Modules/overlapped.c

/*
 * Support for overlapped IO
 *
 * Some code borrowed from Modules/_winapi.c of CPython
 */

/* XXX check overflow and DWORD <-> Py_ssize_t conversions
   Check itemsize */

#include "Python.h"
#include "structmember.h"         // PyMemberDef

#define WINDOWS_LEAN_AND_MEAN
#include <winsock2.h>
#include <ws2tcpip.h>
#include <mswsock.h>

#if defined(MS_WIN32) && !defined(MS_WIN64)
#  define F_POINTER "k"
#  define T_POINTER T_ULONG
#else
#  define F_POINTER "K"
#  define T_POINTER T_ULONGLONG
#endif

/* Compatibility with Python 3.3 */
#if PY_VERSION_HEX < 0x03040000
#    define PyMem_RawMalloc PyMem_Malloc
#    define PyMem_RawFree PyMem_Free
#endif

#define F_HANDLE F_POINTER
#define F_ULONG_PTR F_POINTER
#define F_DWORD "k"
#define F_BOOL "i"
#define F_UINT "I"

#define T_HANDLE T_POINTER

enum {TYPE_NONE, TYPE_NOT_STARTED, TYPE_READ, TYPE_READINTO, TYPE_WRITE,
      TYPE_ACCEPT, TYPE_CONNECT, TYPE_DISCONNECT, TYPE_CONNECT_NAMED_PIPE,
      TYPE_WAIT_NAMED_PIPE_AND_CONNECT, TYPE_TRANSMIT_FILE, TYPE_READ_FROM,
      TYPE_WRITE_TO};

typedef struct {
    PyObject_HEAD
    OVERLAPPED overlapped;
    /* For convenience, we store the file handle too */
    HANDLE handle;
    /* Error returned by last method call */
    DWORD error;
    /* Type of operation */
    DWORD type;
    union {
        /* Buffer allocated by us: TYPE_READ and TYPE_ACCEPT */
        PyObject *allocated_buffer;
        /* Buffer passed by the user: TYPE_WRITE, TYPE_WRITE_TO, and TYPE_READINTO */
        Py_buffer user_buffer;

        /* Data used for reading from a connectionless socket:
           TYPE_READ_FROM */
        struct {
            // A (buffer, (host, port)) tuple
            PyObject *result;
            // The actual read buffer
            PyObject *allocated_buffer;
            struct sockaddr_in6 address;
            int address_length;
        } read_from;
    };
} OverlappedObject;

/*
 * Map Windows error codes to subclasses of OSError
 */

static PyObject *
SetFromWindowsErr(DWORD err)
{
    PyObject *exception_type;

    if (err == 0)
        err = GetLastError();
    switch (err) {
        case ERROR_CONNECTION_REFUSED:
            exception_type = PyExc_ConnectionRefusedError;
            break;
        case ERROR_CONNECTION_ABORTED:
            exception_type = PyExc_ConnectionAbortedError;
            break;
        default:
            exception_type = PyExc_OSError;
    }
    return PyErr_SetExcFromWindowsErr(exception_type, err);
}

/*
 * Some functions should be loaded at runtime
 */

static LPFN_ACCEPTEX Py_AcceptEx = NULL;
static LPFN_CONNECTEX Py_ConnectEx = NULL;
static LPFN_DISCONNECTEX Py_DisconnectEx = NULL;
static LPFN_TRANSMITFILE Py_TransmitFile = NULL;
static BOOL (CALLBACK *Py_CancelIoEx)(HANDLE, LPOVERLAPPED) = NULL;

#define GET_WSA_POINTER(s, x)                                           \
    (SOCKET_ERROR != WSAIoctl(s, SIO_GET_EXTENSION_FUNCTION_POINTER,    \
                              &Guid##x, sizeof(Guid##x), &Py_##x,       \
                              sizeof(Py_##x), &dwBytes, NULL, NULL))

static int
initialize_function_pointers(void)
{
    GUID GuidAcceptEx = WSAID_ACCEPTEX;
    GUID GuidConnectEx = WSAID_CONNECTEX;
    GUID GuidDisconnectEx = WSAID_DISCONNECTEX;
    GUID GuidTransmitFile = WSAID_TRANSMITFILE;
    HINSTANCE hKernel32;
    SOCKET s;
    DWORD dwBytes;

    s = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (s == INVALID_SOCKET) {
        SetFromWindowsErr(WSAGetLastError());
        return -1;
    }

    if (!GET_WSA_POINTER(s, AcceptEx) ||
        !GET_WSA_POINTER(s, ConnectEx) ||
        !GET_WSA_POINTER(s, DisconnectEx) ||
        !GET_WSA_POINTER(s, TransmitFile))
    {
        closesocket(s);
        SetFromWindowsErr(WSAGetLastError());
        return -1;
    }

    closesocket(s);

    /* On WinXP we will have Py_CancelIoEx == NULL */
    Py_BEGIN_ALLOW_THREADS
    hKernel32 = GetModuleHandle("KERNEL32");
    *(FARPROC *)&Py_CancelIoEx = GetProcAddress(hKernel32, "CancelIoEx");
    Py_END_ALLOW_THREADS
    return 0;
}

/*
 * Completion port stuff
 */

PyDoc_STRVAR(
    CreateIoCompletionPort_doc,
    "CreateIoCompletionPort(handle, port, key, concurrency) -> port\n\n"
    "Create a completion port or register a handle with a port.");

static PyObject *
overlapped_CreateIoCompletionPort(PyObject *self, PyObject *args)
{
    HANDLE FileHandle;
    HANDLE ExistingCompletionPort;
    ULONG_PTR CompletionKey;
    DWORD NumberOfConcurrentThreads;
    HANDLE ret;

    if (!PyArg_ParseTuple(args, F_HANDLE F_HANDLE F_ULONG_PTR F_DWORD,
                          &FileHandle, &ExistingCompletionPort, &CompletionKey,
                          &NumberOfConcurrentThreads))
        return NULL;

    Py_BEGIN_ALLOW_THREADS
    ret = CreateIoCompletionPort(FileHandle, ExistingCompletionPort,
                                 CompletionKey, NumberOfConcurrentThreads);
    Py_END_ALLOW_THREADS

    if (ret == NULL)
        return SetFromWindowsErr(0);
    return Py_BuildValue(F_HANDLE, ret);
}

PyDoc_STRVAR(
    GetQueuedCompletionStatus_doc,
    "GetQueuedCompletionStatus(port, msecs) -> (err, bytes, key, address)\n\n"
    "Get a message from completion port.  Wait for up to msecs milliseconds.");

static PyObject *
overlapped_GetQueuedCompletionStatus(PyObject *self, PyObject *args)
{
    HANDLE CompletionPort = NULL;
    DWORD NumberOfBytes = 0;
    ULONG_PTR CompletionKey = 0;
    OVERLAPPED *Overlapped = NULL;
    DWORD Milliseconds;
    DWORD err;
    BOOL ret;

    if (!PyArg_ParseTuple(args, F_HANDLE F_DWORD,
                          &CompletionPort, &Milliseconds))
        return NULL;

    Py_BEGIN_ALLOW_THREADS
    ret = GetQueuedCompletionStatus(CompletionPort, &NumberOfBytes,
                                    &CompletionKey, &Overlapped, Milliseconds);
    Py_END_ALLOW_THREADS

    err = ret ? ERROR_SUCCESS : GetLastError();
    if (Overlapped == NULL) {
        if (err == WAIT_TIMEOUT)
            Py_RETURN_NONE;
        else
            return SetFromWindowsErr(err);
    }
    return Py_BuildValue(F_DWORD F_DWORD F_ULONG_PTR F_POINTER,
                         err, NumberOfBytes, CompletionKey, Overlapped);
}

