Add support for the send/recvmsg API to the socket module. Patch by David Watson and Heiko Wundram. (Closes #6560)

7 files changed, 3167 insertions, 0 deletions

diff --git a/Doc/library/socket.rst b/Doc/library/socket.rst
index f587977..196fe8b 100644
--- a/Doc/library/socket.rst
+++ b/Doc/library/socket.rst
@@ -198,6 +198,7 @@ The module :mod:`socket` exports the following constants and functions:
           SOMAXCONN
           MSG_*
           SOL_*
+          SCM_*
           IPPROTO_*
           IPPORT_*
           INADDR_*
@@ -511,6 +512,49 @@ The module :mod:`socket` exports the following constants and functions:
    Availability: Unix (maybe not all platforms).
 
 
+..
+   XXX: Are sendmsg(), recvmsg() and CMSG_*() available on any
+   non-Unix platforms?  The old (obsolete?) 4.2BSD form of the
+   interface, in which struct msghdr has no msg_control or
+   msg_controllen members, is not currently supported.
+
+.. function:: CMSG_LEN(length)
+
+   Return the total length, without trailing padding, of an ancillary
+   data item with associated data of the given *length*.  This value
+   can often be used as the buffer size for :meth:`~socket.recvmsg` to
+   receive a single item of ancillary data, but :rfc:`3542` requires
+   portable applications to use :func:`CMSG_SPACE` and thus include
+   space for padding, even when the item will be the last in the
+   buffer.  Raises :exc:`OverflowError` if *length* is outside the
+   permissible range of values.
+
+   Availability: most Unix platforms, possibly others.
+
+   .. versionadded:: 3.3
+
+
+.. function:: CMSG_SPACE(length)
+
+   Return the buffer size needed for :meth:`~socket.recvmsg` to
+   receive an ancillary data item with associated data of the given
+   *length*, along with any trailing padding.  The buffer space needed
+   to receive multiple items is the sum of the :func:`CMSG_SPACE`
+   values for their associated data lengths.  Raises
+   :exc:`OverflowError` if *length* is outside the permissible range
+   of values.
+
+   Note that some systems might support ancillary data without
+   providing this function.  Also note that setting the buffer size
+   using the results of this function may not precisely limit the
+   amount of ancillary data that can be received, since additional
+   data may be able to fit into the padding area.
+
+   Availability: most Unix platforms, possibly others.
+
+   .. versionadded:: 3.3
+
+
 .. function:: getdefaulttimeout()
 
    Return the default timeout in seconds (float) for new socket objects. A value
@@ -742,6 +786,109 @@ correspond to Unix system calls applicable to sockets.
    to zero. (The format of *address* depends on the address family --- see above.)
 
 
+.. method:: socket.recvmsg(bufsize[, ancbufsize[, flags]])
+
+   Receive normal data (up to *bufsize* bytes) and ancillary data from
+   the socket.  The *ancbufsize* argument sets the size in bytes of
+   the internal buffer used to receive the ancillary data; it defaults
+   to 0, meaning that no ancillary data will be received.  Appropriate
+   buffer sizes for ancillary data can be calculated using
+   :func:`CMSG_SPACE` or :func:`CMSG_LEN`, and items which do not fit
+   into the buffer might be truncated or discarded.  The *flags*
+   argument defaults to 0 and has the same meaning as for
+   :meth:`recv`.
+
+   The return value is a 4-tuple: ``(data, ancdata, msg_flags,
+   address)``.  The *data* item is a :class:`bytes` object holding the
+   non-ancillary data received.  The *ancdata* item is a list of zero
+   or more tuples ``(cmsg_level, cmsg_type, cmsg_data)`` representing
+   the ancillary data (control messages) received: *cmsg_level* and
+   *cmsg_type* are integers specifying the protocol level and
+   protocol-specific type respectively, and *cmsg_data* is a
+   :class:`bytes` object holding the associated data.  The *msg_flags*
+   item is the bitwise OR of various flags indicating conditions on
+   the received message; see your system documentation for details.
+   If the receiving socket is unconnected, *address* is the address of
+   the sending socket, if available; otherwise, its value is
+   unspecified.
+
+   On some systems, :meth:`sendmsg` and :meth:`recvmsg` can be used to
+   pass file descriptors between processes over an :const:`AF_UNIX`
+   socket.  When this facility is used (it is often restricted to
+   :const:`SOCK_STREAM` sockets), :meth:`recvmsg` will return, in its
+   ancillary data, items of the form ``(socket.SOL_SOCKET,
+   socket.SCM_RIGHTS, fds)``, where *fds* is a :class:`bytes` object
+   representing the new file descriptors as a binary array of the
+   native C :c:type:`int` type.  If :meth:`recvmsg` raises an
+   exception after the system call returns, it will first attempt to
+   close any file descriptors received via this mechanism.
+
+   Some systems do not indicate the truncated length of ancillary data
+   items which have been only partially received.  If an item appears
+   to extend beyond the end of the buffer, :meth:`recvmsg` will issue
+   a :exc:`RuntimeWarning`, and will return the part of it which is
+   inside the buffer provided it has not been truncated before the
+   start of its associated data.
+
+   On systems which support the :const:`SCM_RIGHTS` mechanism, the
+   following function will receive up to *maxfds* file descriptors,
+   returning the message data and a list containing the descriptors
+   (while ignoring unexpected conditions such as unrelated control
+   messages being received).  See also :meth:`sendmsg`. ::
+
+      import socket, array
+
+      def recv_fds(sock, msglen, maxfds):
+          fds = array.array("i")   # Array of ints
+          msg, ancdata, flags, addr = sock.recvmsg(msglen, socket.CMSG_LEN(maxfds * fds.itemsize))
+          for cmsg_level, cmsg_type, cmsg_data in ancdata:
+              if (cmsg_level == socket.SOL_SOCKET and cmsg_type == socket.SCM_RIGHTS):
+                  # Append data, ignoring any truncated integers at the end.
+                  fds.fromstring(cmsg_data[:len(cmsg_data) - (len(cmsg_data) % fds.itemsize)])
+          return msg, list(fds)
+
+   Availability: most Unix platforms, possibly others.
+
+   .. versionadded:: 3.3
+
+
+.. method:: socket.recvmsg_into(buffers[, ancbufsize[, flags]])
+
+   Receive normal data and ancillary data from the socket, behaving as
+   :meth:`recvmsg` would, but scatter the non-ancillary data into a
+   series of buffers instead of returning a new bytes object.  The
+   *buffers* argument must be an iterable of objects that export
+   writable buffers (e.g. :class:`bytearray` objects); these will be
+   filled with successive chunks of the non-ancillary data until it
+   has all been written or there are no more buffers.  The operating
+   system may set a limit (:func:`~os.sysconf` value ``SC_IOV_MAX``)
+   on the number of buffers that can be used.  The *ancbufsize* and
+   *flags* arguments have the same meaning as for :meth:`recvmsg`.
+
+   The return value is a 4-tuple: ``(nbytes, ancdata, msg_flags,
+   address)``, where *nbytes* is the total number of bytes of
+   non-ancillary data written into the buffers, and *ancdata*,
+   *msg_flags* and *address* are the same as for :meth:`recvmsg`.
+
+   Example::
+
+      >>> import socket
+      >>> s1, s2 = socket.socketpair()
+      >>> b1 = bytearray(b'----')
+      >>> b2 = bytearray(b'0123456789')
+      >>> b3 = bytearray(b'--------------')
+      >>> s1.send(b'Mary had a little lamb')
+      22
+      >>> s2.recvmsg_into([b1, memoryview(b2)[2:9], b3])
+      (22, [], 0, None)
+      >>> [b1, b2, b3]
+      [bytearray(b'Mary'), bytearray(b'01 had a 9'), bytearray(b'little lamb---')]
+
+   Availability: most Unix platforms, possibly others.
+
+   .. versionadded:: 3.3
+
+
 .. method:: socket.recvfrom_into(buffer[, nbytes[, flags]])
 
    Receive data from the socket, writing it into *buffer* instead of creating a
@@ -789,6 +936,41 @@ correspond to Unix system calls applicable to sockets.
    above.)
 
 
+.. method:: socket.sendmsg(buffers[, ancdata[, flags[, address]]])
+
+   Send normal and ancillary data to the socket, gathering the
+   non-ancillary data from a series of buffers and concatenating it
+   into a single message.  The *buffers* argument specifies the
+   non-ancillary data as an iterable of buffer-compatible objects
+   (e.g. :class:`bytes` objects); the operating system may set a limit
+   (:func:`~os.sysconf` value ``SC_IOV_MAX``) on the number of buffers
+   that can be used.  The *ancdata* argument specifies the ancillary
+   data (control messages) as an iterable of zero or more tuples
+   ``(cmsg_level, cmsg_type, cmsg_data)``, where *cmsg_level* and
+   *cmsg_type* are integers specifying the protocol level and
+   protocol-specific type respectively, and *cmsg_data* is a
+   buffer-compatible object holding the associated data.  Note that
+   some systems (in particular, systems without :func:`CMSG_SPACE`)
+   might support sending only one control message per call.  The
+   *flags* argument defaults to 0 and has the same meaning as for
+   :meth:`send`.  If *address* is supplied and not ``None``, it sets a
+   destination address for the message.  The return value is the
+   number of bytes of non-ancillary data sent.
+
+   The following function sends the list of file descriptors *fds*
+   over an :const:`AF_UNIX` socket, on systems which support the
+   :const:`SCM_RIGHTS` mechanism.  See also :meth:`recvmsg`. ::
+
+      import socket, array
+
+      def send_fds(sock, msg, fds):
+          return sock.sendmsg([msg], [(socket.SOL_SOCKET, socket.SCM_RIGHTS, array.array("i", fds))])
+
+   Availability: most Unix platforms, possibly others.
+
+   .. versionadded:: 3.3
+
+
 .. method:: socket.setblocking(flag)
 
    Set blocking or non-blocking mode of the socket: if *flag* is false, the
diff --git a/Doc/whatsnew/3.3.rst b/Doc/whatsnew/3.3.rst
index 0edf5ff..3036a3e 100644
--- a/Doc/whatsnew/3.3.rst
+++ b/Doc/whatsnew/3.3.rst
@@ -212,6 +212,18 @@ signal
 * :func:`signal.signal` and :func:`signal.siginterrupt` raise an OSError,
   instead of a RuntimeError: OSError has an errno attribute.
 
