I need to do a C++ assignment, but I'm pretty much new to Win32 IPC.
I did a little bit of research yesterday, but I cannot find the thing I'm searching for.
Basically, I need two programs, the first creates a FileMapping with paging file, waits a buffer, display the buffer, and closes it.
The second connects to the Communication Channel, writes the buffer to the first program, and then closes.
The closest thing I've come to is this resource:
IPC Communication
but the guy there uses pipes instead of communication channels using paging file.
Also, I found that I can open a FileMapping with paging file pretty much like this:
TCHAR szMapFileName[] = _T("Local\\HelloWorld");
HANDLE hMapFile = CreateFileMapping(
INVALID_HANDLE_VALUE,
NULL, // Default security
PAGE_READWRITE, // Read/write access
0, // Max. object size
BUFFER_SIZE, // Buffer size
szMapFileName // Name of mapping object);
If somebody can provide a little help that would be very valuable(maybe a skeleton of an app?). I tried to do some research yesterday, but in vain.
Thanks
CreateFileMapping(), OpenFileMapping() and MapViewOfFile() are indeed the functions you need to call to allocate a shared memory buffer.
The first app must:
Create the File Mapping.
Create a synchronization object, preferably an Event, to wait on it.
Create an additional thread, which will be waiting for the Event to be signaled. Doing this in the main (UI) thread would block the application.
When the Event is signaled, post a custom message (WM_APP+nnn) to the main thread, to display the contents of the buffer.
It's a matter of specs or design what to do after this point, eg exit the application, just not be waiting for the buffer to receive data anymore, clear the event and wait again, etc.
The second app must:
Open the File Mapping and the Event. If failed, display an error message or warning and exit.
Write the data to the shared memory buffer.
Signal the Event.
Exit.
It could be further improved, eg the second app may not write to the buffer if the event is not in the nonsignaled state.
Related
I'm an intermediate C++ programmer, but I'm new to using Windows' API functions.
I'm trying to create a console program that will sit/sleep until either
The user inputs something in the console and presses enter
Serial data is received on a serial port that's already been opened
Searching around, it sounds like the way to do this in Windows is with Events, (which sound like they're the same basic idea as interrupts?)
I found documentation on the WaitCommEvent, and I've read about reading console input buffer events. I'm guessing the function to use is WaitForMultipleObjects, but what handles specifically do I send it so it will wait for both a serial RX event or a console standard input event?
UPDATE:
Thanks for the response!
Currently I've just been using std::cin/cout to read from and write to the console. I looked over the Console API you mentioned and saw the GetStdHandle function which will give the CONIN$ handle you mentioned. Can I just send that CONIN$handle to the wait function instead of using CreateFile and manually using ReadFile/the Console API like you suggested?
For the Serial, I know how to open my serial handle as OVERLAPPED instead of as NONOVERLAPPED, but I'm not sure what you mean by
it is not usually too difficult to modify synchronous I/O code to use
an asynchronous handle
Something like this?
uint32 read(HANDLE serialHandle, uint8* pBuffer, int32 bufferLenght)
{
DWORD dwBytesRead;
if (!ReadFile(SerialHandle, pBuffer, bufferLength, &dwBytesRead, NULL))
{ /*ERROR*/ }
else
{
// Wait on some flag or variable until read is complete
// to make this call synchronous/NONOVERLAPPED ?
return static_cast<uint32>(dwBytesRead);
}
}
What/where would that flag be to wait on until the read is complete?
From Low-Level Console Input Functions on MSDN:
A thread of an application's process can perform a wait operation to wait for input to be available in an input buffer. To initiate a wait operation, specify a handle to the input buffer in a call to any of the wait functions.
So you need to use a console handle, which you can obtain by calling CreateFile on CONIN$. You will also need to use the same handle, either with ReadFile or the console API, to read the console input; using runtime library functions is likely to mess you up due to buffering.