PyDoc_STRVAR(
    PostQueuedCompletionStatus_doc,
    "PostQueuedCompletionStatus(port, bytes, key, address) -> None\n\n"
    "Post a message to completion port.");

static PyObject *
overlapped_PostQueuedCompletionStatus(PyObject *self, PyObject *args)
{
    HANDLE CompletionPort;
    DWORD NumberOfBytes;
    ULONG_PTR CompletionKey;
    OVERLAPPED *Overlapped;
    BOOL ret;

    if (!PyArg_ParseTuple(args, F_HANDLE F_DWORD F_ULONG_PTR F_POINTER,
                          &CompletionPort, &NumberOfBytes, &CompletionKey,
                          &Overlapped))
        return NULL;

    Py_BEGIN_ALLOW_THREADS
    ret = PostQueuedCompletionStatus(CompletionPort, NumberOfBytes,
                                     CompletionKey, Overlapped);
    Py_END_ALLOW_THREADS

    if (!ret)
        return SetFromWindowsErr(0);
    Py_RETURN_NONE;
}

/*
 * Wait for a handle
 */

struct PostCallbackData {
    HANDLE CompletionPort;
    LPOVERLAPPED Overlapped;
};

static VOID CALLBACK
PostToQueueCallback(PVOID lpParameter, BOOLEAN TimerOrWaitFired)
{
    struct PostCallbackData *p = (struct PostCallbackData*) lpParameter;

    PostQueuedCompletionStatus(p->CompletionPort, TimerOrWaitFired,
                               0, p->Overlapped);
    /* ignore possible error! */
    PyMem_RawFree(p);
}

PyDoc_STRVAR(
    RegisterWaitWithQueue_doc,
    "RegisterWaitWithQueue(Object, CompletionPort, Overlapped, Timeout)\n"
    "    -> WaitHandle\n\n"
    "Register wait for Object; when complete CompletionPort is notified.\n");

static PyObject *
overlapped_RegisterWaitWithQueue(PyObject *self, PyObject *args)
{
    HANDLE NewWaitObject;
    HANDLE Object;
    ULONG Milliseconds;
    struct PostCallbackData data, *pdata;

    if (!PyArg_ParseTuple(args, F_HANDLE F_HANDLE F_POINTER F_DWORD,
                          &Object,
                          &data.CompletionPort,
                          &data.Overlapped,
                          &Milliseconds))
        return NULL;

    /* Use PyMem_RawMalloc() rather than PyMem_Malloc(), since
       PostToQueueCallback() will call PyMem_Free() from a new C thread
       which doesn't hold the GIL. */
    pdata = PyMem_RawMalloc(sizeof(struct PostCallbackData));
    if (pdata == NULL)
        return SetFromWindowsErr(0);

    *pdata = data;

    if (!RegisterWaitForSingleObject(
            &NewWaitObject, Object, PostToQueueCallback, pdata, Milliseconds,
            WT_EXECUTEINWAITTHREAD | WT_EXECUTEONLYONCE))
    {
        PyMem_RawFree(pdata);
        return SetFromWindowsErr(0);
    }

    return Py_BuildValue(F_HANDLE, NewWaitObject);
}

PyDoc_STRVAR(
    UnregisterWait_doc,
    "UnregisterWait(WaitHandle) -> None\n\n"
    "Unregister wait handle.\n");

static PyObject *
overlapped_UnregisterWait(PyObject *self, PyObject *args)
{
    HANDLE WaitHandle;
    BOOL ret;

    if (!PyArg_ParseTuple(args, F_HANDLE, &WaitHandle))
        return NULL;

    Py_BEGIN_ALLOW_THREADS
    ret = UnregisterWait(WaitHandle);
    Py_END_ALLOW_THREADS

    if (!ret)
        return SetFromWindowsErr(0);
    Py_RETURN_NONE;
}

PyDoc_STRVAR(
    UnregisterWaitEx_doc,
    "UnregisterWaitEx(WaitHandle, Event) -> None\n\n"
    "Unregister wait handle.\n");

static PyObject *
overlapped_UnregisterWaitEx(PyObject *self, PyObject *args)
{
    HANDLE WaitHandle, Event;
    BOOL ret;

    if (!PyArg_ParseTuple(args, F_HANDLE F_HANDLE, &WaitHandle, &Event))
        return NULL;

    Py_BEGIN_ALLOW_THREADS
    ret = UnregisterWaitEx(WaitHandle, Event);
    Py_END_ALLOW_THREADS

    if (!ret)
        return SetFromWindowsErr(0);
    Py_RETURN_NONE;
}

/*
 * Event functions -- currently only used by tests
 */

PyDoc_STRVAR(
    CreateEvent_doc,
    "CreateEvent(EventAttributes, ManualReset, InitialState, Name)"
    " -> Handle\n\n"
    "Create an event.  EventAttributes must be None.\n");

static PyObject *
overlapped_CreateEvent(PyObject *self, PyObject *args)
{
    PyObject *EventAttributes;
    BOOL ManualReset;
    BOOL InitialState;
    Py_UNICODE *Name;
    HANDLE Event;

    if (!PyArg_ParseTuple(args, "O" F_BOOL F_BOOL "Z",
                          &EventAttributes, &ManualReset,
                          &InitialState, &Name))
        return NULL;

    if (EventAttributes != Py_None) {
        PyErr_SetString(PyExc_ValueError, "EventAttributes must be None");
        return NULL;
    }

    Py_BEGIN_ALLOW_THREADS
    Event = CreateEventW(NULL, ManualReset, InitialState, Name);
    Py_END_ALLOW_THREADS

    if (Event == NULL)
        return SetFromWindowsErr(0);
    return Py_BuildValue(F_HANDLE, Event);
}

PyDoc_STRVAR(
    SetEvent_doc,
    "SetEvent(Handle) -> None\n\n"
    "Set event.\n");

static PyObject *
overlapped_SetEvent(PyObject *self, PyObject *args)
{
    HANDLE Handle;
    BOOL ret;

    if (!PyArg_ParseTuple(args, F_HANDLE, &Handle))
        return NULL;

    Py_BEGIN_ALLOW_THREADS
    ret = SetEvent(Handle);
    Py_END_ALLOW_THREADS

    if (!ret)
        return SetFromWindowsErr(0);
    Py_RETURN_NONE;
}

PyDoc_STRVAR(
    ResetEvent_doc,
    "ResetEvent(Handle) -> None\n\n"
    "Reset event.\n");

static PyObject *
overlapped_ResetEvent(PyObject *self, PyObject *args)
{
    HANDLE Handle;
    BOOL ret;

    if (!PyArg_ParseTuple(args, F_HANDLE, &Handle))
        return NULL;

    Py_BEGIN_ALLOW_THREADS
    ret = ResetEvent(Handle);
    Py_END_ALLOW_THREADS

    if (!ret)
        return SetFromWindowsErr(0);
    Py_RETURN_NONE;
}

/*
 * Bind socket handle to local port without doing slow getaddrinfo()
 */

PyDoc_STRVAR(
    BindLocal_doc,
    "BindLocal(handle, family) -> None\n\n"
    "Bind a socket handle to an arbitrary local port.\n"
    "family should AF_INET or AF_INET6.\n");

static PyObject *
overlapped_BindLocal(PyObject *self, PyObject *args)
{
    SOCKET Socket;
    int Family;
    BOOL ret;

    if (!PyArg_ParseTuple(args, F_HANDLE "i", &Socket, &Family))
        return NULL;

    if (Family == AF_INET) {
        struct sockaddr_in addr;
        memset(&addr, 0, sizeof(addr));
        addr.sin_family = AF_INET;
        addr.sin_port = 0;
        addr.sin_addr.S_un.S_addr = INADDR_ANY;
        ret = bind(Socket, (SOCKADDR*)&addr, sizeof(addr)) != SOCKET_ERROR;
    } else if (Family == AF_INET6) {
        struct sockaddr_in6 addr;
        memset(&addr, 0, sizeof(addr));
        addr.sin6_family = AF_INET6;
        addr.sin6_port = 0;
        addr.sin6_addr = in6addr_any;
        ret = bind(Socket, (SOCKADDR*)&addr, sizeof(addr)) != SOCKET_ERROR;
    } else {
        PyErr_SetString(PyExc_ValueError, "expected tuple of length 2 or 4");
        return NULL;
    }

    if (!ret)
        return SetFromWindowsErr(WSAGetLastError());
    Py_RETURN_NONE;
}