+socket
+------
+
+The :class:`~socket.socket` class now exposes addititonal methods to
+process ancillary data when supported by the underlying platform:
+
+* :func:`~socket.socket.sendmsg`
+* :func:`~socket.socket.recvmsg`
+* :func:`~socket.socket.recvmsg_into`
+
+(Contributed by David Watson in :issue:`6560`, based on an earlier patch
+by Heiko Wundram)
 
 ssl
 ---
diff --git a/Lib/ssl.py b/Lib/ssl.py
index 914e749..1b7416e 100644
--- a/Lib/ssl.py
+++ b/Lib/ssl.py
@@ -355,6 +355,14 @@ class SSLSocket(socket):
         else:
             return socket.sendto(self, data, flags_or_addr, addr)
 
+    def sendmsg(self, *args, **kwargs):
+        self._checkClosed()
+        if self._sslobj:
+            raise ValueError("sendmsg not allowed on instances of %s" %
+                             self.__class__)
+        else:
+            return socket.sendmsg(self, *args, **kwargs)
+
     def sendall(self, data, flags=0):
         self._checkClosed()
         if self._sslobj:
@@ -413,6 +421,22 @@ class SSLSocket(socket):
         else:
             return socket.recvfrom_into(self, buffer, nbytes, flags)
 
+    def recvmsg(self, *args, **kwargs):
+        self._checkClosed()
+        if self._sslobj:
+            raise ValueError("recvmsg not allowed on instances of %s" %
+                             self.__class__)
+        else:
+            return socket.recvmsg(self, *args, **kwargs)
+
+    def recvmsg_into(self, *args, **kwargs):
+        self._checkClosed()
+        if self._sslobj:
+            raise ValueError("recvmsg_into not allowed on instances of %s" %
+                             self.__class__)
+        else:
+            return socket.recvmsg_into(self, *args, **kwargs)
+
     def pending(self):
         self._checkClosed()
         if self._sslobj:
diff --git a/Lib/test/test_socket.py b/Lib/test/test_socket.py
index 4e5085e..bbc9b78 100644
--- a/Lib/test/test_socket.py
+++ b/Lib/test/test_socket.py
@@ -7,6 +7,8 @@ import errno
 import io
 import socket
 import select
+import tempfile
+import _testcapi
 import time
 import traceback
 import queue
@@ -34,6 +36,9 @@ except ImportError:
     thread = None
     threading = None
 
+# Size in bytes of the int type
+SIZEOF_INT = array.array("i").itemsize
+
 class SocketTCPTest(unittest.TestCase):
 
     def setUp(self):
@@ -55,6 +60,26 @@ class SocketUDPTest(unittest.TestCase):
         self.serv.close()
         self.serv = None
 
+class ThreadSafeCleanupTestCase(unittest.TestCase):
+    """Subclass of unittest.TestCase with thread-safe cleanup methods.
+
+    This subclass protects the addCleanup() and doCleanups() methods
+    with a recursive lock.
+    """
+
+    if threading:
+        def __init__(self, *args, **kwargs):
+            super().__init__(*args, **kwargs)
+            self._cleanup_lock = threading.RLock()
+
+        def addCleanup(self, *args, **kwargs):
+            with self._cleanup_lock:
+                return super().addCleanup(*args, **kwargs)
+
+        def doCleanups(self, *args, **kwargs):
+            with self._cleanup_lock:
+                return super().doCleanups(*args, **kwargs)
+
 class ThreadableTest:
     """Threadable Test class
 
@@ -237,6 +262,243 @@ class SocketPairTest(unittest.TestCase, ThreadableTest):
         ThreadableTest.clientTearDown(self)
 
 
+# The following classes are used by the sendmsg()/recvmsg() tests.
+# Combining, for instance, ConnectedStreamTestMixin and TCPTestBase
+# gives a drop-in replacement for SocketConnectedTest, but different
+# address families can be used, and the attributes serv_addr and
+# cli_addr will be set to the addresses of the endpoints.
+
+class SocketTestBase(unittest.TestCase):
+    """A base class for socket tests.
+
+    Subclasses must provide methods newSocket() to return a new socket
+    and bindSock(sock) to bind it to an unused address.
+
+    Creates a socket self.serv and sets self.serv_addr to its address.
+    """
+
+    def setUp(self):
+        self.serv = self.newSocket()
+        self.bindServer()
+
+    def bindServer(self):
+        """Bind server socket and set self.serv_addr to its address."""
+        self.bindSock(self.serv)
+        self.serv_addr = self.serv.getsockname()
+
+    def tearDown(self):
+        self.serv.close()
+        self.serv = None
+
+
+class SocketListeningTestMixin(SocketTestBase):
+    """Mixin to listen on the server socket."""
+
+    def setUp(self):
+        super().setUp()
+        self.serv.listen(1)
+
+
+class ThreadedSocketTestMixin(ThreadSafeCleanupTestCase, SocketTestBase,
+                              ThreadableTest):
+    """Mixin to add client socket and allow client/server tests.
+
+    Client socket is self.cli and its address is self.cli_addr.  See
+    ThreadableTest for usage information.
+    """
+
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        ThreadableTest.__init__(self)
+
+    def clientSetUp(self):
+        self.cli = self.newClientSocket()
+        self.bindClient()
+
+    def newClientSocket(self):
+        """Return a new socket for use as client."""
+        return self.newSocket()
+
+    def bindClient(self):
+        """Bind client socket and set self.cli_addr to its address."""
+        self.bindSock(self.cli)
+        self.cli_addr = self.cli.getsockname()
+
+    def clientTearDown(self):
+        self.cli.close()
+        self.cli = None
+        ThreadableTest.clientTearDown(self)
+
+
+class ConnectedStreamTestMixin(SocketListeningTestMixin,
+                               ThreadedSocketTestMixin):
+    """Mixin to allow client/server stream tests with connected client.
+
+    Server's socket representing connection to client is self.cli_conn
+    and client's connection to server is self.serv_conn.  (Based on
+    SocketConnectedTest.)
+    """
+
+    def setUp(self):
+        super().setUp()
+        # Indicate explicitly we're ready for the client thread to
+        # proceed and then perform the blocking call to accept
+        self.serverExplicitReady()
+        conn, addr = self.serv.accept()
+        self.cli_conn = conn
+
+    def tearDown(self):
+        self.cli_conn.close()
+        self.cli_conn = None
+        super().tearDown()
+
+    def clientSetUp(self):
+        super().clientSetUp()
+        self.cli.connect(self.serv_addr)
+        self.serv_conn = self.cli
+
+    def clientTearDown(self):
+        self.serv_conn.close()
+        self.serv_conn = None
+        super().clientTearDown()
+
+
+class UnixSocketTestBase(SocketTestBase):
+    """Base class for Unix-domain socket tests."""
+
+    # This class is used for file descriptor passing tests, so we
+    # create the sockets in a private directory so that other users
+    # can't send anything that might be problematic for a privileged
+    # user running the tests.
+
+    def setUp(self):
+        self.dir_path = tempfile.mkdtemp()
+        self.addCleanup(os.rmdir, self.dir_path)
+        super().setUp()
+
+    def bindSock(self, sock):
+        path = tempfile.mktemp(dir=self.dir_path)
+        sock.bind(path)
+        self.addCleanup(support.unlink, path)
+
+class UnixStreamBase(UnixSocketTestBase):
+    """Base class for Unix-domain SOCK_STREAM tests."""
+
+    def newSocket(self):
+        return socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
+
+
+class InetTestBase(SocketTestBase):
+    """Base class for IPv4 socket tests."""
+
+    host = HOST
+
+    def setUp(self):
+        super().setUp()
+        self.port = self.serv_addr[1]
+
+    def bindSock(self, sock):
+        support.bind_port(sock, host=self.host)
+
+class TCPTestBase(InetTestBase):
+    """Base class for TCP-over-IPv4 tests."""
+
+    def newSocket(self):
+        return socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+
+class UDPTestBase(InetTestBase):
+    """Base class for UDP-over-IPv4 tests."""
+
+    def newSocket(self):
+        return socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+
+class SCTPStreamBase(InetTestBase):
+    """Base class for SCTP tests in one-to-one (SOCK_STREAM) mode."""
+
+    def newSocket(self):
+        return socket.socket(socket.AF_INET, socket.SOCK_STREAM,
+                             socket.IPPROTO_SCTP)
+
+
+class Inet6TestBase(InetTestBase):
+    """Base class for IPv6 socket tests."""
+
+    # Don't use "localhost" here - it may not have an IPv6 address
+    # assigned to it by default (e.g. in /etc/hosts), and if someone
+    # has assigned it an IPv4-mapped address, then it's unlikely to
+    # work with the full IPv6 API.
+    host = "::1"
+
+class UDP6TestBase(Inet6TestBase):
+    """Base class for UDP-over-IPv6 tests."""
+
+    def newSocket(self):
+        return socket.socket(socket.AF_INET6, socket.SOCK_DGRAM)
+
+
+# Test-skipping decorators for use with ThreadableTest.
+
+def skipWithClientIf(condition, reason):
+    """Skip decorated test if condition is true, add client_skip decorator.
+
+    If the decorated object is not a class, sets its attribute
+    "client_skip" to a decorator which will return an empty function
+    if the test is to be skipped, or the original function if it is
+    not.  This can be used to avoid running the client part of a
+    skipped test when using ThreadableTest.
+    """
+    def client_pass(*args, **kwargs):
+        pass
+    def skipdec(obj):
+        retval = unittest.skip(reason)(obj)
+        if not isinstance(obj, type):
+            retval.client_skip = lambda f: client_pass
+        return retval
+    def noskipdec(obj):
+        if not (isinstance(obj, type) or hasattr(obj, "client_skip")):
+            obj.client_skip = lambda f: f
+        return obj
+    return skipdec if condition else noskipdec
+
+
+def requireAttrs(obj, *attributes):
+    """Skip decorated test if obj is missing any of the given attributes.
+
+    Sets client_skip attribute as skipWithClientIf() does.
+    """
+    missing = [name for name in attributes if not hasattr(obj, name)]
+    return skipWithClientIf(
+        missing, "don't have " + ", ".join(name for name in missing))
+
+
+def requireSocket(*args):
+    """Skip decorated test if a socket cannot be created with given arguments.
+
+    When an argument is given as a string, will use the value of that
+    attribute of the socket module, or skip the test if it doesn't
+    exist.  Sets client_skip attribute as skipWithClientIf() does.
+    """
+    err = None
+    missing = [obj for obj in args if
+               isinstance(obj, str) and not hasattr(socket, obj)]
+    if missing:
+        err = "don't have " + ", ".join(name for name in missing)
+    else:
+        callargs = [getattr(socket, obj) if isinstance(obj, str) else obj
+                    for obj in args]
+        try:
+            s = socket.socket(*callargs)
+        except socket.error as e:
+            # XXX: check errno?
+            err = str(e)
+        else:
+            s.close()
+    return skipWithClientIf(
+        err is not None,
+        "can't create socket({0}): {1}".format(
+            ", ".join(str(o) for o in args), err))
+
+
 #######################################################################
 ## Begin Tests
 
@@ -945,6 +1207,1839 @@ class BasicUDPTest(ThreadedUDPSocketTest):
     def _testRecvFromNegative(self):
         self.cli.sendto(MSG, 0, (HOST, self.port))
 