For the serial port, I believe you will need to use asynchronous I/O. The WaitCommEvent function (when provided with an asynchronous mode handle) accepts an OVERLAPPED structure containing a handle to a manual-reset event object. You would then use the same event handle in the call to WaitForMultipleObjects.
Unfortunately this is an all-or-nothing, so you have to open the COM handle in asynchronous mode and use asynchronous I/O exclusively. (Luckily, it is not usually too difficult to modify synchronous I/O code to use an asynchronous handle, although if there are a lot of I/O calls you might want to write a wrapper function to do the repetitive work of building the OVERLAPPED structure and waiting for the operation to complete.)
This code, which I have no control over, reads a file using overlapped I/O:
// Read file asynchronously
HANDLE hFile = CreateFile(..., FILE_FLAG_OVERLAPPED, ...);
BYTE buffer[10];
OVERLAPPED oRead = { 0 };
ReadFile(hFile, buffer, 10, NULL, &oRead);
// Do work while file is being read
...
// Wait for read to finish
WaitForSingleObject(hFile, INFINITE);
// ReadFile has finished
// buffer now contains data which can be used
...
In another thread (actually in an API hook of ReadFile), I need to signal the hFile to unblock the WaitForSingleObject. Normally Windows (or the device driver handling the ReadFile) does this, but I need to simulate it.
None of the APIs I found that normally do this work with hFile, including ReleaseMutex, ReleaseSemaphore, and SetEvent. They all return Error 6 (handle is invalid). Is there an API that works with a file, named pipe, or communications device?
I know it is not recommended to WaitForSingleObject(hFile), but the above code is a given, and I need to work with it. Thanks!
So far as I know, signaling the file handle takes place internally to Windows, and there is no API even when running in kernel mode. (I believe the file system driver simply tells Windows that the operation is complete and lets Windows figure out how to notify the user-mode process. I may be wrong.)
One resolution would be to issue a genuine ReadFile (a zero-byte read might be sufficient) against the handle in order to signal it.
But it would probably be more sensible to hook WaitForSingleObject, check whether it is being called on the file handle in question, and if so modify the behaviour as appropriate.
My application records audio samples from a microphone connected to my PC. So I chose the Windows WaveInXXX API to do the job.
After reading the documentation I decided to avoid using the callback mechanism with WaveInProc to save me the hassle synchronizing the threads. The whole application is pretty big and I thought this would make debugging simpler. When the application requests a block of samples, I just iterate over my buffer queue, take one out, copy the data, unprepare it, prepare it and add it back to the buffer queue. Basic program structure looks like this, I hope it makes the basic program flow clear:
WaveInOpen()
WaveInStart()
FunctionAddingPreparedBuffersToTheQueue()
while(someConditionThatEventuallyBecomesFalse)
if(NextBufferInQueueIsMarkedDone)
GetDataFromBuffer()
UnpreparePrepareHeaderAndAddBuffer()
else
WaitForAShortTime()
WaveInStop()
WaveInClose()
Now the problem appears: After some time (and I am unable to reproduce the exact condition), WaveInAddBuffer() causes a deadlock although it's in the same thread as all the rest. The header for the buffer that shall be added when the deadlock happens is prepared and dwFlags == WHDR_PREPARED == 2.
Any ideas what could cause this deadlock?
I have not seen such a problem, but a guess might be something like fragmentation related to all the unprepare/prepare cycles. They are not necessary. You can do the prepare once for each buffer and then unprepare when finished recording. (Prepare locks the buffer into physical memory.)
using socket with the overlapped operation selected the event-based completion notification;
Have 2 events, one for data, the other to cancel long send/recv:
HANDLE events[] = { m_hDataEvent, m_hInterruptEvent };
then calling WSASend,
WSASend(m_Socket, &DataBuf, 1, NULL, 0, &SendOverlapped, NULL);
followed by
WSAWaitForMultipleEvents(2, events, FALSE, INFINITE, FALSE);
which is setup to return on any one event signaled.
Now assume send is in progress, and m_hInterruptEvent is signaled.
