Two threads using the same websock handle- does it cause any issue? - c++

We are having a C++ application to send and receive WebSocket messages
One thread to send the message (using WinHttpWebSocketSend)
the second thread to receive (using WinHttpWebSocketReceive)
But the same WebSocket handle is used across these 2 threads. Will it cause any problems? I don't know if we have to handle it another way. It works in our application - we are able to send and receive messages - but I don't know if it will have any problem in the production environment. Any one has better ideas?

Like most platforms, nearly all Windows API system calls do not provide thread barriers beyond preventing simultaneous access to the key parts of the kernel. While I could not say for sure (the documentation doesn't seem to answer your explicit question) I would be surprised if the WinHTTP API provides barriers that prevent multiple threads from stepping on each other (so to speak)--particularly because it's really just a "helper" API that uses the somewhat lower level Winsock stuff directly--and I would take it upon myself to implement the necessary barriers.
I'm also wondering why you're using threads in this manner to begin with. I know essentially nothing about the WinHTTP API, but I did notice WINHTTP_OPTION_ASSURED_NON_BLOCKING_CALLBACKS which leads me to believe that you can implement an asynchronous approach which would prevent any thread-safety issues to begin with (and probably be much faster and memory efficient).
It appears that the callback mechanism for WinHTTP is rather expressive. See WINHTTP_STATUS_CALLBACK. Presumably, you can simply use non-blocking operation, create an event listener, and associate the connection handle with dwContext. No threads involved.

Related

c++ watchdog for 3rd party lib calls

I have a problem with long running boost::regex_match(...) invocation in a threaded process environment. But it could be another lib (API call) having the same problem.
Is there a generic way to set up a watchdog for such?
For non-threaded process alarm() can be used to detect timeout.
However, signals don't play nicely with threads. I can avoid direct use of alarm() in the thread and delegate timer mgt. to a dedicated separate thread and let that one use pthread_kill(...) to address the correct threads (this is just an idea - i didn't yet verify that part).
However, also this only interrupts and detects the situation, but cannot gracefully stop boost::regex_match(...).
I played around with Throwing an exception from within a signal handler using sigsetjmp() and siglongjmp() for the thread using boost::regex_match(..).
But it causes memory leaks in boost::regex_match(...) becausesiglongjmp()` bypasses destructors.
How can i gracefully stop a 3rd party API call - presuming that it's implemented exception safe?
Or does it have to be supported by some "stoppable" feature actively implemented in the 3rd party API? (is there some for the boost library?)
Maybe some strange idea, but:
Code can be implemented to be "thread-safe" and/or "exception-safe".
Would it be an option to define "longjmp-safe"? This could be done by passing an additional token to a lib to let is associate all resource allocations to that token. After longjmp() the client SW could ask the API separately to release those resources.
simpler maybe would just be some central init()/release() or register()/unregister() API call, by which the API could clean-up itself.
In a case where you have to:
monitor exceeding execution time
stop execution of processing
you should simply think for tasks instead of threads.
Using threads is something which sounds like "state of the art" but in practice tasks are very often the better way of implementation. Especially for controlling memory leeks in "undefined" end of execution, confine unwanted memory excess and control stack overruns etc.
In the case you have mentioned I tend to implement that as tasks. IPC works well on all known platforms but is not portable. If portability is no problem, changing to a task based solution is not a big deal.
A hanging task can be killed by a os call and all locks, memory and other resources like ipc/shared memory/pipes etc. will be removed automatically. So this fits much better to your problem and it did not depend on your external and maybe unchangeable third party components.