+
+# Tests for the sendmsg()/recvmsg() interface.  Where possible, the
+# same test code is used with different families and types of socket
+# (e.g. stream, datagram), and tests using recvmsg() are repeated
+# using recvmsg_into().
+#
+# The generic test classes such as SendmsgTests and
+# RecvmsgGenericTests inherit from SendrecvmsgBase and expect to be
+# supplied with sockets cli_sock and serv_sock representing the
+# client's and the server's end of the connection respectively, and
+# attributes cli_addr and serv_addr holding their (numeric where
+# appropriate) addresses.
+#
+# The final concrete test classes combine these with subclasses of
+# SocketTestBase which set up client and server sockets of a specific
+# type, and with subclasses of SendrecvmsgBase such as
+# SendrecvmsgDgramBase and SendrecvmsgConnectedBase which map these
+# sockets to cli_sock and serv_sock and override the methods and
+# attributes of SendrecvmsgBase to fill in destination addresses if
+# needed when sending, check for specific flags in msg_flags, etc.
+#
+# RecvmsgIntoMixin provides a version of doRecvmsg() implemented using
+# recvmsg_into().
+
+# XXX: like the other datagram (UDP) tests in this module, the code
+# here assumes that datagram delivery on the local machine will be
+# reliable.
+
+class SendrecvmsgBase(ThreadSafeCleanupTestCase):
+    # Base class for sendmsg()/recvmsg() tests.
+
+    # Time in seconds to wait before considering a test failed, or
+    # None for no timeout.  Not all tests actually set a timeout.
+    fail_timeout = 3.0
+
+    def setUp(self):
+        self.misc_event = threading.Event()
+        super().setUp()
+
+    def sendToServer(self, msg):
+        # Send msg to the server.
+        return self.cli_sock.send(msg)
+
+    # Tuple of alternative default arguments for sendmsg() when called
+    # via sendmsgToServer() (e.g. to include a destination address).
+    sendmsg_to_server_defaults = ()
+
+    def sendmsgToServer(self, *args):
+        # Call sendmsg() on self.cli_sock with the given arguments,
+        # filling in any arguments which are not supplied with the
+        # corresponding items of self.sendmsg_to_server_defaults, if
+        # any.
+        return self.cli_sock.sendmsg(
+            *(args + self.sendmsg_to_server_defaults[len(args):]))
+
+    def doRecvmsg(self, sock, bufsize, *args):
+        # Call recvmsg() on sock with given arguments and return its
+        # result.  Should be used for tests which can use either
+        # recvmsg() or recvmsg_into() - RecvmsgIntoMixin overrides
+        # this method with one which emulates it using recvmsg_into(),
+        # thus allowing the same test to be used for both methods.
+        result = sock.recvmsg(bufsize, *args)
+        self.registerRecvmsgResult(result)
+        return result
+
+    def registerRecvmsgResult(self, result):
+        # Called by doRecvmsg() with the return value of recvmsg() or
+        # recvmsg_into().  Can be overridden to arrange cleanup based
+        # on the returned ancillary data, for instance.
+        pass
+
+    def checkRecvmsgAddress(self, addr1, addr2):
+        # Called to compare the received address with the address of
+        # the peer.
+        self.assertEqual(addr1, addr2)
+
+    # Flags that are normally unset in msg_flags
+    msg_flags_common_unset = 0
+    for name in ("MSG_CTRUNC", "MSG_OOB"):
+        msg_flags_common_unset |= getattr(socket, name, 0)
+
+    # Flags that are normally set
+    msg_flags_common_set = 0
+
+    # Flags set when a complete record has been received (e.g. MSG_EOR
+    # for SCTP)
+    msg_flags_eor_indicator = 0
+
+    # Flags set when a complete record has not been received
+    # (e.g. MSG_TRUNC for datagram sockets)
+    msg_flags_non_eor_indicator = 0
+
+    def checkFlags(self, flags, eor=None, checkset=0, checkunset=0, ignore=0):
+        # Method to check the value of msg_flags returned by recvmsg[_into]().
+        #
+        # Checks that all bits in msg_flags_common_set attribute are
+        # set in "flags" and all bits in msg_flags_common_unset are
+        # unset.
+        #
+        # The "eor" argument specifies whether the flags should
+        # indicate that a full record (or datagram) has been received.
+        # If "eor" is None, no checks are done; otherwise, checks
+        # that:
+        #
+        #  * if "eor" is true, all bits in msg_flags_eor_indicator are
+        #    set and all bits in msg_flags_non_eor_indicator are unset
+        #
+        #  * if "eor" is false, all bits in msg_flags_non_eor_indicator
+        #    are set and all bits in msg_flags_eor_indicator are unset
+        #
+        # If "checkset" and/or "checkunset" are supplied, they require
+        # the given bits to be set or unset respectively, overriding
+        # what the attributes require for those bits.
+        #
+        # If any bits are set in "ignore", they will not be checked,
+        # regardless of the other inputs.
+        #
+        # Will raise Exception if the inputs require a bit to be both
+        # set and unset, and it is not ignored.
+
+        defaultset = self.msg_flags_common_set
+        defaultunset = self.msg_flags_common_unset
+
+        if eor:
+            defaultset |= self.msg_flags_eor_indicator
+            defaultunset |= self.msg_flags_non_eor_indicator
+        elif eor is not None:
+            defaultset |= self.msg_flags_non_eor_indicator
+            defaultunset |= self.msg_flags_eor_indicator
+
+        # Function arguments override defaults
+        defaultset &= ~checkunset
+        defaultunset &= ~checkset
+
+        # Merge arguments with remaining defaults, and check for conflicts
+        checkset |= defaultset
+        checkunset |= defaultunset
+        inboth = checkset & checkunset & ~ignore
+        if inboth:
+            raise Exception("contradictory set, unset requirements for flags "
+                            "{0:#x}".format(inboth))
+
+        # Compare with given msg_flags value
+        mask = (checkset | checkunset) & ~ignore
+        self.assertEqual(flags & mask, checkset & mask)
+
+
+class RecvmsgIntoMixin(SendrecvmsgBase):
+    # Mixin to implement doRecvmsg() using recvmsg_into().
+
+    def doRecvmsg(self, sock, bufsize, *args):
+        buf = bytearray(bufsize)
+        result = sock.recvmsg_into([buf], *args)
+        self.registerRecvmsgResult(result)
+        self.assertGreaterEqual(result[0], 0)
+        self.assertLessEqual(result[0], bufsize)
+        return (bytes(buf[:result[0]]),) + result[1:]
+
+
+class SendrecvmsgDgramFlagsBase(SendrecvmsgBase):
+    # Defines flags to be checked in msg_flags for datagram sockets.
+
+    @property
+    def msg_flags_non_eor_indicator(self):
+        return super().msg_flags_non_eor_indicator | socket.MSG_TRUNC
+
+
+class SendrecvmsgSCTPFlagsBase(SendrecvmsgBase):
+    # Defines flags to be checked in msg_flags for SCTP sockets.
+
+    @property
+    def msg_flags_eor_indicator(self):
+        return super().msg_flags_eor_indicator | socket.MSG_EOR
+
+
+class SendrecvmsgConnectionlessBase(SendrecvmsgBase):
+    # Base class for tests on connectionless-mode sockets.  Users must
+    # supply sockets on attributes cli and serv to be mapped to
+    # cli_sock and serv_sock respectively.
+
+    @property
+    def serv_sock(self):
+        return self.serv
+
+    @property
+    def cli_sock(self):
+        return self.cli
+
+    @property
+    def sendmsg_to_server_defaults(self):
+        return ([], [], 0, self.serv_addr)
+
+    def sendToServer(self, msg):
+        return self.cli_sock.sendto(msg, self.serv_addr)
+
+
+class SendrecvmsgConnectedBase(SendrecvmsgBase):
+    # Base class for tests on connected sockets.  Users must supply
+    # sockets on attributes serv_conn and cli_conn (representing the
+    # connections *to* the server and the client), to be mapped to
+    # cli_sock and serv_sock respectively.
+
+    @property
+    def serv_sock(self):
+        return self.cli_conn
+
+    @property
+    def cli_sock(self):
+        return self.serv_conn
+
+    def checkRecvmsgAddress(self, addr1, addr2):
+        # Address is currently "unspecified" for a connected socket,
+        # so we don't examine it
+        pass
+
+
+class SendrecvmsgServerTimeoutBase(SendrecvmsgBase):
+    # Base class to set a timeout on server's socket.
+
+    def setUp(self):
+        super().setUp()
+        self.serv_sock.settimeout(self.fail_timeout)
+
+
+class SendmsgTests(SendrecvmsgServerTimeoutBase):
+    # Tests for sendmsg() which can use any socket type and do not
+    # involve recvmsg() or recvmsg_into().
+
+    def testSendmsg(self):
+        # Send a simple message with sendmsg().
+        self.assertEqual(self.serv_sock.recv(len(MSG)), MSG)
+
+    def _testSendmsg(self):
+        self.assertEqual(self.sendmsgToServer([MSG]), len(MSG))
+
+    def testSendmsgDataGenerator(self):
+        # Send from buffer obtained from a generator (not a sequence).
+        self.assertEqual(self.serv_sock.recv(len(MSG)), MSG)
+
+    def _testSendmsgDataGenerator(self):
+        self.assertEqual(self.sendmsgToServer((o for o in [MSG])),
+                         len(MSG))
+
+    def testSendmsgAncillaryGenerator(self):
+        # Gather (empty) ancillary data from a generator.
+        self.assertEqual(self.serv_sock.recv(len(MSG)), MSG)
+
+    def _testSendmsgAncillaryGenerator(self):
+        self.assertEqual(self.sendmsgToServer([MSG], (o for o in [])),
+                         len(MSG))
+
+    def testSendmsgArray(self):
+        # Send data from an array instead of the usual bytes object.
+        self.assertEqual(self.serv_sock.recv(len(MSG)), MSG)
+
+    def _testSendmsgArray(self):
+        self.assertEqual(self.sendmsgToServer([array.array("B", MSG)]),
+                         len(MSG))
+
+    def testSendmsgGather(self):
+        # Send message data from more than one buffer (gather write).
+        self.assertEqual(self.serv_sock.recv(len(MSG)), MSG)
+
+    def _testSendmsgGather(self):
+        self.assertEqual(self.sendmsgToServer([MSG[:3], MSG[3:]]), len(MSG))
+
+    def testSendmsgBadArgs(self):
+        # Check that sendmsg() rejects invalid arguments.
+        self.assertEqual(self.serv_sock.recv(1000), b"done")
+
+    def _testSendmsgBadArgs(self):
+        self.assertRaises(TypeError, self.cli_sock.sendmsg)
+        self.assertRaises(TypeError, self.sendmsgToServer,
+                          b"not in an iterable")
+        self.assertRaises(TypeError, self.sendmsgToServer,
+                          object())
+        self.assertRaises(TypeError, self.sendmsgToServer,
+                          [object()])
+        self.assertRaises(TypeError, self.sendmsgToServer,
+                          [MSG, object()])
+        self.assertRaises(TypeError, self.sendmsgToServer,
+                          [MSG], object())
+        self.assertRaises(TypeError, self.sendmsgToServer,
+                          [MSG], [], object())
+        self.assertRaises(TypeError, self.sendmsgToServer,
+                          [MSG], [], 0, object())
+        self.sendToServer(b"done")
