, as if the socket manages to send the data although there is no Internet connection. Is this logical?
Has anyone come up with the same problem or seen similar behaviour? Or has anyone managed to set a write timeout for a GCDAsyncSocket?
Thanks a lot.
Related
As we all know, in the CAN bus communication protocol, sender know whether the data was successfully sent. I send socketcan data as follows.
ret = write (socket, frame, sizeof (struct can_frame));
However, even if the CAN communication cable is disconnected, the return value of ret is still 16(=sizeof (struct can_frame)).I queried the information and found that the problem was due to the tx_queue of the network stack used by socketcan. When write is called multiple times, the buffer is full and the return value of ret is -1.
But this is not the behavior I expect, I hope that every frame of data sent will immediately get the status of success or failure.
By
echo 0> / sys / class / net / can0 / tx_queue_len
I want to cancel the tx_queue, but it does not work.
What I want to ask is, is there a way to cancel the tx_queue of socketcan, or to get the status of the each sending frame about controller through the API (such as libsocketcan).
Thanks.
You cannot use write() itself to discover whether a CAN frame was successfully put on the bus, because all it does is write the frame to the in-kernel socket buffer. The kernel then moves the frame to the transmit queue of the SocketCAN network interface, followed by the driver moving it to the transmit buffer of the CAN controller, which finally puts the frame on the bus. What you want is a direct write which bypasses all those buffers, but that's not possible with SocketCAN, even if you set the transmit queue length to 0.
However, there is another way to get confirmation. If you enable the CAN_RAW_RECV_OWN_MSGS socket option (see section 4.1.4 and 4.1.7 in the SocketCAN documentation), you will receive frames that were successfully sent. You'll need to use recvmsg() so you get the message flags. msg_flags will have the MSG_CONFIRM bit set for a frames that was successfully sent by the same socket on which it is received. You won't be informed of failures, but you can detect them by using a timeout for the confirmation.
It's not an ideal solution because it mixes the read and write logic in your application. One way to avoid this would be to use two sockets. One for writing and reading MSG_CONFIRM frames, the other for reading all other frames. You could then create a (blocking) write function that does a write() followed by multiple calls to recvmsg() with an appropriate timeout.
Finally, it is useful to enable error frames (through the CAN_RAW_ERR_FILTER socket option). If you send a frame on a socket with a disconnected cable, this will typically result in a bus off state, which will be reported in an error frame.
I create epoll and register some non-blocking sockets which try connect to closed ports on localhost. Why epoll tells me, that i can write to this socket (it give event for one of created socket with eventmask contain EPOLLOUT)? But this socket doesn't open and if i try send something to it i get an error Connection refused.
Another question - what does mean even EPOLLHUP? I thought that this is event for refused connection. But how in this case event can have simultaneously EPOLLHUP and EPOLLOUT events?
Sample code on Python:
import socket
import select
poll = select.epoll()
fd_to_sock = {}
for i in range(1, 3):
s = socket.socket()
s.setblocking(0)
s.connect_ex(('localhost', i))
poll.register(s, select.EPOLLOUT)
fd_to_sock[s.fileno()] = s
print(poll.poll(0.1))
# prints '[(4, 28), (5, 28)]'
All that poll guarantees is that your application won't block after calling corresponding function. So you are getting what you've paid for - you can now rest assured writing to this socket won't block - and it didn't block, did it?
Poll never guarantees that corresponding operation will succeed.
poll/select/epoll return when the file descriptor is "ready" but that just means that the operation will not block (not that you will necessarily be able to write to it successfully).
Likewise for EPOLLIN: for example, it will return ready when a socket is closed; in that case, you won't actually be able to read data from it.
EPOLLHUP means that there was a "hang up" on the connection. That would really only occur once you actually had a connection. Also, the documentation (http://linux.die.net/man/2/epoll_ctl) says that you don't need to include it anyway:
EPOLLHUP
Hang up happened on the associated file descriptor. epoll_wait(2) will always wait for this event; it is not necessary to set it in events.
Is it possible to implement the equivalent of Socket.Poll in async/await paradigm (or BeginXXX/EndXXX async pattern)?
A method which would act like NetworkStream.ReadAsync or Socket.BeginReceive but:
leave the data in the socket buffer
complete after the specified interval of time if no data arrived (leaving the socket in connected state so that the polling operation can be retried)
I need to implement IMAP IDLE so that the client connects to the mail server and then goes into waiting state where it received data from the server. If the server does not send anything within 10 minutes, the code sends ping to the server (without reconnecting, the connection is never closed), and starts waiting for data again.
In my tests, leaving the data in the buffer seems to be possible if I tell Socket.BeginReceive method to read no more than 0 bytes, e.g.:
sock.BeginReceive(b, 0, 0, SocketFlags.None, null, null)
However, not sure if it indeed will work in all cases, maybe I'm missing something. For instance, if the remote server closes the connection, it may send a zero-byte packet and not sure if Socket.BeginReceive will act identically to Socket.Poll in this case or not.
And the main problem is how to stop socket.BeginReceive without closing the socket.
I am make a application base on lwip,the applcation just send data to the server;
When my app works for some times (about 5 hours),I found that the send thread hung in send() function,and after about 30min send() return 0,and my thread run agin;
In the server side ,have make a keepalive,its time is 5min,when my app hungs,5min later the server close the sockect,but my app have not get this,still hungs in send() until 30min get 0 return; why this happen?
1: the up speed is not enough to send data,it will hungs in send?
2: maybe the server have not read data on time,and it make send buff full and hungs?
how can i avoid these peoblems in my code ? I have try to set TCP_NODELAY,SO_SNDTIMEO and select before send,but also have this problem.
send() blocks when the receiver is too far behind the sender. recv() returns zero when the peer has closed the connection, which means you must close the socket and stop reading.
I am attempting to write an FTP Client and I need to print out the server response to my commands. One of these commands is STAT. The server returns the response and as I understand it the response is in the socket buffer which I can read using the read() command. The problem is I only need the response for STAT so I know it will end with END OF STATUS. This is the code I wrote to read the response:
in = read(connFd, &timebuffer, sizeof(timebuffer));;
while(in>0){
printf("%s", timebuffer);
memset(&timebuffer, 0, sizeof timebuffer);
in = read(connFd, &timebuffer, sizeof(timebuffer));
}
memset(&timebuffer, 0, sizeof timebuffer);
The problem I am getting is that once the read() function goes through the buffer and finishes reading the while loop does not terminate and continues infinitely, my program just sits there. I assume it is because the read() function is waiting for data so I was wondering if there is a way to tell read() to stop once the end of the buffer is reached. I thought this would happen automagically since read() would return something x<1 but if it is waiting I understand what the problem is. So how would I fix it? Is there a way to set up a timeout(0) so it would only read data if it is there already? Also I know there are "flags" that I set to 0 but I can't find much info on them. I appreciate any help. Would the only way be to check for "END OF STATUS" string in the buffer? Would I use strstr(buffer)
read is a blocking call (unless you've set the socket to be non-blocking) and so will only return once its received the exact number of bytes you've requested or the socket gets closed.
If the socket is set to be non-blocking then you will get a zero return to "read" but you may get that even when you haven't reached the end of your response because your program will certainly be faster than the network.
As an additional note... You can't use strstr() unless you concatenate all your reads. You could get 1/2 of the terminate message in one read and the remaining in the next read.