/*
 * Windows equivalent of os.strerror() -- compare _ctypes/callproc.c
 */

PyDoc_STRVAR(
    FormatMessage_doc,
    "FormatMessage(error_code) -> error_message\n\n"
    "Return error message for an error code.");

static PyObject *
overlapped_FormatMessage(PyObject *ignore, PyObject *args)
{
    DWORD code, n;
    WCHAR *lpMsgBuf;
    PyObject *res;

    if (!PyArg_ParseTuple(args, F_DWORD, &code))
        return NULL;

    n = FormatMessageW(FORMAT_MESSAGE_ALLOCATE_BUFFER |
                       FORMAT_MESSAGE_FROM_SYSTEM |
                       FORMAT_MESSAGE_IGNORE_INSERTS,
                       NULL,
                       code,
                       MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
                       (LPWSTR) &lpMsgBuf,
                       0,
                       NULL);
    if (n) {
        while (iswspace(lpMsgBuf[n-1]))
            --n;
        lpMsgBuf[n] = L'\0';
        res = Py_BuildValue("u", lpMsgBuf);
    } else {
        res = PyUnicode_FromFormat("unknown error code %u", code);
    }
    LocalFree(lpMsgBuf);
    return res;
}


/*
 * Mark operation as completed - used when reading produces ERROR_BROKEN_PIPE
 */

static void
mark_as_completed(OVERLAPPED *ov)
{
    ov->Internal = 0;
    if (ov->hEvent != NULL)
        SetEvent(ov->hEvent);
}

/*
 * A Python object wrapping an OVERLAPPED structure and other useful data
 * for overlapped I/O
 */

PyDoc_STRVAR(
    Overlapped_doc,
    "Overlapped object");

static PyObject *
Overlapped_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
    OverlappedObject *self;
    HANDLE event = INVALID_HANDLE_VALUE;
    static char *kwlist[] = {"event", NULL};

    if (!PyArg_ParseTupleAndKeywords(args, kwds, "|" F_HANDLE, kwlist, &event))
        return NULL;

    if (event == INVALID_HANDLE_VALUE) {
        event = CreateEvent(NULL, TRUE, FALSE, NULL);
        if (event == NULL)
            return SetFromWindowsErr(0);
    }

    self = PyObject_New(OverlappedObject, type);
    if (self == NULL) {
        if (event != NULL)
            CloseHandle(event);
        return NULL;
    }

    self->handle = NULL;
    self->error = 0;
    self->type = TYPE_NONE;
    self->allocated_buffer = NULL;
    memset(&self->overlapped, 0, sizeof(OVERLAPPED));
    memset(&self->user_buffer, 0, sizeof(Py_buffer));
    if (event)
        self->overlapped.hEvent = event;
    return (PyObject *)self;
}


/* Note (bpo-32710): OverlappedType.tp_clear is not defined to not release
   buffers while overlapped are still running, to prevent a crash. */
static int
Overlapped_clear(OverlappedObject *self)
{
    switch (self->type) {
        case TYPE_READ:
        case TYPE_ACCEPT: {
            Py_CLEAR(self->allocated_buffer);
            break;
        }
        case TYPE_READ_FROM: {
            // An initial call to WSARecvFrom will only allocate the buffer.
            // The result tuple of (message, address) is only
            // allocated _after_ a message has been received.
            if(self->read_from.result) {
                // We've received a message, free the result tuple.
                Py_CLEAR(self->read_from.result);
            }
            if(self->read_from.allocated_buffer) {
                Py_CLEAR(self->read_from.allocated_buffer);
            }
            break;
        }
        case TYPE_WRITE:
        case TYPE_WRITE_TO:
        case TYPE_READINTO: {
            if (self->user_buffer.obj) {
                PyBuffer_Release(&self->user_buffer);
            }
            break;
        }
    }
    self->type = TYPE_NOT_STARTED;
    return 0;
}

static void
Overlapped_dealloc(OverlappedObject *self)
{
    DWORD bytes;
    DWORD olderr = GetLastError();
    BOOL wait = FALSE;
    BOOL ret;

    if (!HasOverlappedIoCompleted(&self->overlapped) &&
        self->type != TYPE_NOT_STARTED)
    {
        if (Py_CancelIoEx && Py_CancelIoEx(self->handle, &self->overlapped))
            wait = TRUE;

        Py_BEGIN_ALLOW_THREADS
        ret = GetOverlappedResult(self->handle, &self->overlapped,
                                  &bytes, wait);
        Py_END_ALLOW_THREADS

        switch (ret ? ERROR_SUCCESS : GetLastError()) {
            case ERROR_SUCCESS:
            case ERROR_NOT_FOUND:
            case ERROR_OPERATION_ABORTED:
                break;
            default:
                PyErr_Format(
                    PyExc_RuntimeError,
                    "%R still has pending operation at "
                    "deallocation, the process may crash", self);
                PyErr_WriteUnraisable(NULL);
        }
    }

    if (self->overlapped.hEvent != NULL) {
        CloseHandle(self->overlapped.hEvent);
    }

    Overlapped_clear(self);
    PyObject_Del(self);
    SetLastError(olderr);
}


/* Convert IPv4 sockaddr to a Python str. */

static PyObject *
make_ipv4_addr(const struct sockaddr_in *addr)
{
        char buf[INET_ADDRSTRLEN];
        if (inet_ntop(AF_INET, &addr->sin_addr, buf, sizeof(buf)) == NULL) {
                PyErr_SetFromErrno(PyExc_OSError);
                return NULL;
        }
        return PyUnicode_FromString(buf);
}

/* Convert IPv6 sockaddr to a Python str. */

static PyObject *
make_ipv6_addr(const struct sockaddr_in6 *addr)
{
        char buf[INET6_ADDRSTRLEN];
        if (inet_ntop(AF_INET6, &addr->sin6_addr, buf, sizeof(buf)) == NULL) {
                PyErr_SetFromErrno(PyExc_OSError);
                return NULL;
        }
        return PyUnicode_FromString(buf);
}

static PyObject*
unparse_address(LPSOCKADDR Address, DWORD Length)
{
        /* The function is adopted from mocketmodule.c makesockaddr()*/

    switch(Address->sa_family) {
        case AF_INET: {
            const struct sockaddr_in *a = (const struct sockaddr_in *)Address;
            PyObject *addrobj = make_ipv4_addr(a);
            PyObject *ret = NULL;
            if (addrobj) {
                ret = Py_BuildValue("Oi", addrobj, ntohs(a->sin_port));
                Py_DECREF(addrobj);
            }
            return ret;
        }
        case AF_INET6: {
            const struct sockaddr_in6 *a = (const struct sockaddr_in6 *)Address;
            PyObject *addrobj = make_ipv6_addr(a);
            PyObject *ret = NULL;
            if (addrobj) {
                ret = Py_BuildValue("OiII",
                                    addrobj,
                                    ntohs(a->sin6_port),
                                    ntohl(a->sin6_flowinfo),
                                    a->sin6_scope_id);
                Py_DECREF(addrobj);
            }
            return ret;
        }
        default: {
            PyErr_SetString(PyExc_ValueError, "recvfrom returned unsupported address family");
            return NULL;
        }
    }
}

PyDoc_STRVAR(
    Overlapped_cancel_doc,
    "cancel() -> None\n\n"
    "Cancel overlapped operation");

static PyObject *
Overlapped_cancel(OverlappedObject *self, PyObject *Py_UNUSED(ignored))
{
    BOOL ret = TRUE;

    if (self->type == TYPE_NOT_STARTED
        || self->type == TYPE_WAIT_NAMED_PIPE_AND_CONNECT)
        Py_RETURN_NONE;

    if (!HasOverlappedIoCompleted(&self->overlapped)) {
        Py_BEGIN_ALLOW_THREADS
        if (Py_CancelIoEx)
            ret = Py_CancelIoEx(self->handle, &self->overlapped);
        else
            ret = CancelIo(self->handle);
        Py_END_ALLOW_THREADS
    }