+
+    def testSendmsgBadCmsg(self):
+        # Check that invalid ancillary data items are rejected.
+        self.assertEqual(self.serv_sock.recv(1000), b"done")
+
+    def _testSendmsgBadCmsg(self):
+        self.assertRaises(TypeError, self.sendmsgToServer,
+                          [MSG], [object()])
+        self.assertRaises(TypeError, self.sendmsgToServer,
+                          [MSG], [(object(), 0, b"data")])
+        self.assertRaises(TypeError, self.sendmsgToServer,
+                          [MSG], [(0, object(), b"data")])
+        self.assertRaises(TypeError, self.sendmsgToServer,
+                          [MSG], [(0, 0, object())])
+        self.assertRaises(TypeError, self.sendmsgToServer,
+                          [MSG], [(0, 0)])
+        self.assertRaises(TypeError, self.sendmsgToServer,
+                          [MSG], [(0, 0, b"data", 42)])
+        self.sendToServer(b"done")
+
+    @requireAttrs(socket, "CMSG_SPACE")
+    def testSendmsgBadMultiCmsg(self):
+        # Check that invalid ancillary data items are rejected when
+        # more than one item is present.
+        self.assertEqual(self.serv_sock.recv(1000), b"done")
+
+    @testSendmsgBadMultiCmsg.client_skip
+    def _testSendmsgBadMultiCmsg(self):
+        self.assertRaises(TypeError, self.sendmsgToServer,
+                          [MSG], [0, 0, b""])
+        self.assertRaises(TypeError, self.sendmsgToServer,
+                          [MSG], [(0, 0, b""), object()])
+        self.sendToServer(b"done")
+
+    def testSendmsgExcessCmsgReject(self):
+        # Check that sendmsg() rejects excess ancillary data items
+        # when the number that can be sent is limited.
+        self.assertEqual(self.serv_sock.recv(1000), b"done")
+
+    def _testSendmsgExcessCmsgReject(self):
+        if not hasattr(socket, "CMSG_SPACE"):
+            # Can only send one item
+            with self.assertRaises(socket.error) as cm:
+                self.sendmsgToServer([MSG], [(0, 0, b""), (0, 0, b"")])
+            self.assertIsNone(cm.exception.errno)
+        self.sendToServer(b"done")
+
+    def testSendmsgAfterClose(self):
+        # Check that sendmsg() fails on a closed socket.
+        pass
+
+    def _testSendmsgAfterClose(self):
+        self.cli_sock.close()
+        self.assertRaises(socket.error, self.sendmsgToServer, [MSG])
+
+
+class SendmsgStreamTests(SendmsgTests):
+    # Tests for sendmsg() which require a stream socket and do not
+    # involve recvmsg() or recvmsg_into().
+
+    def testSendmsgExplicitNoneAddr(self):
+        # Check that peer address can be specified as None.
+        self.assertEqual(self.serv_sock.recv(len(MSG)), MSG)
+
+    def _testSendmsgExplicitNoneAddr(self):
+        self.assertEqual(self.sendmsgToServer([MSG], [], 0, None), len(MSG))
+
+    def testSendmsgTimeout(self):
+        # Check that timeout works with sendmsg().
+        self.assertEqual(self.serv_sock.recv(512), b"a"*512)
+        self.assertTrue(self.misc_event.wait(timeout=self.fail_timeout))
+
+    def _testSendmsgTimeout(self):
+        try:
+            self.cli_sock.settimeout(0.03)
+            with self.assertRaises(socket.timeout):
+                while True:
+                    self.sendmsgToServer([b"a"*512])
+        finally:
+            self.misc_event.set()
+
+    # XXX: would be nice to have more tests for sendmsg flags argument.
+
+    # Linux supports MSG_DONTWAIT when sending, but in general, it
+    # only works when receiving.  Could add other platforms if they
+    # support it too.
+    @skipWithClientIf(sys.platform not in {"linux2"},
+                      "MSG_DONTWAIT not known to work on this platform when "
+                      "sending")
+    def testSendmsgDontWait(self):
+        # Check that MSG_DONTWAIT in flags causes non-blocking behaviour.
+        self.assertEqual(self.serv_sock.recv(512), b"a"*512)
+        self.assertTrue(self.misc_event.wait(timeout=self.fail_timeout))
+
+    @testSendmsgDontWait.client_skip
+    def _testSendmsgDontWait(self):
+        try:
+            with self.assertRaises(socket.error) as cm:
+                while True:
+                    self.sendmsgToServer([b"a"*512], [], socket.MSG_DONTWAIT)
+            self.assertIn(cm.exception.errno,
+                          (errno.EAGAIN, errno.EWOULDBLOCK))
+        finally:
+            self.misc_event.set()
+
+
+class SendmsgConnectionlessTests(SendmsgTests):
+    # Tests for sendmsg() which require a connectionless-mode
+    # (e.g. datagram) socket, and do not involve recvmsg() or
+    # recvmsg_into().
+
+    def testSendmsgNoDestAddr(self):
+        # Check that sendmsg() fails when no destination address is
+        # given for unconnected socket.
+        pass
+
+    def _testSendmsgNoDestAddr(self):
+        self.assertRaises(socket.error, self.cli_sock.sendmsg,
+                          [MSG])
+        self.assertRaises(socket.error, self.cli_sock.sendmsg,
+                          [MSG], [], 0, None)
+
+
+class RecvmsgGenericTests(SendrecvmsgBase):
+    # Tests for recvmsg() which can also be emulated using
+    # recvmsg_into(), and can use any socket type.
+
+    def testRecvmsg(self):
+        # Receive a simple message with recvmsg[_into]().
+        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock, len(MSG))
+        self.assertEqual(msg, MSG)
+        self.checkRecvmsgAddress(addr, self.cli_addr)
+        self.assertEqual(ancdata, [])
+        self.checkFlags(flags, eor=True)
+
+    def _testRecvmsg(self):
+        self.sendToServer(MSG)
+
+    def testRecvmsgExplicitDefaults(self):
+        # Test recvmsg[_into]() with default arguments provided explicitly.
+        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock,
+                                                   len(MSG), 0, 0)
+        self.assertEqual(msg, MSG)
+        self.checkRecvmsgAddress(addr, self.cli_addr)
+        self.assertEqual(ancdata, [])
+        self.checkFlags(flags, eor=True)
+
+    def _testRecvmsgExplicitDefaults(self):
+        self.sendToServer(MSG)
+
+    def testRecvmsgShorter(self):
+        # Receive a message smaller than buffer.
+        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock,
+                                                   len(MSG) + 42)
+        self.assertEqual(msg, MSG)
+        self.checkRecvmsgAddress(addr, self.cli_addr)
+        self.assertEqual(ancdata, [])
+        self.checkFlags(flags, eor=True)
+
+    def _testRecvmsgShorter(self):
+        self.sendToServer(MSG)
+
+    def testRecvmsgTrunc(self):
+        # Receive part of message, check for truncation indicators.
+        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock,
+                                                   len(MSG) - 3)
+        self.assertEqual(msg, MSG[:-3])
+        self.checkRecvmsgAddress(addr, self.cli_addr)
+        self.assertEqual(ancdata, [])
+        self.checkFlags(flags, eor=False)
+
+    def _testRecvmsgTrunc(self):
+        self.sendToServer(MSG)
+
+    def testRecvmsgShortAncillaryBuf(self):
+        # Test ancillary data buffer too small to hold any ancillary data.
+        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock,
+                                                   len(MSG), 1)
+        self.assertEqual(msg, MSG)
+        self.checkRecvmsgAddress(addr, self.cli_addr)
+        self.assertEqual(ancdata, [])
+        self.checkFlags(flags, eor=True)
+
+    def _testRecvmsgShortAncillaryBuf(self):
+        self.sendToServer(MSG)
+
+    def testRecvmsgLongAncillaryBuf(self):
+        # Test large ancillary data buffer.
+        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock,
+                                                   len(MSG), 10240)
+        self.assertEqual(msg, MSG)
+        self.checkRecvmsgAddress(addr, self.cli_addr)
+        self.assertEqual(ancdata, [])
+        self.checkFlags(flags, eor=True)
+
+    def _testRecvmsgLongAncillaryBuf(self):
+        self.sendToServer(MSG)
+
+    def testRecvmsgAfterClose(self):
+        # Check that recvmsg[_into]() fails on a closed socket.
+        self.serv_sock.close()
+        self.assertRaises(socket.error, self.doRecvmsg, self.serv_sock, 1024)
+
+    def _testRecvmsgAfterClose(self):
+        pass
+
+    def testRecvmsgTimeout(self):
+        # Check that timeout works.
+        try:
+            self.serv_sock.settimeout(0.03)
+            self.assertRaises(socket.timeout,
+                              self.doRecvmsg, self.serv_sock, len(MSG))
+        finally:
+            self.misc_event.set()
+
+    def _testRecvmsgTimeout(self):
+        self.assertTrue(self.misc_event.wait(timeout=self.fail_timeout))
+
+    @requireAttrs(socket, "MSG_PEEK")
+    def testRecvmsgPeek(self):
+        # Check that MSG_PEEK in flags enables examination of pending
+        # data without consuming it.
+
+        # Receive part of data with MSG_PEEK.
+        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock,
+                                                   len(MSG) - 3, 0,
+                                                   socket.MSG_PEEK)
+        self.assertEqual(msg, MSG[:-3])
+        self.checkRecvmsgAddress(addr, self.cli_addr)
+        self.assertEqual(ancdata, [])
+        # Ignoring MSG_TRUNC here (so this test is the same for stream
+        # and datagram sockets).  Some wording in POSIX seems to
+        # suggest that it needn't be set when peeking, but that may
+        # just be a slip.
+        self.checkFlags(flags, eor=False,
+                        ignore=getattr(socket, "MSG_TRUNC", 0))
+
+        # Receive all data with MSG_PEEK.
+        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock,
+                                                   len(MSG), 0,
+                                                   socket.MSG_PEEK)
+        self.assertEqual(msg, MSG)
+        self.checkRecvmsgAddress(addr, self.cli_addr)
+        self.assertEqual(ancdata, [])
+        self.checkFlags(flags, eor=True)
+
+        # Check that the same data can still be received normally.
+        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock, len(MSG))
+        self.assertEqual(msg, MSG)
+        self.checkRecvmsgAddress(addr, self.cli_addr)
+        self.assertEqual(ancdata, [])
+        self.checkFlags(flags, eor=True)
+
+    @testRecvmsgPeek.client_skip
+    def _testRecvmsgPeek(self):
+        self.sendToServer(MSG)
+
+    @requireAttrs(socket.socket, "sendmsg")
+    def testRecvmsgFromSendmsg(self):
+        # Test receiving with recvmsg[_into]() when message is sent
+        # using sendmsg().
+        self.serv_sock.settimeout(self.fail_timeout)
+        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock, len(MSG))
+        self.assertEqual(msg, MSG)
+        self.checkRecvmsgAddress(addr, self.cli_addr)
+        self.assertEqual(ancdata, [])
+        self.checkFlags(flags, eor=True)
+
+    @testRecvmsgFromSendmsg.client_skip
+    def _testRecvmsgFromSendmsg(self):
+        self.assertEqual(self.sendmsgToServer([MSG[:3], MSG[3:]]), len(MSG))
+
+
+class RecvmsgGenericStreamTests(RecvmsgGenericTests):