C++ Fastest Way to Hit a URL

I'm trying to ping a URL on a server in the middle of my high-performance C++ application, where every millisecond is critical. I don't care about the return data from the query... I just need to send a HTTP request to a specific URL (to cause it to load), and I'm trying to find the most effective, non-blocking method to accomplish this.
My application uses Boost::ASIO, but most methods to do this seem to involve building and tearing down sockets each time (which might unfortunately be necessary), but I'm hoping there's a basic C/C++ socket one-liner that won't cause any overhead, memory leaks, blocking, etc. Just quickly open a socket, shoot the HTTP request off, and move along.
And this will need to happen thousands of times per second, so sockets and overhead is important (don't want to flood the OS).
Anyone have any advice on the most efficient way to accomplish this?
Thanks so much!
With thousands of notifications sent per second, I can't imagine opening a socket connection for each one. That would probably be too inefficient due to the overhead. So, as Casey suggested, try using a dedicated connection.
Since it sounds like you are doing quite a bit of processing on your main thread, you might consider creating a worker thread for the socket work. You will probably need to use thread synchronization objects like a mutex or critical section to single thread the code - at least when updating a container (probably a queue) from your main thread and reading it from the worker thread.

Can I use Boost::Asio and not worry about network programming problems?

I have to make a server in my current project but I don't have any or little experience in this area. My question is, can I just use Asio in my project and it will simply handle any problems a normal server has to face (partial reads, multithreading problems, ...)?
(My server will have to handle hundreds of clients at the same time)
ASIO takes care of the low-level socket programming and polling code. You still have to provide all the functionality to process raw network data. Ultimately, you get an unpredictable number of bytes from the network any time a read callback is called, and it is up to you to take those bytes and reconstruct your application message from them.
But indeed, as far as receiving an unspecified number of bytes is concerned, you won't have to worry about how that is implemented.
Multithreading is "easy" in the sense that you can run the ASIO processor multiple times concurrently, but it is your responsibility to provide a read callback that can deal with being run multiple times at once.
Asio is intentionally not multithreaded. It handles concurrency by multiplexing via the operating system's select(), kqueue, epoll, or other mechanism.
As for partial receives, there is no automatic way to get TCP to respect message boundaries. Asio can't do anything about that, so you'll need some technique at the application level to indicate completion. HTTP traditionally handles this by closing the socket when it's finished, though it's also possible to pre-send the size of the message.

VNC viewer implementation

Our team is implementing a VNC viewer (=VNC client) on Windows. The protocol (called RFB) is stateful, meaning that the viewer has to read 1 byte, see what it is, then read either 3 or 10 bytes more, parse them, and so on.
We've decided to use asynchronous sockets and a single (UI) thread. Consequently, there are 2 ways to go:
1) state machine -- if we get a block on socket reading, just remember the current state and quit. Later on, a socket notification will arrive and the interrupted logic will resume from the proper stage;
2) inner message loop -- once we determine that reading from the socket would block, we enter an inner message loop and spin there until all the necessary data is finally received.
UI is not thus frozen in case of a block.
As experience showed, the second approach is bad, as any message can come while we're in the inner message loop. I cannot tell the full story here, but it simply is not reliable enough. Crashes and kludges.
The first option seems to be quite acceptable, but it is not easy to program in such a style. One has to remember the state of an algorithm and values of all the local variables required for further processing.
This is quite possible to use multiple threads, but we just thought that the problems in this case would be even much harder: synchronization of frame-buffer access, multi-threading issues, etc. Moreover, even in this variant it seems necessary to use asynchronous sockets as well.
So, what way is in your opinion the best ?
The problem is quite a general one. This is the problem of organizing asynchronous communication through stateful protocols.
Edit 1: We use C++ and MFC as UI framework.
I've done a few parallel computing projects and it seems that MPI (Message Passing Interface) might be helpful to your VNC project. You're probably not so interested in the parallel computing power provided by MPI, but you may want to use the simplified socket-like interface for asynchronous communication over a network.
http://www.open-mpi.org/
You can find other implementations of MPI and tons of use examples from google.
Don't bother with CSocket, you'll move to CAsyncSocket in the end because of the extra control you get (interrupting, shutting down etc.). I'd also recommend using a separate thread to manage the communication, it adds complexity but keeping the UI responsive should be a top priority.
I think you will find that your design will be simplified greatly by using a separate thread to handle a blocking socket.
The main reason for this is you don't need to spin and wait. The UI remains responsive while the network thread(s) block when it has nothing to do and comes back when it has stuff to do. You are effectively offloading a large portion of your overhead to the OS.
Remember, RFB does not require a whole lot of state info to work. Because client to server messages are short; there is nothing requiring you to receive a frame buffer before you send your next pointer input.
My point being is messages in RFB can be intermixed; the server will work on your schedule.
Now, Windows provides easy to use synchronization API's that while not always the most efficient, are more than enough for your purposes and will ease getting a proof of concept up and going.
Take a look at Windows Synchronization and specifically Critical Sections
Just my 2cents, I've implemented both a vnc server and client on windows, these were my impressions.