    /* CancelIoEx returns ERROR_NOT_FOUND if the I/O completed in-between */
    if (!ret && GetLastError() != ERROR_NOT_FOUND)
        return SetFromWindowsErr(0);
    Py_RETURN_NONE;
}

PyDoc_STRVAR(
    Overlapped_getresult_doc,
    "getresult(wait=False) -> result\n\n"
    "Retrieve result of operation.  If wait is true then it blocks\n"
    "until the operation is finished.  If wait is false and the\n"
    "operation is still pending then an error is raised.");

static PyObject *
Overlapped_getresult(OverlappedObject *self, PyObject *args)
{
    BOOL wait = FALSE;
    DWORD transferred = 0;
    BOOL ret;
    DWORD err;
    PyObject *addr;

    if (!PyArg_ParseTuple(args, "|" F_BOOL, &wait))
        return NULL;

    if (self->type == TYPE_NONE) {
        PyErr_SetString(PyExc_ValueError, "operation not yet attempted");
        return NULL;
    }

    if (self->type == TYPE_NOT_STARTED) {
        PyErr_SetString(PyExc_ValueError, "operation failed to start");
        return NULL;
    }

    Py_BEGIN_ALLOW_THREADS
    ret = GetOverlappedResult(self->handle, &self->overlapped, &transferred,
                              wait);
    Py_END_ALLOW_THREADS

    self->error = err = ret ? ERROR_SUCCESS : GetLastError();
    switch (err) {
        case ERROR_SUCCESS:
        case ERROR_MORE_DATA:
            break;
        case ERROR_BROKEN_PIPE:
            if (self->type == TYPE_READ || self->type == TYPE_READINTO) {
                break;
            }
            else if (self->type == TYPE_READ_FROM &&
                     (self->read_from.result != NULL ||
                      self->read_from.allocated_buffer != NULL))
            {
                break;
            }
            /* fall through */
        default:
            return SetFromWindowsErr(err);
    }

    switch (self->type) {
        case TYPE_READ:
            assert(PyBytes_CheckExact(self->allocated_buffer));
            if (transferred != PyBytes_GET_SIZE(self->allocated_buffer) &&
                _PyBytes_Resize(&self->allocated_buffer, transferred))
                return NULL;

            Py_INCREF(self->allocated_buffer);
            return self->allocated_buffer;
        case TYPE_READ_FROM:
            assert(PyBytes_CheckExact(self->read_from.allocated_buffer));

            if (transferred != PyBytes_GET_SIZE(
                    self->read_from.allocated_buffer) &&
                _PyBytes_Resize(&self->read_from.allocated_buffer, transferred))
            {
                return NULL;
            }

            // unparse the address
            addr = unparse_address((SOCKADDR*)&self->read_from.address,
                                   self->read_from.address_length);

            if (addr == NULL) {
                return NULL;
            }

            // The result is a two item tuple: (message, address)
            self->read_from.result = PyTuple_New(2);
            if (self->read_from.result == NULL) {
                Py_CLEAR(addr);
                return NULL;
            }

            // first item: message
            Py_INCREF(self->read_from.allocated_buffer);
            PyTuple_SET_ITEM(self->read_from.result, 0,
                             self->read_from.allocated_buffer);
            // second item: address
            PyTuple_SET_ITEM(self->read_from.result, 1, addr);

            Py_INCREF(self->read_from.result);
            return self->read_from.result;
        default:
            return PyLong_FromUnsignedLong((unsigned long) transferred);
    }
}

static PyObject *
do_ReadFile(OverlappedObject *self, HANDLE handle,
            char *bufstart, DWORD buflen)
{
    DWORD nread;
    int ret;
    DWORD err;

    Py_BEGIN_ALLOW_THREADS
    ret = ReadFile(handle, bufstart, buflen, &nread,
                   &self->overlapped);
    Py_END_ALLOW_THREADS

    self->error = err = ret ? ERROR_SUCCESS : GetLastError();
    switch (err) {
        case ERROR_BROKEN_PIPE:
            mark_as_completed(&self->overlapped);
            return SetFromWindowsErr(err);
        case ERROR_SUCCESS:
        case ERROR_MORE_DATA:
        case ERROR_IO_PENDING:
            Py_RETURN_NONE;
        default:
            Overlapped_clear(self);
            return SetFromWindowsErr(err);
    }
}

PyDoc_STRVAR(
    Overlapped_ReadFile_doc,
    "ReadFile(handle, size) -> Overlapped[message]\n\n"
    "Start overlapped read");

static PyObject *
Overlapped_ReadFile(OverlappedObject *self, PyObject *args)
{
    HANDLE handle;
    DWORD size;
    PyObject *buf;

    if (!PyArg_ParseTuple(args, F_HANDLE F_DWORD, &handle, &size))
        return NULL;

    if (self->type != TYPE_NONE) {
        PyErr_SetString(PyExc_ValueError, "operation already attempted");
        return NULL;
    }

#if SIZEOF_SIZE_T <= SIZEOF_LONG
    size = Py_MIN(size, (DWORD)PY_SSIZE_T_MAX);
#endif
    buf = PyBytes_FromStringAndSize(NULL, Py_MAX(size, 1));
    if (buf == NULL)
        return NULL;

    self->type = TYPE_READ;
    self->handle = handle;
    self->allocated_buffer = buf;

    return do_ReadFile(self, handle, PyBytes_AS_STRING(buf), size);
}

PyDoc_STRVAR(
    Overlapped_ReadFileInto_doc,
    "ReadFileInto(handle, buf) -> Overlapped[bytes_transferred]\n\n"
    "Start overlapped receive");

static PyObject *
Overlapped_ReadFileInto(OverlappedObject *self, PyObject *args)
{
    HANDLE handle;
    PyObject *bufobj;

    if (!PyArg_ParseTuple(args, F_HANDLE "O", &handle, &bufobj))
        return NULL;

    if (self->type != TYPE_NONE) {
        PyErr_SetString(PyExc_ValueError, "operation already attempted");
        return NULL;
    }

    if (!PyArg_Parse(bufobj, "y*", &self->user_buffer))
        return NULL;

#if SIZEOF_SIZE_T > SIZEOF_LONG
    if (self->user_buffer.len > (Py_ssize_t)ULONG_MAX) {
        PyBuffer_Release(&self->user_buffer);
        PyErr_SetString(PyExc_ValueError, "buffer too large");
        return NULL;
    }
#endif

    self->type = TYPE_READINTO;
    self->handle = handle;

    return do_ReadFile(self, handle, self->user_buffer.buf,
                       (DWORD)self->user_buffer.len);
}

static PyObject *
do_WSARecv(OverlappedObject *self, HANDLE handle,
           char *bufstart, DWORD buflen, DWORD flags)
{
    DWORD nread;
    WSABUF wsabuf;
    int ret;
    DWORD err;

    wsabuf.buf = bufstart;
    wsabuf.len = buflen;

    Py_BEGIN_ALLOW_THREADS
    ret = WSARecv((SOCKET)handle, &wsabuf, 1, &nread, &flags,
                  &self->overlapped, NULL);
    Py_END_ALLOW_THREADS

    self->error = err = (ret < 0 ? WSAGetLastError() : ERROR_SUCCESS);
    switch (err) {
        case ERROR_BROKEN_PIPE:
            mark_as_completed(&self->overlapped);
            return SetFromWindowsErr(err);
        case ERROR_SUCCESS:
        case ERROR_MORE_DATA:
        case ERROR_IO_PENDING:
            Py_RETURN_NONE;
        default:
            Overlapped_clear(self);
            return SetFromWindowsErr(err);
    }
}