+    # Tests which require a stream socket and can use either recvmsg()
+    # or recvmsg_into().
+
+    def testRecvmsgEOF(self):
+        # Receive end-of-stream indicator (b"", peer socket closed).
+        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock, 1024)
+        self.assertEqual(msg, b"")
+        self.checkRecvmsgAddress(addr, self.cli_addr)
+        self.assertEqual(ancdata, [])
+        self.checkFlags(flags, eor=None) # Might not have end-of-record marker
+
+    def _testRecvmsgEOF(self):
+        self.cli_sock.close()
+
+    def testRecvmsgOverflow(self):
+        # Receive a message in more than one chunk.
+        seg1, ancdata, flags, addr = self.doRecvmsg(self.serv_sock,
+                                                    len(MSG) - 3)
+        self.checkRecvmsgAddress(addr, self.cli_addr)
+        self.assertEqual(ancdata, [])
+        self.checkFlags(flags, eor=False)
+
+        seg2, ancdata, flags, addr = self.doRecvmsg(self.serv_sock, 1024)
+        self.checkRecvmsgAddress(addr, self.cli_addr)
+        self.assertEqual(ancdata, [])
+        self.checkFlags(flags, eor=True)
+
+        msg = seg1 + seg2
+        self.assertEqual(msg, MSG)
+
+    def _testRecvmsgOverflow(self):
+        self.sendToServer(MSG)
+
+
+class RecvmsgTests(RecvmsgGenericTests):
+    # Tests for recvmsg() which can use any socket type.
+
+    def testRecvmsgBadArgs(self):
+        # Check that recvmsg() rejects invalid arguments.
+        self.assertRaises(TypeError, self.serv_sock.recvmsg)
+        self.assertRaises(ValueError, self.serv_sock.recvmsg,
+                          -1, 0, 0)
+        self.assertRaises(ValueError, self.serv_sock.recvmsg,
+                          len(MSG), -1, 0)
+        self.assertRaises(TypeError, self.serv_sock.recvmsg,
+                          [bytearray(10)], 0, 0)
+        self.assertRaises(TypeError, self.serv_sock.recvmsg,
+                          object(), 0, 0)
+        self.assertRaises(TypeError, self.serv_sock.recvmsg,
+                          len(MSG), object(), 0)
+        self.assertRaises(TypeError, self.serv_sock.recvmsg,
+                          len(MSG), 0, object())
+
+        msg, ancdata, flags, addr = self.serv_sock.recvmsg(len(MSG), 0, 0)
+        self.assertEqual(msg, MSG)
+        self.checkRecvmsgAddress(addr, self.cli_addr)
+        self.assertEqual(ancdata, [])
+        self.checkFlags(flags, eor=True)
+
+    def _testRecvmsgBadArgs(self):
+        self.sendToServer(MSG)
+
+
+class RecvmsgIntoTests(RecvmsgIntoMixin, RecvmsgGenericTests):
+    # Tests for recvmsg_into() which can use any socket type.
+
+    def testRecvmsgIntoBadArgs(self):
+        # Check that recvmsg_into() rejects invalid arguments.
+        buf = bytearray(len(MSG))
+        self.assertRaises(TypeError, self.serv_sock.recvmsg_into)
+        self.assertRaises(TypeError, self.serv_sock.recvmsg_into,
+                          len(MSG), 0, 0)
+        self.assertRaises(TypeError, self.serv_sock.recvmsg_into,
+                          buf, 0, 0)
+        self.assertRaises(TypeError, self.serv_sock.recvmsg_into,
+                          [object()], 0, 0)
+        self.assertRaises(TypeError, self.serv_sock.recvmsg_into,
+                          [b"I'm not writable"], 0, 0)
+        self.assertRaises(TypeError, self.serv_sock.recvmsg_into,
+                          [buf, object()], 0, 0)
+        self.assertRaises(ValueError, self.serv_sock.recvmsg_into,
+                          [buf], -1, 0)
+        self.assertRaises(TypeError, self.serv_sock.recvmsg_into,
+                          [buf], object(), 0)
+        self.assertRaises(TypeError, self.serv_sock.recvmsg_into,
+                          [buf], 0, object())
+
+        nbytes, ancdata, flags, addr = self.serv_sock.recvmsg_into([buf], 0, 0)
+        self.assertEqual(nbytes, len(MSG))
+        self.assertEqual(buf, bytearray(MSG))
+        self.checkRecvmsgAddress(addr, self.cli_addr)
+        self.assertEqual(ancdata, [])
+        self.checkFlags(flags, eor=True)
+
+    def _testRecvmsgIntoBadArgs(self):
+        self.sendToServer(MSG)
+
+    def testRecvmsgIntoGenerator(self):
+        # Receive into buffer obtained from a generator (not a sequence).
+        buf = bytearray(len(MSG))
+        nbytes, ancdata, flags, addr = self.serv_sock.recvmsg_into(
+            (o for o in [buf]))
+        self.assertEqual(nbytes, len(MSG))
+        self.assertEqual(buf, bytearray(MSG))
+        self.checkRecvmsgAddress(addr, self.cli_addr)
+        self.assertEqual(ancdata, [])
+        self.checkFlags(flags, eor=True)
+
+    def _testRecvmsgIntoGenerator(self):
+        self.sendToServer(MSG)
+
+    def testRecvmsgIntoArray(self):
+        # Receive into an array rather than the usual bytearray.
+        buf = array.array("B", [0] * len(MSG))
+        nbytes, ancdata, flags, addr = self.serv_sock.recvmsg_into([buf])
+        self.assertEqual(nbytes, len(MSG))
+        self.assertEqual(buf.tostring(), MSG)
+        self.checkRecvmsgAddress(addr, self.cli_addr)
+        self.assertEqual(ancdata, [])
+        self.checkFlags(flags, eor=True)
+
+    def _testRecvmsgIntoArray(self):
+        self.sendToServer(MSG)
+
+    def testRecvmsgIntoScatter(self):
+        # Receive into multiple buffers (scatter write).
+        b1 = bytearray(b"----")
+        b2 = bytearray(b"0123456789")
+        b3 = bytearray(b"--------------")
+        nbytes, ancdata, flags, addr = self.serv_sock.recvmsg_into(
+            [b1, memoryview(b2)[2:9], b3])
+        self.assertEqual(nbytes, len(b"Mary had a little lamb"))
+        self.assertEqual(b1, bytearray(b"Mary"))
+        self.assertEqual(b2, bytearray(b"01 had a 9"))
+        self.assertEqual(b3, bytearray(b"little lamb---"))
+        self.checkRecvmsgAddress(addr, self.cli_addr)
+        self.assertEqual(ancdata, [])
+        self.checkFlags(flags, eor=True)
+
+    def _testRecvmsgIntoScatter(self):
+        self.sendToServer(b"Mary had a little lamb")
+
+
+class CmsgMacroTests(unittest.TestCase):
+    # Test the functions CMSG_LEN() and CMSG_SPACE().  Tests
+    # assumptions used by sendmsg() and recvmsg[_into](), which share
+    # code with these functions.
+
+    # Match the definition in socketmodule.c
+    socklen_t_limit = min(0x7fffffff, _testcapi.INT_MAX)
+
+    @requireAttrs(socket, "CMSG_LEN")
+    def testCMSG_LEN(self):
+        # Test CMSG_LEN() with various valid and invalid values,
+        # checking the assumptions used by recvmsg() and sendmsg().
+        toobig = self.socklen_t_limit - socket.CMSG_LEN(0) + 1
+        values = list(range(257)) + list(range(toobig - 257, toobig))
+
+        # struct cmsghdr has at least three members, two of which are ints
+        self.assertGreater(socket.CMSG_LEN(0), array.array("i").itemsize * 2)
+        for n in values:
+            ret = socket.CMSG_LEN(n)
+            # This is how recvmsg() calculates the data size
+            self.assertEqual(ret - socket.CMSG_LEN(0), n)
+            self.assertLessEqual(ret, self.socklen_t_limit)
+
+        self.assertRaises(OverflowError, socket.CMSG_LEN, -1)
+        # sendmsg() shares code with these functions, and requires
+        # that it reject values over the limit.
+        self.assertRaises(OverflowError, socket.CMSG_LEN, toobig)
+        self.assertRaises(OverflowError, socket.CMSG_LEN, sys.maxsize)
+
+    @requireAttrs(socket, "CMSG_SPACE")
+    def testCMSG_SPACE(self):
+        # Test CMSG_SPACE() with various valid and invalid values,
+        # checking the assumptions used by sendmsg().
+        toobig = self.socklen_t_limit - socket.CMSG_SPACE(1) + 1
+        values = list(range(257)) + list(range(toobig - 257, toobig))
+
+        last = socket.CMSG_SPACE(0)
+        # struct cmsghdr has at least three members, two of which are ints
+        self.assertGreater(last, array.array("i").itemsize * 2)
+        for n in values:
+            ret = socket.CMSG_SPACE(n)
+            self.assertGreaterEqual(ret, last)
+            self.assertGreaterEqual(ret, socket.CMSG_LEN(n))
+            self.assertGreaterEqual(ret, n + socket.CMSG_LEN(0))
+            self.assertLessEqual(ret, self.socklen_t_limit)
+            last = ret
+
+        self.assertRaises(OverflowError, socket.CMSG_SPACE, -1)
+        # sendmsg() shares code with these functions, and requires
+        # that it reject values over the limit.
+        self.assertRaises(OverflowError, socket.CMSG_SPACE, toobig)
+        self.assertRaises(OverflowError, socket.CMSG_SPACE, sys.maxsize)
+
+
+class SCMRightsTest(SendrecvmsgServerTimeoutBase):
+    # Tests for file descriptor passing on Unix-domain sockets.
+
+    # Invalid file descriptor value that's unlikely to evaluate to a
+    # real FD even if one of its bytes is replaced with a different
+    # value (which shouldn't actually happen).
+    badfd = -0x5555
+
+    def newFDs(self, n):
+        # Return a list of n file descriptors for newly-created files
+        # containing their list indices as ASCII numbers.
+        fds = []
+        for i in range(n):
+            fd, path = tempfile.mkstemp()
+            self.addCleanup(os.unlink, path)
+            self.addCleanup(os.close, fd)
+            os.write(fd, str(i).encode())
+            fds.append(fd)
+        return fds
+
+    def checkFDs(self, fds):
+        # Check that the file descriptors in the given list contain
+        # their correct list indices as ASCII numbers.
+        for n, fd in enumerate(fds):
+            os.lseek(fd, 0, os.SEEK_SET)
+            self.assertEqual(os.read(fd, 1024), str(n).encode())
+
+    def registerRecvmsgResult(self, result):
+        self.addCleanup(self.closeRecvmsgFDs, result)
+
+    def closeRecvmsgFDs(self, recvmsg_result):
+        # Close all file descriptors specified in the ancillary data
+        # of the given return value from recvmsg() or recvmsg_into().
+        for cmsg_level, cmsg_type, cmsg_data in recvmsg_result[1]:
+            if (cmsg_level == socket.SOL_SOCKET and
+                    cmsg_type == socket.SCM_RIGHTS):
+                fds = array.array("i")
+                fds.fromstring(cmsg_data[:
+                        len(cmsg_data) - (len(cmsg_data) % fds.itemsize)])
+                for fd in fds:
+                    os.close(fd)
+
+    def createAndSendFDs(self, n):
+        # Send n new file descriptors created by newFDs() to the
+        # server, with the constant MSG as the non-ancillary data.
+        self.assertEqual(
+            self.sendmsgToServer([MSG],
+                                 [(socket.SOL_SOCKET,
+                                   socket.SCM_RIGHTS,
+                                   array.array("i", self.newFDs(n)))]),
+            len(MSG))
+
+    def checkRecvmsgFDs(self, numfds, result, maxcmsgs=1, ignoreflags=0):
+        # Check that constant MSG was received with numfds file
+        # descriptors in a maximum of maxcmsgs control messages (which
+        # must contain only complete integers).  By default, check
+        # that MSG_CTRUNC is unset, but ignore any flags in
+        # ignoreflags.
+        msg, ancdata, flags, addr = result
+        self.assertEqual(msg, MSG)