WSAWaitForMultipleEvents returns, technically the function calling send can return as well and delete internally allocated buffers.
What is not clear to me, the WSASend may still be working in background, and deleting buffers will cause data corruption in best case.
What would be the proper way to stop the background Send/Receive, if the socket needs to be used for something else immediately?
I looked at the CancelIO(), but the MSDN never mentions it in relation to Sockets, does it work with file based IO only?
It makes no sense to try to cancel it once sent. Even if you succeeded you would have a problem because the receiving application would not have any idea that the transmission was interrupted. Your new message will be mistaken for the end of the old message.
If you feel the need to cancel long sends, you should probably look at your application design.
Send in chunks and check for cancellation in between chunks. Ensure you have a way of communicating to the receiver that the transmission was cancelled.
Close the socket to cancel. Again, ensure the client has a way to know that this is an interrupted transmission (for example if the client knows the total length in advance they will recognise an interrupted transmission).
Just wait for it to succeed in the background and don't worry. If you have urgent messages use a separate connection for them.
For your particular question "What would be the proper way to stop the background Send/Receive, if the socket needs to be used for something else immediately", the answer is: Sockets are cheap - Just use two - one for the slow transmission the other for the urgent messages.
When I run this program
OVERLAPPED o;
int main()
{
..
CreateIoCompletionPort(....);
for (int i = 0; i<10; i++)
{
WriteFile(..,&o);
OVERLAPPED* po;
GetQueuedCompletionStatus(..,&po);
}
}
it seems that the WriteFile didn't return until the writing job is done. At the same time , GetQueuedCompletionStatus() gets called. The behavior is like a synchronous IO operation rather than an asynch-IO operation.
Why is that?
If the file handle and volume have write caching enabled, the file operation may complete with just a memory copy to cache, to be flushed lazily later. Since there is no actual IO taking place, there's no reason to do async IO in that case.
Internally, each IO operation is represented by an IRP (IO request packet). It is created by the kernel and given to the filesystem to handle the request, where it passes down through layered drivers until the request becomes an actual disk controller command. That driver will make the request, mark the IRP as pending and return control of the thread. If the handle was opened for overlapped IO, the kernel gives control back to your program immediately. Otherwise, the kernel will wait for the IRP to complete before returning.
Not all IO operations make it all the way to the disk, however. The filesystem may determine that the write should be cached, and not written until later. There is even a special path for operations that can be satisfied entirely using the cache, called fast IO. Even if you make an asynchronous request, fast IO is always synchronous because it's just copying data into and out of cache.
Process monitor, in advanced output mode, displays the different modes and will show blank in the status field while an IRP is pending.
There is a limit to how much data is allowed to be outstanding in the write cache. Once it fills up, the write operations will not complete immediately. Try writing a lot of data at once, with may operations.
I wrote a blog posting a while back entitled "When are asynchronous file writes not asynchronous" and the answer was, unfortunately, "most of the time". See the posting here: http://www.lenholgate.com/blog/2008/02/when-are-asynchronous-file-writes-not-asynchronous.html
The gist of it is:
For security reasons Windows extends files in a synchronous manner
You can attempt to work around this by setting the end of the file to a large value before you start and then trimming the file to the correct size when you finish.
You can tell the cache manager to use your buffers and not its, by using FILE_FLAG_NO_BUFFERING
At least it's not as bad as if you're forced to use FILE_FLAG_WRITE_THROUGH
If GetQueuedCompletionStatus is being called, then the call to WriteFile is synchronous (and it has returned), but it can still modify &o even after it's returned if it is asynchronous.
from this page in MSDN:
For asynchronous write operations,
hFile can be any handle opened with
the CreateFile function using the
FILE_FLAG_OVERLAPPED flag or a socket
handle returned by the socket or
accept function.
also, from this page:
If a handle is provided, it has to
have been opened for overlapped I/O
completion. For example, you must
specify the FILE_FLAG_OVERLAPPED flag
when using the CreateFile function to
obtain the handle.