I know it's probably a basic question, but I'd like to hear the best way to realize it.
So to the problem. I have a driver thread using a call to select, and I have a GUI thread that needs sometimes to interrupt select by writing to some file descriptor within the same process (GUI FD or something). I used pipe in UNIX, but I'm not experienced in sockets for Windows, so I'm not sure what kind of FD I should use. Example is greatly appreciated but not required ).
Thanks.
select() is not the best to implement asynchronous I/O under Windows. unfortunately, the select() call on windows only works with socket handles and not with pipe or fie handles.
you should have a look at overlapped I/O.
by using an event in your overlapped structure, you can have a behaviour close to select(). any event on a socket will trigger an event, which you can wait on by using WaitForMultipleObjects(). now, your GUI thread can signal the I/O thread by setting a specific (separate) event, which you create using the CreateEvent() call.
You could set the timeout shorter on the select and then loop round if it was a timeout. Or you could send a simple packet to the socket being selected causing it to wake up.
Related
I have iocp running and working (mostly) -- but should calling CloseHandle() on a handle cause it to complete?
e.g., I've called ReadFile() and it's now waiting for input to read. At another point I'm calling CloseHandle() from another thread in the pool used to service iocp completion packets. I expect for there to be a completion on the previous ReadFile() call with an ERROR_INVALID_HANDLE, but I'm not seeing that. Instead, it never seems to return. Could this be a sign that something else is holding a reference to the handle? If so, how would you suggest figuring that out/debugging it?
Any suggestions?
Thanks!
I've created two threads A & B using CreateThread windows API. I'm trying to send the data from thread A to B.
I know I can use Event object and wait for the Event object in another using "WaitForSingleObject" method. What this event does all is just signal the thread. That's it! But how I can send a data. Also I don't want thread B to wait till thread A signals. It has it own job to do. I can't make it wait.
I can't find a Windows function that will allow me to send data to / from the worker thread and main thread referencing the worker thread either by thread ID or by the returned HANDLE. I do not want to introduce the MFC dependency in my project and would like to hear any suggestions as to how others would or have done in this situation. Thanks in advance for any help!
First of all, you should keep in mind that Windows provides a number of mechanisms to deal with threading for you: I/O Completion Ports, old thread pools and new thread pools. Depending on what you're doing any of them might be useful for your purposes.
As to "sending" data from one thread to another, you have a couple of choices. Windows message queues are thread-safe, and a a thread (even if it doesn't have a window) can have a message queue, which you can post messages to using PostThreadMessage.
I've also posted code for a thread-safe queue in another answer.
As far as having the thread continue executing, but take note when a change has happened, the typical method is to have it call WaitForSingleObject with a timeout value of 0, then check the return value -- if it's WAIT_OBJECT_0, the Event (or whatever) has been set, so it needs to take note of the change. If it's WAIT_TIMEOUT, there's been no change, and it can continue executing. Either way, WaitForSingleObject returns immediately.
Since the two threads are in the same process (at least that's what it sounds like), then it is not necessary to "send" data. They can share it (e.g., a simple global variable). You do need to synchronize access to it via either an event, semaphore, mutex, etc.
Depending on what you are doing, it can be very simple.
Thread1Func() {
Set some global data
Signal semaphore to indicate it is available
}
Thread2Func() {
WaitForSingleObject to check/wait if data is available
use the data
}
If you are concerned with minimizing Windows dependencies, and assuming you are coding in C++, then I recommend using Boost.Threads, which is a pretty nice, Posix-like C++ threading interface. This will give you easy portability between Windows and Linux.
If you go this route, then use a mutex to protect any data shared across threads, and a condition variable (combined with the mutex) to signal one thread from the other.
Don´t use a mutexes when only working in one single process, beacuse it has more overhead (since it is a system-wide defined object)... Place a critical section around Your data and try to enter it (as Jerry Coffin did in his code around for the thread safe queue).
Is there any scalable Win32 API (like IOCP not like select) that gives you reactor style
operations on sockets? AFAIK IOCP allows you to receive notification on completed operations
like data read or written (proactor) but I'm looking for reactor style of operations: I
need to get notification when the socket is readable or writable (reactor).
Something similar to epoll, kqueue, /dev/poll ?
Is there such API in Win32? If so where can I find a manual on it?
** Clarification:** I need select like api for sockets that is as scalable as IOCP, or I'm looking for a way to use IOCP in reactor like operations.