+        self.checkRecvmsgAddress(addr, self.cli_addr)
+        self.checkFlags(flags, eor=True, checkunset=socket.MSG_CTRUNC,
+                        ignore=ignoreflags)
+
+        self.assertIsInstance(ancdata, list)
+        self.assertLessEqual(len(ancdata), maxcmsgs)
+        fds = array.array("i")
+        for item in ancdata:
+            self.assertIsInstance(item, tuple)
+            cmsg_level, cmsg_type, cmsg_data = item
+            self.assertEqual(cmsg_level, socket.SOL_SOCKET)
+            self.assertEqual(cmsg_type, socket.SCM_RIGHTS)
+            self.assertIsInstance(cmsg_data, bytes)
+            self.assertEqual(len(cmsg_data) % SIZEOF_INT, 0)
+            fds.fromstring(cmsg_data)
+
+        self.assertEqual(len(fds), numfds)
+        self.checkFDs(fds)
+
+    def testFDPassSimple(self):
+        # Pass a single FD (array read from bytes object).
+        self.checkRecvmsgFDs(1, self.doRecvmsg(self.serv_sock,
+                                               len(MSG), 10240))
+
+    def _testFDPassSimple(self):
+        self.assertEqual(
+            self.sendmsgToServer(
+                [MSG],
+                [(socket.SOL_SOCKET,
+                  socket.SCM_RIGHTS,
+                  array.array("i", self.newFDs(1)).tostring())]),
+            len(MSG))
+
+    def testMultipleFDPass(self):
+        # Pass multiple FDs in a single array.
+        self.checkRecvmsgFDs(4, self.doRecvmsg(self.serv_sock,
+                                               len(MSG), 10240))
+
+    def _testMultipleFDPass(self):
+        self.createAndSendFDs(4)
+
+    @requireAttrs(socket, "CMSG_SPACE")
+    def testFDPassCMSG_SPACE(self):
+        # Test using CMSG_SPACE() to calculate ancillary buffer size.
+        self.checkRecvmsgFDs(
+            4, self.doRecvmsg(self.serv_sock, len(MSG),
+                              socket.CMSG_SPACE(4 * SIZEOF_INT)))
+
+    @testFDPassCMSG_SPACE.client_skip
+    def _testFDPassCMSG_SPACE(self):
+        self.createAndSendFDs(4)
+
+    def testFDPassCMSG_LEN(self):
+        # Test using CMSG_LEN() to calculate ancillary buffer size.
+        self.checkRecvmsgFDs(1,
+                             self.doRecvmsg(self.serv_sock, len(MSG),
+                                            socket.CMSG_LEN(4 * SIZEOF_INT)),
+                             # RFC 3542 says implementations may set
+                             # MSG_CTRUNC if there isn't enough space
+                             # for trailing padding.
+                             ignoreflags=socket.MSG_CTRUNC)
+
+    def _testFDPassCMSG_LEN(self):
+        self.createAndSendFDs(1)
+
+    @requireAttrs(socket, "CMSG_SPACE")
+    def testFDPassSeparate(self):
+        # Pass two FDs in two separate arrays.  Arrays may be combined
+        # into a single control message by the OS.
+        self.checkRecvmsgFDs(2,
+                             self.doRecvmsg(self.serv_sock, len(MSG), 10240),
+                             maxcmsgs=2)
+
+    @testFDPassSeparate.client_skip
+    def _testFDPassSeparate(self):
+        fd0, fd1 = self.newFDs(2)
+        self.assertEqual(
+            self.sendmsgToServer([MSG], [(socket.SOL_SOCKET,
+                                          socket.SCM_RIGHTS,
+                                          array.array("i", [fd0])),
+                                         (socket.SOL_SOCKET,
+                                          socket.SCM_RIGHTS,
+                                          array.array("i", [fd1]))]),
+            len(MSG))
+
+    @requireAttrs(socket, "CMSG_SPACE")
+    def testFDPassSeparateMinSpace(self):
+        # Pass two FDs in two separate arrays, receiving them into the
+        # minimum space for two arrays.
+        self.checkRecvmsgFDs(2,
+                             self.doRecvmsg(self.serv_sock, len(MSG),
+                                            socket.CMSG_SPACE(SIZEOF_INT) +
+                                            socket.CMSG_LEN(SIZEOF_INT)),
+                             maxcmsgs=2, ignoreflags=socket.MSG_CTRUNC)
+
+    @testFDPassSeparateMinSpace.client_skip
+    def _testFDPassSeparateMinSpace(self):
+        fd0, fd1 = self.newFDs(2)
+        self.assertEqual(
+            self.sendmsgToServer([MSG], [(socket.SOL_SOCKET,
+                                          socket.SCM_RIGHTS,
+                                          array.array("i", [fd0])),
+                                         (socket.SOL_SOCKET,
+                                          socket.SCM_RIGHTS,
+                                          array.array("i", [fd1]))]),
+            len(MSG))
+
+    def sendAncillaryIfPossible(self, msg, ancdata):
+        # Try to send msg and ancdata to server, but if the system
+        # call fails, just send msg with no ancillary data.
+        try:
+            nbytes = self.sendmsgToServer([msg], ancdata)
+        except socket.error as e:
+            # Check that it was the system call that failed
+            self.assertIsInstance(e.errno, int)
+            nbytes = self.sendmsgToServer([msg])
+        self.assertEqual(nbytes, len(msg))
+
+    def testFDPassEmpty(self):
+        # Try to pass an empty FD array.  Can receive either no array
+        # or an empty array.
+        self.checkRecvmsgFDs(0, self.doRecvmsg(self.serv_sock,
+                                               len(MSG), 10240),
+                             ignoreflags=socket.MSG_CTRUNC)
+
+    def _testFDPassEmpty(self):
+        self.sendAncillaryIfPossible(MSG, [(socket.SOL_SOCKET,
+                                            socket.SCM_RIGHTS,
+                                            b"")])
+
+    def testFDPassPartialInt(self):
+        # Try to pass a truncated FD array.
+        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock,
+                                                   len(MSG), 10240)
+        self.assertEqual(msg, MSG)
+        self.checkRecvmsgAddress(addr, self.cli_addr)
+        self.checkFlags(flags, eor=True, ignore=socket.MSG_CTRUNC)
+        self.assertLessEqual(len(ancdata), 1)
+        for cmsg_level, cmsg_type, cmsg_data in ancdata:
+            self.assertEqual(cmsg_level, socket.SOL_SOCKET)
+            self.assertEqual(cmsg_type, socket.SCM_RIGHTS)
+            self.assertLess(len(cmsg_data), SIZEOF_INT)
+
+    def _testFDPassPartialInt(self):
+        self.sendAncillaryIfPossible(
+            MSG,
+            [(socket.SOL_SOCKET,
+              socket.SCM_RIGHTS,
+              array.array("i", [self.badfd]).tostring()[:-1])])
+
+    @requireAttrs(socket, "CMSG_SPACE")
+    def testFDPassPartialIntInMiddle(self):
+        # Try to pass two FD arrays, the first of which is truncated.
+        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock,
+                                                   len(MSG), 10240)
+        self.assertEqual(msg, MSG)
+        self.checkRecvmsgAddress(addr, self.cli_addr)
+        self.checkFlags(flags, eor=True, ignore=socket.MSG_CTRUNC)
+        self.assertLessEqual(len(ancdata), 2)
+        fds = array.array("i")
+        # Arrays may have been combined in a single control message
+        for cmsg_level, cmsg_type, cmsg_data in ancdata:
+            self.assertEqual(cmsg_level, socket.SOL_SOCKET)
+            self.assertEqual(cmsg_type, socket.SCM_RIGHTS)
+            fds.fromstring(cmsg_data[:
+                    len(cmsg_data) - (len(cmsg_data) % fds.itemsize)])
+        self.assertLessEqual(len(fds), 2)
+        self.checkFDs(fds)
+
+    @testFDPassPartialIntInMiddle.client_skip
+    def _testFDPassPartialIntInMiddle(self):
+        fd0, fd1 = self.newFDs(2)
+        self.sendAncillaryIfPossible(
+            MSG,
+            [(socket.SOL_SOCKET,
+              socket.SCM_RIGHTS,
+              array.array("i", [fd0, self.badfd]).tostring()[:-1]),
+             (socket.SOL_SOCKET,
+              socket.SCM_RIGHTS,
+              array.array("i", [fd1]))])
+
+    def checkTruncatedHeader(self, result, ignoreflags=0):
+        # Check that no ancillary data items are returned when data is
+        # truncated inside the cmsghdr structure.
+        msg, ancdata, flags, addr = result
+        self.assertEqual(msg, MSG)
+        self.checkRecvmsgAddress(addr, self.cli_addr)
+        self.assertEqual(ancdata, [])
+        self.checkFlags(flags, eor=True, checkset=socket.MSG_CTRUNC,
+                        ignore=ignoreflags)
+
+    def testCmsgTruncNoBufSize(self):
+        # Check that no ancillary data is received when no buffer size
+        # is specified.
+        self.checkTruncatedHeader(self.doRecvmsg(self.serv_sock, len(MSG)),
+                                  # BSD seems to set MSG_CTRUNC only
+                                  # if an item has been partially
+                                  # received.
+                                  ignoreflags=socket.MSG_CTRUNC)
+
+    def _testCmsgTruncNoBufSize(self):
+        self.createAndSendFDs(1)
+
+    def testCmsgTrunc0(self):
+        # Check that no ancillary data is received when buffer size is 0.
+        self.checkTruncatedHeader(self.doRecvmsg(self.serv_sock, len(MSG), 0),
+                                  ignoreflags=socket.MSG_CTRUNC)
+
+    def _testCmsgTrunc0(self):
+        self.createAndSendFDs(1)
+
+    # Check that no ancillary data is returned for various non-zero
+    # (but still too small) buffer sizes.
+
+    def testCmsgTrunc1(self):
+        self.checkTruncatedHeader(self.doRecvmsg(self.serv_sock, len(MSG), 1))
+
+    def _testCmsgTrunc1(self):
+        self.createAndSendFDs(1)
+
+    def testCmsgTrunc2Int(self):
+        # The cmsghdr structure has at least three members, two of
+        # which are ints, so we still shouldn't see any ancillary
+        # data.
+        self.checkTruncatedHeader(self.doRecvmsg(self.serv_sock, len(MSG),
+                                                 SIZEOF_INT * 2))
+
+    def _testCmsgTrunc2Int(self):
+        self.createAndSendFDs(1)
+
+    def testCmsgTruncLen0Minus1(self):
+        self.checkTruncatedHeader(self.doRecvmsg(self.serv_sock, len(MSG),
+                                                 socket.CMSG_LEN(0) - 1))
+
+    def _testCmsgTruncLen0Minus1(self):
+        self.createAndSendFDs(1)
+
+    # The following tests try to truncate the control message in the
+    # middle of the FD array.
+
+    def checkTruncatedArray(self, ancbuf, maxdata, mindata=0):
+        # Check that file descriptor data is truncated to between
+        # mindata and maxdata bytes when received with buffer size
+        # ancbuf, and that any complete file descriptor numbers are
+        # valid.
+        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock,
+                                                   len(MSG), ancbuf)
+        self.assertEqual(msg, MSG)
+        self.checkRecvmsgAddress(addr, self.cli_addr)
+        self.checkFlags(flags, eor=True, checkset=socket.MSG_CTRUNC)
+
+        if mindata == 0 and ancdata == []:
+            return
+        self.assertEqual(len(ancdata), 1)
+        cmsg_level, cmsg_type, cmsg_data = ancdata[0]
+        self.assertEqual(cmsg_level, socket.SOL_SOCKET)
