CODEWITHSUNDEEP
c++asynchronousnamed-pipes
Frank
Frank

Reputation: 2066

ReadFile using NamedPipe blocks though using overlapped

I am using NamedPipes to communicate between different modules of our windows application. At one point it is possible that a read operation could take long time so we would like to add a timeout. I added the OVERLAPPED-flag like this:

pipeHandle = CreateFile(
            pipename,
            PIPE_ACCESS_DUPLEX | FILE_FLAG_OVERLAPPED,
            0,
            nullptr,
            OPEN_EXISTING,
            0,
            nullptr);

Then the read operation looks like this:

    OVERLAPPED overlapped;
        ZeroMemory(&overlapped, sizeof(OVERLAPPED));
        overlapped.hEvent = CreateEvent(nullptr, TRUE, FALSE, nullptr);
    successful = ReadFile(
            pipeHandle,
            buffer,
            4096 * sizeof(wchar_t),
            &numBytesRead,
            &overlapped
        );
    if (successful == FALSE)
    {
        LOG("Reading was not successful");
        if (GetLastError() != ERROR_IO_PENDING)
        {
            // The function call failed. ToDo: recovery.
            LOG_LAST_ERROR("Failed because of Error: ");
            return ERROR_READ_FAULT;
        }
        else
        {
            // TODO: add a timeout...
            successful = GetOverlappedResult(pipeHandle, &overlapped, &numBytesRead, TRUE);
        }
    }
    else
    {
        LOG("Reading was successful");
        // I/O completed:
        return ERROR_SUCCESS;
    }

Now I would expect that call to ReadFile would return immediatley so I could handle possible timeouts, but instead it blocks till the call returns.

What am I doing wrong?

Upvotes: 0

Views: 622

Answers (1)

jlahd
jlahd

Reputation: 6303

In your code, the FILE_FLAG_OVERLAPPED is in the dwDesiredAccess parameter, but it should be specified in the dwFlagsAndAttributes parameter.

So, this might work better:

pipeHandle = CreateFile(
        pipename,
        PIPE_ACCESS_DUPLEX,
        0,
        nullptr,
        OPEN_EXISTING,
        FILE_FLAG_OVERLAPPED,
        nullptr);

Upvotes: 1

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