Help with Asynchronous I/O

Question

I have been doing a lot of searching and still can't seem to figure out how to fix my issue. I am writing a GUI program (in WinAPI so no MFC please) to communicate with another program (command line based). I am using anonymous pipes since everything is local (but perhaps named pipes would be better?) which then I use CreateProcess(); to run the program I am trying to get the output from.

Now, I just moved from synchronous to asynchronous a few hours ago and am running into a few problems (even though synchronous did not do what I wanted anyway). The first problem is that I am still running into the same problem I had while running synchronous i/o; if I run my "read" (or "write" for that matter) function more than once, the program will freeze. I cannot have this because the purpose of the program is to periodically update to the GUI what the output of the command prompt is.

The second problem, and ultimately more serious is a new one with my asynchronous i/o; it does not read the entire output like my synchronous did. It reads until the program I am reading from sends the return character (or else it is simply coincidental that that is the point where it happens to stop reading). I feel like perhaps I am not fully understanding the capabilities of the OVERLAP, but I'm feeling bogged down by reading so much of the MSDN right now, so perhaps I am overlooking some important aspects of it.

So basically, the code below is the bare minimum of what I am doing. I have tried using various for() and while() loop techniques to try to get through all of the output data, but it seems to be no go. Note that BUFSIZE is defined at 0x1000 which is in fact more than the little test program I wrote for this needs even.

    ::ReadFile(_hChild_Out_Rd, chBuf, BUFSIZE, &dwRead, &o1);
chBuf[dwRead] = '\0';
::SetDlgItemTextA(global,IDO_WORLDOUT,chBuf);

So does anybody have any ideas?

Thanks so much for your help!

Regards,
Dennis M.

Answer 1

That looks like synchronous code. With asynchronous (OVERLAPPED) I/O, you can't use the buffer until the operation completes. Set the hEvent member of the OVERLAPPED structure, and change your main loop from PeekMessage to MsgWaitForMultipleObjects so that your program can respond to I/O events. Then you can wait on both the OVERLAPPED operation handle, and also the process handle, so you know when the other program exited.

Answer 2

You may want to look into memory mapped files, you can construct something in memory that is a queue and write from one place and read from the other. In a project I did I had a circular queue where one process was writing to the queue and the other was reading whenever it had time.

You create a memory mapped file like this:

SECURITY_ATTRIBUTES sa;
sa.bInheritHandle = TRUE;
sa.nLength = sizeof( sa );
sa.lpSecurityDescriptor = NULL;

hdFile = CreateFileMapping(INVALID_HANDLE_VALUE,&sa,
   PAGE_READWRITE,0,dwBufSize, L"SomeName");

Then you get a pointer to the shared memory like this

pOutSharedMemory=MapViewOfFile(hdFile,FILE_MAP_ALL_ACCESS,0,0,dwBufSize);

now you can map pointers to the memory

e.g. a couple of counters to keep track of where you are in memory (within dwBufSize)

pdwReadOffset = reinterpret_cast<DWORD*>(pOutSharedMemory)
pdwWriteOffset = pdwReadOffset + sizeof(DWORD)

then have some struct which you then read/write to the memory using memmove

you would have to take care of synchronization in place using critical sections etc.