PyDoc_STRVAR(
    Overlapped_WSARecv_doc,
    "RecvFile(handle, size, flags) -> Overlapped[message]\n\n"
    "Start overlapped receive");

static PyObject *
Overlapped_WSARecv(OverlappedObject *self, PyObject *args)
{
    HANDLE handle;
    DWORD size;
    DWORD flags = 0;
    PyObject *buf;

    if (!PyArg_ParseTuple(args, F_HANDLE F_DWORD "|" F_DWORD,
                          &handle, &size, &flags))
        return NULL;

    if (self->type != TYPE_NONE) {
        PyErr_SetString(PyExc_ValueError, "operation already attempted");
        return NULL;
    }

#if SIZEOF_SIZE_T <= SIZEOF_LONG
    size = Py_MIN(size, (DWORD)PY_SSIZE_T_MAX);
#endif
    buf = PyBytes_FromStringAndSize(NULL, Py_MAX(size, 1));
    if (buf == NULL)
        return NULL;

    self->type = TYPE_READ;
    self->handle = handle;
    self->allocated_buffer = buf;

    return do_WSARecv(self, handle, PyBytes_AS_STRING(buf), size, flags);
}

PyDoc_STRVAR(
    Overlapped_WSARecvInto_doc,
    "WSARecvInto(handle, buf, flags) -> Overlapped[bytes_transferred]\n\n"
    "Start overlapped receive");

static PyObject *
Overlapped_WSARecvInto(OverlappedObject *self, PyObject *args)
{
    HANDLE handle;
    PyObject *bufobj;
    DWORD flags;

    if (!PyArg_ParseTuple(args, F_HANDLE "O" F_DWORD,
                          &handle, &bufobj, &flags))
        return NULL;

    if (self->type != TYPE_NONE) {
        PyErr_SetString(PyExc_ValueError, "operation already attempted");
        return NULL;
    }

    if (!PyArg_Parse(bufobj, "y*", &self->user_buffer))
        return NULL;

#if SIZEOF_SIZE_T > SIZEOF_LONG
    if (self->user_buffer.len > (Py_ssize_t)ULONG_MAX) {
        PyBuffer_Release(&self->user_buffer);
        PyErr_SetString(PyExc_ValueError, "buffer too large");
        return NULL;
    }
#endif

    self->type = TYPE_READINTO;
    self->handle = handle;

    return do_WSARecv(self, handle, self->user_buffer.buf,
                      (DWORD)self->user_buffer.len, flags);
}

PyDoc_STRVAR(
    Overlapped_WriteFile_doc,
    "WriteFile(handle, buf) -> Overlapped[bytes_transferred]\n\n"
    "Start overlapped write");

static PyObject *
Overlapped_WriteFile(OverlappedObject *self, PyObject *args)
{
    HANDLE handle;
    PyObject *bufobj;
    DWORD written;
    BOOL ret;
    DWORD err;

    if (!PyArg_ParseTuple(args, F_HANDLE "O", &handle, &bufobj))
        return NULL;

    if (self->type != TYPE_NONE) {
        PyErr_SetString(PyExc_ValueError, "operation already attempted");
        return NULL;
    }

    if (!PyArg_Parse(bufobj, "y*", &self->user_buffer))
        return NULL;

#if SIZEOF_SIZE_T > SIZEOF_LONG
    if (self->user_buffer.len > (Py_ssize_t)ULONG_MAX) {
        PyBuffer_Release(&self->user_buffer);
        PyErr_SetString(PyExc_ValueError, "buffer too large");
        return NULL;
    }
#endif

    self->type = TYPE_WRITE;
    self->handle = handle;

    Py_BEGIN_ALLOW_THREADS
    ret = WriteFile(handle, self->user_buffer.buf,
                    (DWORD)self->user_buffer.len,
                    &written, &self->overlapped);
    Py_END_ALLOW_THREADS

    self->error = err = ret ? ERROR_SUCCESS : GetLastError();
    switch (err) {
        case ERROR_SUCCESS:
        case ERROR_IO_PENDING:
            Py_RETURN_NONE;
        default:
            Overlapped_clear(self);
            return SetFromWindowsErr(err);
    }
}

PyDoc_STRVAR(
    Overlapped_WSASend_doc,
    "WSASend(handle, buf, flags) -> Overlapped[bytes_transferred]\n\n"
    "Start overlapped send");

static PyObject *
Overlapped_WSASend(OverlappedObject *self, PyObject *args)
{
    HANDLE handle;
    PyObject *bufobj;
    DWORD flags;
    DWORD written;
    WSABUF wsabuf;
    int ret;
    DWORD err;

    if (!PyArg_ParseTuple(args, F_HANDLE "O" F_DWORD,
                          &handle, &bufobj, &flags))
        return NULL;

    if (self->type != TYPE_NONE) {
        PyErr_SetString(PyExc_ValueError, "operation already attempted");
        return NULL;
    }

    if (!PyArg_Parse(bufobj, "y*", &self->user_buffer))
        return NULL;

#if SIZEOF_SIZE_T > SIZEOF_LONG
    if (self->user_buffer.len > (Py_ssize_t)ULONG_MAX) {
        PyBuffer_Release(&self->user_buffer);
        PyErr_SetString(PyExc_ValueError, "buffer too large");
        return NULL;
    }
#endif

    self->type = TYPE_WRITE;
    self->handle = handle;
    wsabuf.len = (DWORD)self->user_buffer.len;
    wsabuf.buf = self->user_buffer.buf;

    Py_BEGIN_ALLOW_THREADS
    ret = WSASend((SOCKET)handle, &wsabuf, 1, &written, flags,
                  &self->overlapped, NULL);
    Py_END_ALLOW_THREADS

    self->error = err = (ret < 0 ? WSAGetLastError() : ERROR_SUCCESS);
    switch (err) {
        case ERROR_SUCCESS:
        case ERROR_IO_PENDING:
            Py_RETURN_NONE;
        default:
            Overlapped_clear(self);
            return SetFromWindowsErr(err);
    }
}

PyDoc_STRVAR(
    Overlapped_AcceptEx_doc,
    "AcceptEx(listen_handle, accept_handle) -> Overlapped[address_as_bytes]\n\n"
    "Start overlapped wait for client to connect");

static PyObject *
Overlapped_AcceptEx(OverlappedObject *self, PyObject *args)
{
    SOCKET ListenSocket;
    SOCKET AcceptSocket;
    DWORD BytesReceived;
    DWORD size;
    PyObject *buf;
    BOOL ret;
    DWORD err;

    if (!PyArg_ParseTuple(args, F_HANDLE F_HANDLE,
                          &ListenSocket, &AcceptSocket))
        return NULL;

    if (self->type != TYPE_NONE) {
        PyErr_SetString(PyExc_ValueError, "operation already attempted");
        return NULL;
    }

    size = sizeof(struct sockaddr_in6) + 16;
    buf = PyBytes_FromStringAndSize(NULL, size*2);
    if (!buf)
        return NULL;

    self->type = TYPE_ACCEPT;
    self->handle = (HANDLE)ListenSocket;
    self->allocated_buffer = buf;

