如何确定ReadFile函数超时？

Question

When I do not set timeout settings via GetTimeout & SetTimeout after I connect my USB-UART converter to my laptop, the ReadFile becomes blocking. However, once I set them via SetTimeOut, the ReadFile is not blocking anymore and returns true even if the given number of bytes were not read.

When ReadFile returns true but the dwRead parameter says 0 since no data was going into my PC via serial port, I conclude that the ReadFile function must have timed out. However, using GetLastError returns 0. How can I be verify in my C++ program that the ReadFile has actually timedout?

Answer 1

When using SetCommTimeouts() with ReadFile() , the COMMTIMEOUTS documentation says:

ReadIntervalTimeout
The maximum time allowed to elapse before the arrival of the next byte on the communications line, in milliseconds. If the interval between the arrival of any two bytes exceeds this amount, the ReadFile operation is completed and any buffered data is returned . A value of zero indicates that interval time-outs are not used.

If a timeout occurs, the read is complete ( ReadFile() returns TRUE), and the number of bytes that had been buffered before the timeout elapsed is reflected in your dwRead variable. So, you will know if a timeout occurred if dwRead is less than the number of bytes you asked ReadFile() to read. If there was no buffered data, dwRead will be 0. If dwRead is equal to the number of bytes you asked for, there was no timeout since ReadFile() exited when the last requested byte was read.

Answer 2

most simplest and accurate way use NT api NtReadFile and check final status of operation. timeout <=> iosb.Status == STATUS_TIMEOUT. kernel32 api ReadFile - lost info for this status(STATUS_TIMEOUT), because it check only for STATUS_PENDING and 0 > status

for synchronous work, can use code like this:

void SyncTest(POBJECT_ATTRIBUTES poa)
{
    HANDLE hFile;
    IO_STATUS_BLOCK iosb;
    NTSTATUS status = NtOpenFile(&hFile, FILE_ALL_ACCESS, poa, &iosb, FILE_SHARE_VALID_FLAGS, FILE_SYNCHRONOUS_IO_NONALERT);
    if (0 <= status)
    {
        SERIAL_TIMEOUTS st = { 4000, 1, 0, 1, 0 };
        status = NtDeviceIoControlFile(hFile, 0, 0, 0, &iosb, IOCTL_SERIAL_SET_TIMEOUTS, &st, sizeof(st), 0, 0);
        DbgPrint("time %x[%x,%p]\n", status, iosb.Status, iosb.Information);
        if (0 <= status)
        {
            UCHAR buf[256];
            status = NtReadFile(hFile, 0, 0, 0, &iosb, buf, sizeof(buf), 0, 0);
            DbgPrint("read %x [%x,%p]\n", status, iosb.Status, iosb.Information);
            //assert(status == iosb.Status);
            if (status == STATUS_TIMEOUT)
            {
                DbgPrint("timeout\n");
            }
        }
        NtClose(hFile);
    }
}

for asynchronous:

class __declspec(novtable) IoObject 
{
    friend class UserIrp;
protected:
    HANDLE _hFile;
private:
    LONG _dwRef;

protected:
    virtual ~IoObject()
    {
        if (_hFile) NtClose(_hFile);
    }
    virtual void OnIoComplete(NTSTATUS status, ULONG_PTR Information, ULONG code, PVOID pv) = 0;
public:

    NTSTATUS BindIoCompletion();

    void AddRef()
    {
        InterlockedIncrement(&_dwRef);
    }

    void Release()
    {
        if (!InterlockedDecrement(&_dwRef)) delete this;
    }

    IoObject()
    {
        _hFile = 0;
        _dwRef = 1;
    }
};

class UserIrp : public IO_STATUS_BLOCK
{
    friend IoObject;

    IoObject* _pObj;
    PVOID _pv;
    LONG _dwRef;
    ULONG _code;

    static VOID WINAPI OnComplete(NTSTATUS Status, ULONG_PTR Information, UserIrp* This)
    {
        This->_pObj->OnIoComplete(Status, Information, This->_code, This->_pv);
        This->Release();
    }

    ~UserIrp()
    {
        _pObj->Release();
    }

public:

    NTSTATUS CheckStatus(NTSTATUS status)
    {
        if (NT_ERROR(status))
        {
            OnComplete(status, Information, this);
        }

        return status;
    }

    void AddRef()
    {
        InterlockedIncrement(&_dwRef);
    }

    void Release()
    {
        if (!InterlockedDecrement(&_dwRef)) delete this;
    }

    UserIrp(IoObject* pObj, ULONG code, PVOID pv) : _pObj(pObj), _dwRef(1), _code(code), _pv(pv)
    {
        pObj->AddRef();
    }
};

NTSTATUS IoObject::BindIoCompletion()
{
    return RtlSetIoCompletionCallback(_hFile, (LPOVERLAPPED_COMPLETION_ROUTINE)UserIrp::OnComplete, 0);
}

class MySerial : public IoObject
{
    void OnIoComplete(NTSTATUS status, ULONG_PTR Information, ULONG code, PVOID pv)
    {
        DbgPrint("OnIoComplete(%x, %p, %.4s, %p)\n", status, Information, &code, pv);

        switch (code)
        {
        case 'time':
            if (0 <= status)
            {
                if (PUCHAR buf = new UCHAR[256])
                {
                    if (UserIrp* Irp = new UserIrp(this, 'read', buf))
                    {
                        static LARGE_INTEGER ByteOffset;
                        status = Irp->CheckStatus(NtReadFile(_hFile, 0, 0, Irp, Irp, buf, 256, &ByteOffset, 0));
                        DbgPrint("read begin = %x\n", status);
                        return ;
                    }
                    delete buf;
                }
            }
            break;
        case 'read':
            DbgPrint("read end(%x, %p)\n", status, Information);
            if (status == STATUS_TIMEOUT)
            {
                DbgPrint("timeout\n");
            }
            delete pv;
            break;
        }
    }

    virtual ~MySerial()
    {
        DbgPrint("--MySerial<%p>\n", this);
    }

public:

    MySerial()
    {
        DbgPrint("++MySerial<%p>\n", this);
    }

    NTSTATUS Open(POBJECT_ATTRIBUTES poa)
    {
        IO_STATUS_BLOCK iosb;
        return NtOpenFile(&_hFile, FILE_ALL_ACCESS, poa, &iosb, FILE_SHARE_VALID_FLAGS, 0);
    }

    NTSTATUS SetTimeouts(ULONG ms)
    {
        if (UserIrp* Irp = new UserIrp(this, 'time', 0))
        {
            SERIAL_TIMEOUTS st = { ms, 1, 0, 1, 0 };
            return Irp->CheckStatus(ZwDeviceIoControlFile(_hFile, 0, 0, Irp, Irp, IOCTL_SERIAL_SET_TIMEOUTS, &st, sizeof(st), 0, 0));
        }

        return STATUS_INSUFFICIENT_RESOURCES;
    }
};

void AsyncTest(POBJECT_ATTRIBUTES poa)
{
    if (MySerial* p = new MySerial)
    {
        if (0 <= p->Open(poa) && 0 <= p->BindIoCompletion())
        {
            NTSTATUS status = p->SetTimeouts(4000);
            DbgPrint("set timeout=%x\n", status);
        }
        p->Release();
    }
}