Even more clarification: IOCP allows you to receive an notification on completion of given operation. For example:
WSARecv(buffer,...); // start reading
WSAWaitForMultipleEvents(...); // wait when read is done
So I get notication after operation is done -- proctor style of operations.
What I need is something like that:
WSARecv( NOTHING ); // start waiting for readability (not actual read)
WSAWaitForMultipleEvents(...); // wait until read would not block
// Now WSARecv would not block
WSARecv(buffer,...); // now actual non-blocking read
How can I do this?
You want to look at the WSAAsyncSelect API. It uses a Windows message queue to signal that a handle is read for read/write/whatever, so it doesn't have the concurrency benefits of IOCP, but it allows you to implement a standard reactor pattern without having a limit to the number of handles (like WSAWaitForMultipleEvents).
I'm confused, isn't Reactor pattern where the thread blocks waiting on multiple event sources? That would be select(), which windows supports. The Proactor pattern is where there is a single callback per call, that you can do via ReadFileEx/WriteFileEx.
Not possible.
I've checked Boost.Asio sources that do have reactor style operations and use IOCP. For all reactor style operations separate thread with select is used instead of IOCP.
Have you tried passing zero nNumberOfBytesToRead to, for example ReadFile(socket_fd, ..)?
Maybe it will help to get the "read ready" event.
I have an application that message-loops using MsgWaitForMultipleObjects to catch additional events while pumping ui messages. It seems that as soon as a window is being moved or resized, DefWindowProc begins it's own message loop until the mouse is being released. This situation prevents from the outer loop to catch the additional messages in the meantime.
I'd hate to multi-thread the application just because of this. Is there any other way to solve it?
MsgWaitForMultipleObjects has very few uses in a traditional multi threaded program. It has some use in Games - where traditional non client frame elements are omitted and APIs like "MessageBox" and "DoDragDrop" are avoided...
Usually it finds its best use in "UI worker" threads that don't host visible windows, but use the message queue as an inter thread messaging system and also need to wait on kernel handles.
In your case, making a second thread does not seem avoidable. Ironically, PostThreadMessage + MsgWaitForMultipleObjects will probably be the easiest way to set up a reliable communication mechanism between the GUI thread and your "ui" worker thread.
There are several additional places in the Windows API that will enter their own message loop. If you need to continue handling your messages during these times then you'll need a separate thread.
What happens when you call WaitForSingleObject() on a handle you've created with CreateFile() or _get_osfhandle()?
For reasons not worth explaining I would like to use WaitForSingleObject() to wait on a HANDLE that I've created with _get_osfhandle(fd), where fd comes from a regular call to _open(). Is this possible?
I have tried it in practice, and on some machines it works as expected (the HANDLE is always in the signaled state because you can read more data from it), and on some machines WaitForSingleObject() will block indefinitely if you let it.
The MSDN page for WaitForSingleObject() says that the only supported things that it handles are "change notifications, console input, events, memory resource notifications, mutex, processes, semaphores, threads, and waitable timers."
Additionally, would it be different if I used CreateFile() instead of _get_osfhandle() on a CRT file descriptor?
Don't do it. As you can see, it has undefined behavior.
Even when the behavior is defined, it's defined in such a way as to be relatively not useful unless you don't like writing additional code. It is signaled when any asynchronous I/O operation on that handle completes, which does not generalize to tracking which I/O operation finished.
Why are you trying to wait on a file handle? Clearly the intent matters when you are doing something that isn't even supported well enough to not block indefinitely.
I found the following links. The concensus seems to me, don't do it.
Asynch IO explorer
Waiting on a file handle
When an I/O operation is started on an
asynchronous handle, the handle goes
into a non-signaled state. Therefore,
when used in the context of a
WaitForSingleObject or
WaitForMultipleObjects operation, the
file handle will become signaled when
the I/O operation completes. However,
Microsoft actively discourages this
technique; it does not generalize if
there exists more than one pending I/O
operation; the handle would become
signaled if any I/O operation
completed. Therefore, although this
technique is feasible, it is not
considered best practice.
Egghead Cafe:
Use ReadDirectoryChangesW in
overlapped mode. WaitForSingleObject
can wait on the event in the
OVERLAPPED struct.
You can also use the API
WaitForSingleObject() to wait on a
file change if you use the following
change notification function:
FindFirstChangeNotification()
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/fileio/fs/findfirstchangenotification.asp
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dllproc/base/waitforsingleobject.asp
An interesting note on "evilness" of ReadDirectoryChangesW:
http://blogs.msdn.com/ericgu/archive/2005/10/07/478396.aspx