    Py_BEGIN_ALLOW_THREADS
    ret = Py_AcceptEx(ListenSocket, AcceptSocket, PyBytes_AS_STRING(buf),
                      0, size, size, &BytesReceived, &self->overlapped);
    Py_END_ALLOW_THREADS

    self->error = err = ret ? ERROR_SUCCESS : WSAGetLastError();
    switch (err) {
        case ERROR_SUCCESS:
        case ERROR_IO_PENDING:
            Py_RETURN_NONE;
        default:
            Overlapped_clear(self);
            return SetFromWindowsErr(err);
    }
}


static int
parse_address(PyObject *obj, SOCKADDR *Address, int Length)
{
    Py_UNICODE *Host;
    unsigned short Port;
    unsigned long FlowInfo;
    unsigned long ScopeId;

    memset(Address, 0, Length);

    if (PyArg_ParseTuple(obj, "uH", &Host, &Port))
    {
        Address->sa_family = AF_INET;
        if (WSAStringToAddressW(Host, AF_INET, NULL, Address, &Length) < 0) {
            SetFromWindowsErr(WSAGetLastError());
            return -1;
        }
        ((SOCKADDR_IN*)Address)->sin_port = htons(Port);
        return Length;
    }
    else if (PyArg_ParseTuple(obj,
                              "uHkk;ConnectEx(): illegal address_as_bytes "
                              "argument", &Host, &Port, &FlowInfo, &ScopeId))
    {
        PyErr_Clear();
        Address->sa_family = AF_INET6;
        if (WSAStringToAddressW(Host, AF_INET6, NULL, Address, &Length) < 0) {
            SetFromWindowsErr(WSAGetLastError());
            return -1;
        }
        ((SOCKADDR_IN6*)Address)->sin6_port = htons(Port);
        ((SOCKADDR_IN6*)Address)->sin6_flowinfo = FlowInfo;
        ((SOCKADDR_IN6*)Address)->sin6_scope_id = ScopeId;
        return Length;
    }

    return -1;
}

PyDoc_STRVAR(
    Overlapped_ConnectEx_doc,
    "ConnectEx(client_handle, address_as_bytes) -> Overlapped[None]\n\n"
    "Start overlapped connect.  client_handle should be unbound.");

static PyObject *
Overlapped_ConnectEx(OverlappedObject *self, PyObject *args)
{
    SOCKET ConnectSocket;
    PyObject *AddressObj;
    char AddressBuf[sizeof(struct sockaddr_in6)];
    SOCKADDR *Address = (SOCKADDR*)AddressBuf;
    int Length;
    BOOL ret;
    DWORD err;

    if (!PyArg_ParseTuple(args, F_HANDLE "O!:ConnectEx",
                          &ConnectSocket, &PyTuple_Type, &AddressObj))
    {
        return NULL;
    }

    if (self->type != TYPE_NONE) {
        PyErr_SetString(PyExc_ValueError, "operation already attempted");
        return NULL;
    }

    Length = sizeof(AddressBuf);
    Length = parse_address(AddressObj, Address, Length);
    if (Length < 0)
        return NULL;

    self->type = TYPE_CONNECT;
    self->handle = (HANDLE)ConnectSocket;

    Py_BEGIN_ALLOW_THREADS
    ret = Py_ConnectEx(ConnectSocket, Address, Length,
                       NULL, 0, NULL, &self->overlapped);
    Py_END_ALLOW_THREADS

    self->error = err = ret ? ERROR_SUCCESS : WSAGetLastError();
    switch (err) {
        case ERROR_SUCCESS:
        case ERROR_IO_PENDING:
            Py_RETURN_NONE;
        default:
            Overlapped_clear(self);
            return SetFromWindowsErr(err);
    }
}

PyDoc_STRVAR(
    Overlapped_DisconnectEx_doc,
    "DisconnectEx(handle, flags) -> Overlapped[None]\n\n"
    "Start overlapped connect.  client_handle should be unbound.");

static PyObject *
Overlapped_DisconnectEx(OverlappedObject *self, PyObject *args)
{
    SOCKET Socket;
    DWORD flags;
    BOOL ret;
    DWORD err;

    if (!PyArg_ParseTuple(args, F_HANDLE F_DWORD, &Socket, &flags))
        return NULL;

    if (self->type != TYPE_NONE) {
        PyErr_SetString(PyExc_ValueError, "operation already attempted");
        return NULL;
    }

    self->type = TYPE_DISCONNECT;
    self->handle = (HANDLE)Socket;

    Py_BEGIN_ALLOW_THREADS
    ret = Py_DisconnectEx(Socket, &self->overlapped, flags, 0);
    Py_END_ALLOW_THREADS

    self->error = err = ret ? ERROR_SUCCESS : WSAGetLastError();
    switch (err) {
        case ERROR_SUCCESS:
        case ERROR_IO_PENDING:
            Py_RETURN_NONE;
        default:
            Overlapped_clear(self);
            return SetFromWindowsErr(err);
    }
}

PyDoc_STRVAR(
    Overlapped_TransmitFile_doc,
    "TransmitFile(socket, file, offset, offset_high, "
    "count_to_write, count_per_send, flags) "
    "-> Overlapped[None]\n\n"
    "Transmit file data over a connected socket.");

static PyObject *
Overlapped_TransmitFile(OverlappedObject *self, PyObject *args)
{
    SOCKET Socket;
    HANDLE File;
    DWORD offset;
    DWORD offset_high;
    DWORD count_to_write;
    DWORD count_per_send;
    DWORD flags;
    BOOL ret;
    DWORD err;

    if (!PyArg_ParseTuple(args,
                          F_HANDLE F_HANDLE F_DWORD F_DWORD
                          F_DWORD F_DWORD F_DWORD,
                          &Socket, &File, &offset, &offset_high,
                          &count_to_write, &count_per_send,
                          &flags))
        return NULL;

    if (self->type != TYPE_NONE) {
        PyErr_SetString(PyExc_ValueError, "operation already attempted");
        return NULL;
    }

    self->type = TYPE_TRANSMIT_FILE;
    self->handle = (HANDLE)Socket;
    self->overlapped.Offset = offset;
    self->overlapped.OffsetHigh = offset_high;

    Py_BEGIN_ALLOW_THREADS
    ret = Py_TransmitFile(Socket, File, count_to_write, count_per_send,
                          &self->overlapped,
                          NULL, flags);
    Py_END_ALLOW_THREADS

    self->error = err = ret ? ERROR_SUCCESS : WSAGetLastError();
    switch (err) {
        case ERROR_SUCCESS:
        case ERROR_IO_PENDING:
            Py_RETURN_NONE;
        default:
            Overlapped_clear(self);
            return SetFromWindowsErr(err);
    }
}

PyDoc_STRVAR(
    Overlapped_ConnectNamedPipe_doc,
    "ConnectNamedPipe(handle) -> Overlapped[None]\n\n"
    "Start overlapped wait for a client to connect.");

static PyObject *
Overlapped_ConnectNamedPipe(OverlappedObject *self, PyObject *args)
{
    HANDLE Pipe;
    BOOL ret;
    DWORD err;

    if (!PyArg_ParseTuple(args, F_HANDLE, &Pipe))
        return NULL;

    if (self->type != TYPE_NONE) {
        PyErr_SetString(PyExc_ValueError, "operation already attempted");
        return NULL;
    }

    self->type = TYPE_CONNECT_NAMED_PIPE;
    self->handle = Pipe;

    Py_BEGIN_ALLOW_THREADS
    ret = ConnectNamedPipe(Pipe, &self->overlapped);
    Py_END_ALLOW_THREADS

    self->error = err = ret ? ERROR_SUCCESS : GetLastError();
    switch (err) {
        case ERROR_PIPE_CONNECTED:
            mark_as_completed(&self->overlapped);
            Py_RETURN_TRUE;
        case ERROR_SUCCESS:
        case ERROR_IO_PENDING:
            Py_RETURN_FALSE;
        default:
            Overlapped_clear(self);
            return SetFromWindowsErr(err);
    }
}

PyDoc_STRVAR(
    ConnectPipe_doc,
    "ConnectPipe(addr) -> pipe_handle\n\n"
    "Connect to the pipe for asynchronous I/O (overlapped).");

static PyObject *
overlapped_ConnectPipe(PyObject *self, PyObject *args)
{
    PyObject *AddressObj;
    wchar_t *Address;
    HANDLE PipeHandle;

    if (!PyArg_ParseTuple(args, "U",  &AddressObj))
        return NULL;

    Address = PyUnicode_AsWideCharString(AddressObj, NULL);
    if (Address == NULL)
        return NULL;

    Py_BEGIN_ALLOW_THREADS
    PipeHandle = CreateFileW(Address,
                             GENERIC_READ | GENERIC_WRITE,
                             0, NULL, OPEN_EXISTING,
                             FILE_FLAG_OVERLAPPED, NULL);
    Py_END_ALLOW_THREADS