+        self.assertEqual(cmsg_type, socket.SCM_RIGHTS)
+        self.assertGreaterEqual(len(cmsg_data), mindata)
+        self.assertLessEqual(len(cmsg_data), maxdata)
+        fds = array.array("i")
+        fds.fromstring(cmsg_data[:
+                len(cmsg_data) - (len(cmsg_data) % fds.itemsize)])
+        self.checkFDs(fds)
+
+    def testCmsgTruncLen0(self):
+        self.checkTruncatedArray(ancbuf=socket.CMSG_LEN(0), maxdata=0)
+
+    def _testCmsgTruncLen0(self):
+        self.createAndSendFDs(1)
+
+    def testCmsgTruncLen0Plus1(self):
+        self.checkTruncatedArray(ancbuf=socket.CMSG_LEN(0) + 1, maxdata=1)
+
+    def _testCmsgTruncLen0Plus1(self):
+        self.createAndSendFDs(2)
+
+    def testCmsgTruncLen1(self):
+        self.checkTruncatedArray(ancbuf=socket.CMSG_LEN(SIZEOF_INT),
+                                 maxdata=SIZEOF_INT)
+
+    def _testCmsgTruncLen1(self):
+        self.createAndSendFDs(2)
+
+    def testCmsgTruncLen2Minus1(self):
+        self.checkTruncatedArray(ancbuf=socket.CMSG_LEN(2 * SIZEOF_INT) - 1,
+                                 maxdata=(2 * SIZEOF_INT) - 1)
+
+    def _testCmsgTruncLen2Minus1(self):
+        self.createAndSendFDs(2)
+
+
+class RFC3542AncillaryTest(SendrecvmsgServerTimeoutBase):
+    # Test sendmsg() and recvmsg[_into]() using the ancillary data
+    # features of the RFC 3542 Advanced Sockets API for IPv6.
+    # Currently we can only handle certain data items (e.g. traffic
+    # class, hop limit, MTU discovery and fragmentation settings)
+    # without resorting to unportable means such as the struct module,
+    # but the tests here are aimed at testing the ancillary data
+    # handling in sendmsg() and recvmsg() rather than the IPv6 API
+    # itself.
+
+    # Test value to use when setting hop limit of packet
+    hop_limit = 2
+
+    # Test value to use when setting traffic class of packet.
+    # -1 means "use kernel default".
+    traffic_class = -1
+
+    def ancillaryMapping(self, ancdata):
+        # Given ancillary data list ancdata, return a mapping from
+        # pairs (cmsg_level, cmsg_type) to corresponding cmsg_data.
+        # Check that no (level, type) pair appears more than once.
+        d = {}
+        for cmsg_level, cmsg_type, cmsg_data in ancdata:
+            self.assertNotIn((cmsg_level, cmsg_type), d)
+            d[(cmsg_level, cmsg_type)] = cmsg_data
+        return d
+
+    def checkHopLimit(self, ancbufsize, maxhop=255, ignoreflags=0):
+        # Receive hop limit into ancbufsize bytes of ancillary data
+        # space.  Check that data is MSG, ancillary data is not
+        # truncated (but ignore any flags in ignoreflags), and hop
+        # limit is between 0 and maxhop inclusive.
+        self.serv_sock.setsockopt(socket.IPPROTO_IPV6,
+                                  socket.IPV6_RECVHOPLIMIT, 1)
+        self.misc_event.set()
+        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock,
+                                                   len(MSG), ancbufsize)
+
+        self.assertEqual(msg, MSG)
+        self.checkRecvmsgAddress(addr, self.cli_addr)
+        self.checkFlags(flags, eor=True, checkunset=socket.MSG_CTRUNC,
+                        ignore=ignoreflags)
+
+        self.assertEqual(len(ancdata), 1)
+        self.assertIsInstance(ancdata[0], tuple)
+        cmsg_level, cmsg_type, cmsg_data = ancdata[0]
+        self.assertEqual(cmsg_level, socket.IPPROTO_IPV6)
+        self.assertEqual(cmsg_type, socket.IPV6_HOPLIMIT)
+        self.assertIsInstance(cmsg_data, bytes)
+        self.assertEqual(len(cmsg_data), SIZEOF_INT)
+        a = array.array("i")
+        a.fromstring(cmsg_data)
+        self.assertGreaterEqual(a[0], 0)
+        self.assertLessEqual(a[0], maxhop)
+
+    @requireAttrs(socket, "IPV6_RECVHOPLIMIT", "IPV6_HOPLIMIT")
+    def testRecvHopLimit(self):
+        # Test receiving the packet hop limit as ancillary data.
+        self.checkHopLimit(ancbufsize=10240)
+
+    @testRecvHopLimit.client_skip
+    def _testRecvHopLimit(self):
+        # Need to wait until server has asked to receive ancillary
+        # data, as implementations are not required to buffer it
+        # otherwise.
+        self.assertTrue(self.misc_event.wait(timeout=self.fail_timeout))
+        self.sendToServer(MSG)
+
+    @requireAttrs(socket, "CMSG_SPACE", "IPV6_RECVHOPLIMIT", "IPV6_HOPLIMIT")
+    def testRecvHopLimitCMSG_SPACE(self):
+        # Test receiving hop limit, using CMSG_SPACE to calculate buffer size.
+        self.checkHopLimit(ancbufsize=socket.CMSG_SPACE(SIZEOF_INT))
+
+    @testRecvHopLimitCMSG_SPACE.client_skip
+    def _testRecvHopLimitCMSG_SPACE(self):
+        self.assertTrue(self.misc_event.wait(timeout=self.fail_timeout))
+        self.sendToServer(MSG)
+
+    # Could test receiving into buffer sized using CMSG_LEN, but RFC
+    # 3542 says portable applications must provide space for trailing
+    # padding.  Implementations may set MSG_CTRUNC if there isn't
+    # enough space for the padding.
+
+    @requireAttrs(socket.socket, "sendmsg")
+    @requireAttrs(socket, "IPV6_RECVHOPLIMIT", "IPV6_HOPLIMIT")
+    def testSetHopLimit(self):
+        # Test setting hop limit on outgoing packet and receiving it
+        # at the other end.
+        self.checkHopLimit(ancbufsize=10240, maxhop=self.hop_limit)
+
+    @testSetHopLimit.client_skip
+    def _testSetHopLimit(self):
+        self.assertTrue(self.misc_event.wait(timeout=self.fail_timeout))
+        self.assertEqual(
+            self.sendmsgToServer([MSG],
+                                 [(socket.IPPROTO_IPV6, socket.IPV6_HOPLIMIT,
+                                   array.array("i", [self.hop_limit]))]),
+            len(MSG))
+
+    def checkTrafficClassAndHopLimit(self, ancbufsize, maxhop=255,
+                                     ignoreflags=0):
+        # Receive traffic class and hop limit into ancbufsize bytes of
+        # ancillary data space.  Check that data is MSG, ancillary
+        # data is not truncated (but ignore any flags in ignoreflags),
+        # and traffic class and hop limit are in range (hop limit no
+        # more than maxhop).
+        self.serv_sock.setsockopt(socket.IPPROTO_IPV6,
+                                  socket.IPV6_RECVHOPLIMIT, 1)
+        self.serv_sock.setsockopt(socket.IPPROTO_IPV6,
+                                  socket.IPV6_RECVTCLASS, 1)
+        self.misc_event.set()
+        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock,
+                                                   len(MSG), ancbufsize)
+
+        self.assertEqual(msg, MSG)
+        self.checkRecvmsgAddress(addr, self.cli_addr)
+        self.checkFlags(flags, eor=True, checkunset=socket.MSG_CTRUNC,
+                        ignore=ignoreflags)
+        self.assertEqual(len(ancdata), 2)
+        ancmap = self.ancillaryMapping(ancdata)
+
+        tcdata = ancmap[(socket.IPPROTO_IPV6, socket.IPV6_TCLASS)]
+        self.assertEqual(len(tcdata), SIZEOF_INT)
+        a = array.array("i")
+        a.fromstring(tcdata)
+        self.assertGreaterEqual(a[0], 0)
+        self.assertLessEqual(a[0], 255)
+
+        hldata = ancmap[(socket.IPPROTO_IPV6, socket.IPV6_HOPLIMIT)]
+        self.assertEqual(len(hldata), SIZEOF_INT)
+        a = array.array("i")
+        a.fromstring(hldata)
+        self.assertGreaterEqual(a[0], 0)
+        self.assertLessEqual(a[0], maxhop)
+
+    @requireAttrs(socket, "IPV6_RECVHOPLIMIT", "IPV6_HOPLIMIT",
+                  "IPV6_RECVTCLASS", "IPV6_TCLASS")
+    def testRecvTrafficClassAndHopLimit(self):
+        # Test receiving traffic class and hop limit as ancillary data.
+        self.checkTrafficClassAndHopLimit(ancbufsize=10240)
+
+    @testRecvTrafficClassAndHopLimit.client_skip
+    def _testRecvTrafficClassAndHopLimit(self):
+        self.assertTrue(self.misc_event.wait(timeout=self.fail_timeout))
+        self.sendToServer(MSG)
+
+    @requireAttrs(socket, "CMSG_SPACE", "IPV6_RECVHOPLIMIT", "IPV6_HOPLIMIT",
+                  "IPV6_RECVTCLASS", "IPV6_TCLASS")
+    def testRecvTrafficClassAndHopLimitCMSG_SPACE(self):
+        # Test receiving traffic class and hop limit, using
+        # CMSG_SPACE() to calculate buffer size.
+        self.checkTrafficClassAndHopLimit(
+            ancbufsize=socket.CMSG_SPACE(SIZEOF_INT) * 2)
+
+    @testRecvTrafficClassAndHopLimitCMSG_SPACE.client_skip
+    def _testRecvTrafficClassAndHopLimitCMSG_SPACE(self):
+        self.assertTrue(self.misc_event.wait(timeout=self.fail_timeout))
+        self.sendToServer(MSG)
+
+    @requireAttrs(socket.socket, "sendmsg")
+    @requireAttrs(socket, "CMSG_SPACE", "IPV6_RECVHOPLIMIT", "IPV6_HOPLIMIT",
+                  "IPV6_RECVTCLASS", "IPV6_TCLASS")
+    def testSetTrafficClassAndHopLimit(self):
+        # Test setting traffic class and hop limit on outgoing packet,
+        # and receiving them at the other end.
+        self.checkTrafficClassAndHopLimit(ancbufsize=10240,
+                                          maxhop=self.hop_limit)
+
+    @testSetTrafficClassAndHopLimit.client_skip
+    def _testSetTrafficClassAndHopLimit(self):
+        self.assertTrue(self.misc_event.wait(timeout=self.fail_timeout))
+        self.assertEqual(
+            self.sendmsgToServer([MSG],
+                                 [(socket.IPPROTO_IPV6, socket.IPV6_TCLASS,
+                                   array.array("i", [self.traffic_class])),
+                                  (socket.IPPROTO_IPV6, socket.IPV6_HOPLIMIT,
+                                   array.array("i", [self.hop_limit]))]),
+            len(MSG))
+
+    @requireAttrs(socket.socket, "sendmsg")
+    @requireAttrs(socket, "CMSG_SPACE", "IPV6_RECVHOPLIMIT", "IPV6_HOPLIMIT",
+                  "IPV6_RECVTCLASS", "IPV6_TCLASS")
+    def testOddCmsgSize(self):
+        # Try to send ancillary data with first item one byte too
+        # long.  Fall back to sending with correct size if this fails,
+        # and check that second item was handled correctly.
+        self.checkTrafficClassAndHopLimit(ancbufsize=10240,
+                                          maxhop=self.hop_limit)
+
+    @testOddCmsgSize.client_skip
+    def _testOddCmsgSize(self):
+        self.assertTrue(self.misc_event.wait(timeout=self.fail_timeout))
+        try:
+            nbytes = self.sendmsgToServer(
+                [MSG],
+                [(socket.IPPROTO_IPV6, socket.IPV6_TCLASS,
+                  array.array("i", [self.traffic_class]).tostring() + b"\x00"),
+                 (socket.IPPROTO_IPV6, socket.IPV6_HOPLIMIT,
+                  array.array("i", [self.hop_limit]))])
+        except socket.error as e:
+            self.assertIsInstance(e.errno, int)
+            nbytes = self.sendmsgToServer(
+                [MSG],
+                [(socket.IPPROTO_IPV6, socket.IPV6_TCLASS,
+                  array.array("i", [self.traffic_class])),
+                 (socket.IPPROTO_IPV6, socket.IPV6_HOPLIMIT,
+                  array.array("i", [self.hop_limit]))])
+            self.assertEqual(nbytes, len(MSG))
+
+    # Tests for proper handling of truncated ancillary data
+
+    def checkHopLimitTruncatedHeader(self, ancbufsize, ignoreflags=0):