Most suitable asynchronous socket model for an instant messenger client?

I'm working on an instant messenger client in C++ (Win32) and I'm experimenting with different asynchronous socket models. So far I've been using WSAAsyncSelect for receiving notifications via my main window. However, I've been experiencing some unexpected results with Winsock spawning additionally 5-6 threads (in addition to the initial thread created when calling WSAAsyncSelect) for one single socket.
I have plans to revamp the client to support additional protocols via DLL:s, and I'm afraid that my current solution won't be suitable based on my experiences with WSAAsyncSelect in addition to me being negative towards mixing network with UI code (in the message loop).
I'm looking for advice on what a suitable asynchronous socket model could be for a multi-protocol IM client which needs to be able to handle roughly 10-20+ connections (depending on amount of protocols and protocol design etc.), while not using an excessive amount of threads -- I am very interested in performance and keeping the resource usage down.
I've been looking on IO Completion Ports, but from what I've gathered, it seems overkill. I'd very much appreciate some input on what a suitable socket solution could be!
Thanks in advance! :-)
There are four basic ways to handle multiple concurrent sockets.
Multiplexing, that is using select() to poll the sockets.
AsyncSelect which is basically what you're doing with WSAAsyncSelect.
Worker Threads, creating a single thread for each connection.
IO Completion Ports, or IOCP. dp mentions them above, but basically they are an OS specific way to handle asynchronous I/O, which has very good performance, but it is a little more confusing.
Which you choose often depends on where you plan to go. If you plan to port the application to other platforms, you may want to choose #1 or #3, since select is not terribly different from other models used on other OS's, and most other OS's also have the concept of threads (though they may operate differently). IOCP is typically windows specific (although Linux now has some async I/O functions as well).
If your app is Windows only, then you basically want to choose the best model for what you're doing. This would likely be either #3 or #4. #4 is the most efficient, as it calls back into your application (similar, but with better peformance and fewer issues to WSAsyncSelect).
The big thing you have to deal with when using threads (either IOCP or WorkerThreads) is marshaling the data back to a thread that can update the UI, since you can't call UI functions on worker threads. Ultimately, this will involve some messaging back and forth in most cases.
If you were developing this in Managed code, i'd tell you to look at Jeffrey Richter's AysncEnumerator, but you've chose C++ which has it's pros and cons. Lots of people have written various network libraries for C++, maybe you should spend some time researching some of them.
consider to use the ASIO library you can find in boost (www.boost.org).
Just use synchronous models. Modern operating systems handle multiple threads quite well. Async IO is really needed in rare situations, mostly on servers.
In some ways IO Completion Ports (IOCP) are overkill but to be honest I find the model for asynchronous sockets easier to use than the alternatives (select, non-blocking sockets, Overlapped IO, etc.).
The IOCP API could be clearer but once you get past it it's actually easier to use I think. Back when, the biggest obstacle was platform support (it needed an NT based OS -- i.e., Windows 9x did not support IOCP). With that restriction long gone, I'd consider it.
If you do decide to use IOCP (which, IMHO, is the best option if you're writing for Windows) then I've got some free code available which takes away a lot of the work that you need to do.
Latest version of the code and links to the original articles are available from here.
And my views on how my framework compares to Boost::ASIO can be found here: http://www.lenholgate.com/blog/2008/09/how-does-the-socket-server-framework-compare-to-boostasio.html.