    PyMem_Free(Address);
    if (PipeHandle == INVALID_HANDLE_VALUE)
        return SetFromWindowsErr(0);
    return Py_BuildValue(F_HANDLE, PipeHandle);
}

static PyObject*
Overlapped_getaddress(OverlappedObject *self)
{
    return PyLong_FromVoidPtr(&self->overlapped);
}

static PyObject*
Overlapped_getpending(OverlappedObject *self)
{
    return PyBool_FromLong(!HasOverlappedIoCompleted(&self->overlapped) &&
                           self->type != TYPE_NOT_STARTED);
}

static int
Overlapped_traverse(OverlappedObject *self, visitproc visit, void *arg)
{
    switch (self->type) {
    case TYPE_READ:
    case TYPE_ACCEPT:
        Py_VISIT(self->allocated_buffer);
        break;
    case TYPE_WRITE:
    case TYPE_WRITE_TO:
    case TYPE_READINTO:
        if (self->user_buffer.obj) {
            Py_VISIT(&self->user_buffer.obj);
        }
        break;
    case TYPE_READ_FROM:
        if(self->read_from.result) {
            Py_VISIT(self->read_from.result);
        }
        if(self->read_from.allocated_buffer) {
            Py_VISIT(self->read_from.allocated_buffer);
        }
    }
    return 0;
}

// UDP functions

PyDoc_STRVAR(
    WSAConnect_doc,
    "WSAConnect(client_handle, address_as_bytes) -> Overlapped[None]\n\n"
    "Bind a remote address to a connectionless (UDP) socket");

/*
 * Note: WSAConnect does not support Overlapped I/O so this function should
 * _only_ be used for connectionless sockets (UDP).
 */
static PyObject *
overlapped_WSAConnect(PyObject *self, PyObject *args)
{
    SOCKET ConnectSocket;
    PyObject *AddressObj;
    char AddressBuf[sizeof(struct sockaddr_in6)];
    SOCKADDR *Address = (SOCKADDR*)AddressBuf;
    int Length;
    int err;

    if (!PyArg_ParseTuple(args, F_HANDLE "O", &ConnectSocket, &AddressObj)) {
        return NULL;
    }

    Length = sizeof(AddressBuf);
    Length = parse_address(AddressObj, Address, Length);
    if (Length < 0) {
        return NULL;
    }

    Py_BEGIN_ALLOW_THREADS
    // WSAConnect does not support overlapped I/O so this call will
    // successfully complete immediately.
    err = WSAConnect(ConnectSocket, Address, Length,
                        NULL, NULL, NULL, NULL);
    Py_END_ALLOW_THREADS

    if (err == 0) {
        Py_RETURN_NONE;
    }
    else {
        return SetFromWindowsErr(WSAGetLastError());
    }
}

PyDoc_STRVAR(
    Overlapped_WSASendTo_doc,
    "WSASendTo(handle, buf, flags, address_as_bytes) -> "
    "Overlapped[bytes_transferred]\n\n"
    "Start overlapped sendto over a connectionless (UDP) socket");

static PyObject *
Overlapped_WSASendTo(OverlappedObject *self, PyObject *args)
{
    HANDLE handle;
    PyObject *bufobj;
    DWORD flags;
    PyObject *AddressObj;
    char AddressBuf[sizeof(struct sockaddr_in6)];
    SOCKADDR *Address = (SOCKADDR*)AddressBuf;
    int AddressLength;
    DWORD written;
    WSABUF wsabuf;
    int ret;
    DWORD err;

    if (!PyArg_ParseTuple(args, F_HANDLE "O" F_DWORD "O",
                          &handle, &bufobj, &flags, &AddressObj))
    {
        return NULL;
    }

    // Parse the "to" address
    AddressLength = sizeof(AddressBuf);
    AddressLength = parse_address(AddressObj, Address, AddressLength);
    if (AddressLength < 0) {
        return NULL;
    }

    if (self->type != TYPE_NONE) {
        PyErr_SetString(PyExc_ValueError, "operation already attempted");
        return NULL;
    }

    if (!PyArg_Parse(bufobj, "y*", &self->user_buffer)) {
        return NULL;
    }

#if SIZEOF_SIZE_T > SIZEOF_LONG
    if (self->user_buffer.len > (Py_ssize_t)ULONG_MAX) {
        PyBuffer_Release(&self->user_buffer);
        PyErr_SetString(PyExc_ValueError, "buffer too large");
        return NULL;
    }
#endif

    self->type = TYPE_WRITE_TO;
    self->handle = handle;
    wsabuf.len = (DWORD)self->user_buffer.len;
    wsabuf.buf = self->user_buffer.buf;

    Py_BEGIN_ALLOW_THREADS
    ret = WSASendTo((SOCKET)handle, &wsabuf, 1, &written, flags,
                    Address, AddressLength, &self->overlapped, NULL);
    Py_END_ALLOW_THREADS

    self->error = err = (ret == SOCKET_ERROR ? WSAGetLastError() :
                                               ERROR_SUCCESS);

    switch(err) {
        case ERROR_SUCCESS:
        case ERROR_IO_PENDING:
            Py_RETURN_NONE;
        default:
            self->type = TYPE_NOT_STARTED;
            return SetFromWindowsErr(err);
    }
}



PyDoc_STRVAR(
    Overlapped_WSARecvFrom_doc,
    "RecvFile(handle, size, flags) -> Overlapped[(message, (host, port))]\n\n"
    "Start overlapped receive");

static PyObject *
Overlapped_WSARecvFrom(OverlappedObject *self, PyObject *args)
{
    HANDLE handle;
    DWORD size;
    DWORD flags = 0;
    DWORD nread;
    PyObject *buf;
    WSABUF wsabuf;
    int ret;
    DWORD err;

    if (!PyArg_ParseTuple(args, F_HANDLE F_DWORD "|" F_DWORD,
                          &handle, &size, &flags))
    {
        return NULL;
    }

    if (self->type != TYPE_NONE) {
        PyErr_SetString(PyExc_ValueError, "operation already attempted");
        return NULL;
    }

#if SIZEOF_SIZE_T <= SIZEOF_LONG
    size = Py_MIN(size, (DWORD)PY_SSIZE_T_MAX);
#endif
    buf = PyBytes_FromStringAndSize(NULL, Py_MAX(size, 1));
    if (buf == NULL) {
        return NULL;
    }

    wsabuf.len = size;
    wsabuf.buf = PyBytes_AS_STRING(buf);

    self->type = TYPE_READ_FROM;
    self->handle = handle;
    self->read_from.allocated_buffer = buf;
    memset(&self->read_from.address, 0, sizeof(self->read_from.address));
    self->read_from.address_length = sizeof(self->read_from.address);

    Py_BEGIN_ALLOW_THREADS
    ret = WSARecvFrom((SOCKET)handle, &wsabuf, 1, &nread, &flags,
                      (SOCKADDR*)&self->read_from.address,
                      &self->read_from.address_length,
                      &self->overlapped, NULL);
    Py_END_ALLOW_THREADS

    self->error = err = (ret < 0 ? WSAGetLastError() : ERROR_SUCCESS);

    switch(err) {
    case ERROR_BROKEN_PIPE:
        mark_as_completed(&self->overlapped);
        return SetFromWindowsErr(err);
    case ERROR_SUCCESS:
    case ERROR_MORE_DATA:
    case ERROR_IO_PENDING:
        Py_RETURN_NONE;
    default:
        self->type = TYPE_NOT_STARTED;
        return SetFromWindowsErr(err);
    }
}


static PyMethodDef Overlapped_methods[] = {
    {"getresult", (PyCFunction) Overlapped_getresult,
     METH_VARARGS, Overlapped_getresult_doc},
    {"cancel", (PyCFunction) Overlapped_cancel,
     METH_NOARGS, Overlapped_cancel_doc},
    {"ReadFile", (PyCFunction) Overlapped_ReadFile,
     METH_VARARGS, Overlapped_ReadFile_doc},
    {"ReadFileInto", (PyCFunction) Overlapped_ReadFileInto,
     METH_VARARGS, Overlapped_ReadFileInto_doc},
    {"WSARecv", (PyCFunction) Overlapped_WSARecv,
     METH_VARARGS, Overlapped_WSARecv_doc},
    {"WSARecvInto", (PyCFunction) Overlapped_WSARecvInto,
     METH_VARARGS, Overlapped_WSARecvInto_doc},
    {"WriteFile", (PyCFunction) Overlapped_WriteFile,
     METH_VARARGS, Overlapped_WriteFile_doc},
    {"WSASend", (PyCFunction) Overlapped_WSASend,
     METH_VARARGS, Overlapped_WSASend_doc},
    {"AcceptEx", (PyCFunction) Overlapped_AcceptEx,
     METH_VARARGS, Overlapped_AcceptEx_doc},
    {"ConnectEx", (PyCFunction) Overlapped_ConnectEx,
     METH_VARARGS, Overlapped_ConnectEx_doc},
    {"DisconnectEx", (PyCFunction) Overlapped_DisconnectEx,
     METH_VARARGS, Overlapped_DisconnectEx_doc},
    {"TransmitFile", (PyCFunction) Overlapped_TransmitFile,
     METH_VARARGS, Overlapped_TransmitFile_doc},
    {"ConnectNamedPipe", (PyCFunction) Overlapped_ConnectNamedPipe,
     METH_VARARGS, Overlapped_ConnectNamedPipe_doc},
    {"WSARecvFrom", (PyCFunction) Overlapped_WSARecvFrom,
     METH_VARARGS, Overlapped_WSARecvFrom_doc },
    {"WSASendTo", (PyCFunction) Overlapped_WSASendTo,
     METH_VARARGS, Overlapped_WSASendTo_doc },
    {NULL}
};

static PyMemberDef Overlapped_members[] = {
    {"error", T_ULONG,
     offsetof(OverlappedObject, error),
     READONLY, "Error from last operation"},
    {"event", T_HANDLE,
     offsetof(OverlappedObject, overlapped) + offsetof(OVERLAPPED, hEvent),
     READONLY, "Overlapped event handle"},
    {NULL}
};

static PyGetSetDef Overlapped_getsets[] = {
    {"address", (getter)Overlapped_getaddress, NULL,
     "Address of overlapped structure"},
    {"pending", (getter)Overlapped_getpending, NULL,
     "Whether the operation is pending"},
    {NULL},
};

PyTypeObject OverlappedType = {
    PyVarObject_HEAD_INIT(NULL, 0)
    /* tp_name           */ "_overlapped.Overlapped",
    /* tp_basicsize      */ sizeof(OverlappedObject),
    /* tp_itemsize       */ 0,
    /* tp_dealloc        */ (destructor) Overlapped_dealloc,
    /* tp_vectorcall_offset */ 0,
    /* tp_getattr        */ 0,
    /* tp_setattr        */ 0,
    /* tp_as_async       */ 0,
    /* tp_repr           */ 0,
    /* tp_as_number      */ 0,
    /* tp_as_sequence    */ 0,
    /* tp_as_mapping     */ 0,
    /* tp_hash           */ 0,
    /* tp_call           */ 0,
    /* tp_str            */ 0,
    /* tp_getattro       */ 0,
    /* tp_setattro       */ 0,
    /* tp_as_buffer      */ 0,
    /* tp_flags          */ Py_TPFLAGS_DEFAULT,
    /* tp_doc            */ "OVERLAPPED structure wrapper",
    /* tp_traverse       */ (traverseproc)Overlapped_traverse,
    /* tp_clear          */ 0,
    /* tp_richcompare    */ 0,
    /* tp_weaklistoffset */ 0,
    /* tp_iter           */ 0,
    /* tp_iternext       */ 0,
    /* tp_methods        */ Overlapped_methods,
    /* tp_members        */ Overlapped_members,
    /* tp_getset         */ Overlapped_getsets,
    /* tp_base           */ 0,
    /* tp_dict           */ 0,
    /* tp_descr_get      */ 0,
    /* tp_descr_set      */ 0,
    /* tp_dictoffset     */ 0,
    /* tp_init           */ 0,
    /* tp_alloc          */ 0,
    /* tp_new            */ Overlapped_new,
};

static PyMethodDef overlapped_functions[] = {
    {"CreateIoCompletionPort", overlapped_CreateIoCompletionPort,
     METH_VARARGS, CreateIoCompletionPort_doc},
    {"GetQueuedCompletionStatus", overlapped_GetQueuedCompletionStatus,
     METH_VARARGS, GetQueuedCompletionStatus_doc},
    {"PostQueuedCompletionStatus", overlapped_PostQueuedCompletionStatus,
     METH_VARARGS, PostQueuedCompletionStatus_doc},
    {"FormatMessage", overlapped_FormatMessage,
     METH_VARARGS, FormatMessage_doc},
    {"BindLocal", overlapped_BindLocal,
     METH_VARARGS, BindLocal_doc},
    {"RegisterWaitWithQueue", overlapped_RegisterWaitWithQueue,
     METH_VARARGS, RegisterWaitWithQueue_doc},
    {"UnregisterWait", overlapped_UnregisterWait,
     METH_VARARGS, UnregisterWait_doc},
    {"UnregisterWaitEx", overlapped_UnregisterWaitEx,
     METH_VARARGS, UnregisterWaitEx_doc},
    {"CreateEvent", overlapped_CreateEvent,
     METH_VARARGS, CreateEvent_doc},
    {"SetEvent", overlapped_SetEvent,
     METH_VARARGS, SetEvent_doc},
    {"ResetEvent", overlapped_ResetEvent,
     METH_VARARGS, ResetEvent_doc},
    {"ConnectPipe", overlapped_ConnectPipe,
     METH_VARARGS, ConnectPipe_doc},
    {"WSAConnect", overlapped_WSAConnect,
     METH_VARARGS, WSAConnect_doc},
    {NULL}
};

static struct PyModuleDef overlapped_module = {
    PyModuleDef_HEAD_INIT,
    "_overlapped",
    NULL,
    -1,
    overlapped_functions,
    NULL,
    NULL,
    NULL,
    NULL
};

#define WINAPI_CONSTANT(fmt, con) \
    PyDict_SetItemString(d, #con, Py_BuildValue(fmt, con))

PyMODINIT_FUNC
PyInit__overlapped(void)
{
    PyObject *m, *d;

    /* Ensure WSAStartup() called before initializing function pointers */
    m = PyImport_ImportModule("_socket");
    if (!m)
        return NULL;
    Py_DECREF(m);

    if (initialize_function_pointers() < 0)
        return NULL;

    m = PyModule_Create(&overlapped_module);
    if (PyModule_AddType(m, &OverlappedType) < 0) {
        return NULL;
    }

    d = PyModule_GetDict(m);

    WINAPI_CONSTANT(F_DWORD,  ERROR_IO_PENDING);
    WINAPI_CONSTANT(F_DWORD,  ERROR_NETNAME_DELETED);
    WINAPI_CONSTANT(F_DWORD,  ERROR_OPERATION_ABORTED);
    WINAPI_CONSTANT(F_DWORD,  ERROR_SEM_TIMEOUT);
    WINAPI_CONSTANT(F_DWORD,  ERROR_PIPE_BUSY);
    WINAPI_CONSTANT(F_DWORD,  INFINITE);
    WINAPI_CONSTANT(F_HANDLE, INVALID_HANDLE_VALUE);
    WINAPI_CONSTANT(F_HANDLE, NULL);
    WINAPI_CONSTANT(F_DWORD,  SO_UPDATE_ACCEPT_CONTEXT);
    WINAPI_CONSTANT(F_DWORD,  SO_UPDATE_CONNECT_CONTEXT);
    WINAPI_CONSTANT(F_DWORD,  TF_REUSE_SOCKET);

    return m;
}