+        # Receive hop limit into ancbufsize bytes of ancillary data
+        # space, which should be too small to contain the ancillary
+        # data header (if ancbufsize is None, pass no second argument
+        # to recvmsg()).  Check that data is MSG, MSG_CTRUNC is set
+        # (unless included in ignoreflags), and no ancillary data is
+        # returned.
+        self.serv_sock.setsockopt(socket.IPPROTO_IPV6,
+                                  socket.IPV6_RECVHOPLIMIT, 1)
+        self.misc_event.set()
+        args = () if ancbufsize is None else (ancbufsize,)
+        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock,
+                                                   len(MSG), *args)
+
+        self.assertEqual(msg, MSG)
+        self.checkRecvmsgAddress(addr, self.cli_addr)
+        self.assertEqual(ancdata, [])
+        self.checkFlags(flags, eor=True, checkset=socket.MSG_CTRUNC,
+                        ignore=ignoreflags)
+
+    @requireAttrs(socket, "IPV6_RECVHOPLIMIT", "IPV6_HOPLIMIT")
+    def testCmsgTruncNoBufSize(self):
+        # Check that no ancillary data is received when no ancillary
+        # buffer size is provided.
+        self.checkHopLimitTruncatedHeader(ancbufsize=None,
+                                          # BSD seems to set
+                                          # MSG_CTRUNC only if an item
+                                          # has been partially
+                                          # received.
+                                          ignoreflags=socket.MSG_CTRUNC)
+
+    @testCmsgTruncNoBufSize.client_skip
+    def _testCmsgTruncNoBufSize(self):
+        self.assertTrue(self.misc_event.wait(timeout=self.fail_timeout))
+        self.sendToServer(MSG)
+
+    @requireAttrs(socket, "IPV6_RECVHOPLIMIT", "IPV6_HOPLIMIT")
+    def testSingleCmsgTrunc0(self):
+        # Check that no ancillary data is received when ancillary
+        # buffer size is zero.
+        self.checkHopLimitTruncatedHeader(ancbufsize=0,
+                                          ignoreflags=socket.MSG_CTRUNC)
+
+    @testSingleCmsgTrunc0.client_skip
+    def _testSingleCmsgTrunc0(self):
+        self.assertTrue(self.misc_event.wait(timeout=self.fail_timeout))
+        self.sendToServer(MSG)
+
+    # Check that no ancillary data is returned for various non-zero
+    # (but still too small) buffer sizes.
+
+    @requireAttrs(socket, "IPV6_RECVHOPLIMIT", "IPV6_HOPLIMIT")
+    def testSingleCmsgTrunc1(self):
+        self.checkHopLimitTruncatedHeader(ancbufsize=1)
+
+    @testSingleCmsgTrunc1.client_skip
+    def _testSingleCmsgTrunc1(self):
+        self.assertTrue(self.misc_event.wait(timeout=self.fail_timeout))
+        self.sendToServer(MSG)
+
+    @requireAttrs(socket, "IPV6_RECVHOPLIMIT", "IPV6_HOPLIMIT")
+    def testSingleCmsgTrunc2Int(self):
+        self.checkHopLimitTruncatedHeader(ancbufsize=2 * SIZEOF_INT)
+
+    @testSingleCmsgTrunc2Int.client_skip
+    def _testSingleCmsgTrunc2Int(self):
+        self.assertTrue(self.misc_event.wait(timeout=self.fail_timeout))
+        self.sendToServer(MSG)
+
+    @requireAttrs(socket, "IPV6_RECVHOPLIMIT", "IPV6_HOPLIMIT")
+    def testSingleCmsgTruncLen0Minus1(self):
+        self.checkHopLimitTruncatedHeader(ancbufsize=socket.CMSG_LEN(0) - 1)
+
+    @testSingleCmsgTruncLen0Minus1.client_skip
+    def _testSingleCmsgTruncLen0Minus1(self):
+        self.assertTrue(self.misc_event.wait(timeout=self.fail_timeout))
+        self.sendToServer(MSG)
+
+    @requireAttrs(socket, "IPV6_RECVHOPLIMIT", "IPV6_HOPLIMIT")
+    def testSingleCmsgTruncInData(self):
+        # Test truncation of a control message inside its associated
+        # data.  The message may be returned with its data truncated,
+        # or not returned at all.
+        self.serv_sock.setsockopt(socket.IPPROTO_IPV6,
+                                  socket.IPV6_RECVHOPLIMIT, 1)
+        self.misc_event.set()
+        msg, ancdata, flags, addr = self.doRecvmsg(
+            self.serv_sock, len(MSG), socket.CMSG_LEN(SIZEOF_INT) - 1)
+
+        self.assertEqual(msg, MSG)
+        self.checkRecvmsgAddress(addr, self.cli_addr)
+        self.checkFlags(flags, eor=True, checkset=socket.MSG_CTRUNC)
+
+        self.assertLessEqual(len(ancdata), 1)
+        if ancdata:
+            cmsg_level, cmsg_type, cmsg_data = ancdata[0]
+            self.assertEqual(cmsg_level, socket.IPPROTO_IPV6)
+            self.assertEqual(cmsg_type, socket.IPV6_HOPLIMIT)
+            self.assertLess(len(cmsg_data), SIZEOF_INT)
+
+    @testSingleCmsgTruncInData.client_skip
+    def _testSingleCmsgTruncInData(self):
+        self.assertTrue(self.misc_event.wait(timeout=self.fail_timeout))
+        self.sendToServer(MSG)
+
+    def checkTruncatedSecondHeader(self, ancbufsize, ignoreflags=0):
+        # Receive traffic class and hop limit into ancbufsize bytes of
+        # ancillary data space, which should be large enough to
+        # contain the first item, but too small to contain the header
+        # of the second.  Check that data is MSG, MSG_CTRUNC is set
+        # (unless included in ignoreflags), and only one ancillary
+        # data item is returned.
+        self.serv_sock.setsockopt(socket.IPPROTO_IPV6,
+                                  socket.IPV6_RECVHOPLIMIT, 1)
+        self.serv_sock.setsockopt(socket.IPPROTO_IPV6,
+                                  socket.IPV6_RECVTCLASS, 1)
+        self.misc_event.set()
+        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock,
+                                                   len(MSG), ancbufsize)
+
+        self.assertEqual(msg, MSG)
+        self.checkRecvmsgAddress(addr, self.cli_addr)
+        self.checkFlags(flags, eor=True, checkset=socket.MSG_CTRUNC,
+                        ignore=ignoreflags)
+
+        self.assertEqual(len(ancdata), 1)
+        cmsg_level, cmsg_type, cmsg_data = ancdata[0]
+        self.assertEqual(cmsg_level, socket.IPPROTO_IPV6)
+        self.assertIn(cmsg_type, {socket.IPV6_TCLASS, socket.IPV6_HOPLIMIT})
+        self.assertEqual(len(cmsg_data), SIZEOF_INT)
+        a = array.array("i")
+        a.fromstring(cmsg_data)
+        self.assertGreaterEqual(a[0], 0)
+        self.assertLessEqual(a[0], 255)
+
+    # Try the above test with various buffer sizes.
+
+    @requireAttrs(socket, "CMSG_SPACE", "IPV6_RECVHOPLIMIT", "IPV6_HOPLIMIT",
+                  "IPV6_RECVTCLASS", "IPV6_TCLASS")
+    def testSecondCmsgTrunc0(self):
+        self.checkTruncatedSecondHeader(socket.CMSG_SPACE(SIZEOF_INT),
+                                        ignoreflags=socket.MSG_CTRUNC)
+
+    @testSecondCmsgTrunc0.client_skip
+    def _testSecondCmsgTrunc0(self):
+        self.assertTrue(self.misc_event.wait(timeout=self.fail_timeout))
+        self.sendToServer(MSG)
+
+    @requireAttrs(socket, "CMSG_SPACE", "IPV6_RECVHOPLIMIT", "IPV6_HOPLIMIT",
+                  "IPV6_RECVTCLASS", "IPV6_TCLASS")
+    def testSecondCmsgTrunc1(self):
+        self.checkTruncatedSecondHeader(socket.CMSG_SPACE(SIZEOF_INT) + 1)
+
+    @testSecondCmsgTrunc1.client_skip
+    def _testSecondCmsgTrunc1(self):
+        self.assertTrue(self.misc_event.wait(timeout=self.fail_timeout))
+        self.sendToServer(MSG)
+
+    @requireAttrs(socket, "CMSG_SPACE", "IPV6_RECVHOPLIMIT", "IPV6_HOPLIMIT",
+                  "IPV6_RECVTCLASS", "IPV6_TCLASS")
+    def testSecondCmsgTrunc2Int(self):
+        self.checkTruncatedSecondHeader(socket.CMSG_SPACE(SIZEOF_INT) +
+                                        2 * SIZEOF_INT)
+
+    @testSecondCmsgTrunc2Int.client_skip
+    def _testSecondCmsgTrunc2Int(self):
+        self.assertTrue(self.misc_event.wait(timeout=self.fail_timeout))
+        self.sendToServer(MSG)
+
+    @requireAttrs(socket, "CMSG_SPACE", "IPV6_RECVHOPLIMIT", "IPV6_HOPLIMIT",
+                  "IPV6_RECVTCLASS", "IPV6_TCLASS")
+    def testSecondCmsgTruncLen0Minus1(self):
+        self.checkTruncatedSecondHeader(socket.CMSG_SPACE(SIZEOF_INT) +
+                                        socket.CMSG_LEN(0) - 1)
+
+    @testSecondCmsgTruncLen0Minus1.client_skip
+    def _testSecondCmsgTruncLen0Minus1(self):
+        self.assertTrue(self.misc_event.wait(timeout=self.fail_timeout))
+        self.sendToServer(MSG)
+
+    @requireAttrs(socket, "CMSG_SPACE", "IPV6_RECVHOPLIMIT", "IPV6_HOPLIMIT",
+                  "IPV6_RECVTCLASS", "IPV6_TCLASS")
+    def testSecomdCmsgTruncInData(self):
+        # Test truncation of the second of two control messages inside
+        # its associated data.
+        self.serv_sock.setsockopt(socket.IPPROTO_IPV6,
+                                  socket.IPV6_RECVHOPLIMIT, 1)
+        self.serv_sock.setsockopt(socket.IPPROTO_IPV6,
+                                  socket.IPV6_RECVTCLASS, 1)
+        self.misc_event.set()
+        msg, ancdata, flags, addr = self.doRecvmsg(
+            self.serv_sock, len(MSG),
+            socket.CMSG_SPACE(SIZEOF_INT) + socket.CMSG_LEN(SIZEOF_INT) - 1)
+
+        self.assertEqual(msg, MSG)
+        self.checkRecvmsgAddress(addr, self.cli_addr)
+        self.checkFlags(flags, eor=True, checkset=socket.MSG_CTRUNC)
+
+        cmsg_types = {socket.IPV6_TCLASS, socket.IPV6_HOPLIMIT}
+
+        cmsg_level, cmsg_type, cmsg_data = ancdata.pop(0)
+        self.assertEqual(cmsg_level, socket.IPPROTO_IPV6)
+        cmsg_types.remove(cmsg_type)
+        self.assertEqual(len(cmsg_data), SIZEOF_INT)
+        a = array.array("i")
+        a.fromstring(cmsg_data)
+        self.assertGreaterEqual(a[0], 0)
+        self.assertLessEqual(a[0], 255)
+
+        if ancdata:
+            cmsg_level, cmsg_type, cmsg_data = ancdata.pop(0)
+            self.assertEqual(cmsg_level, socket.IPPROTO_IPV6)
+            cmsg_types.remove(cmsg_type)
+            self.assertLess(len(cmsg_data), SIZEOF_INT)
+
+        self.assertEqual(ancdata, [])
+
+    @testSecomdCmsgTruncInData.client_skip
+    def _testSecomdCmsgTruncInData(self):
+        self.assertTrue(self.misc_event.wait(timeout=self.fail_timeout))
+        self.sendToServer(MSG)
+
+
+# Derive concrete test classes for different socket types.
+
+class SendrecvmsgUDPTestBase(SendrecvmsgDgramFlagsBase,
+                             SendrecvmsgConnectionlessBase,
+                             ThreadedSocketTestMixin, UDPTestBase):
+    pass
+
+@requireAttrs(socket.socket, "sendmsg")
+@unittest.skipUnless(thread, 'Threading required for this test.')
+class SendmsgUDPTest(SendmsgConnectionlessTests, SendrecvmsgUDPTestBase):
+    pass
+
+@requireAttrs(socket.socket, "recvmsg")
+@unittest.skipUnless(thread, 'Threading required for this test.')
+class RecvmsgUDPTest(RecvmsgTests, SendrecvmsgUDPTestBase):
+    pass
+
+@requireAttrs(socket.socket, "recvmsg_into")
+@unittest.skipUnless(thread, 'Threading required for this test.')
+class RecvmsgIntoUDPTest(RecvmsgIntoTests, SendrecvmsgUDPTestBase):
+    pass
+
+
+class SendrecvmsgUDP6TestBase(SendrecvmsgDgramFlagsBase,
+                              SendrecvmsgConnectionlessBase,
+                              ThreadedSocketTestMixin, UDP6TestBase):
+    pass
+
+@requireAttrs(socket.socket, "sendmsg")
+@unittest.skipUnless(socket.has_ipv6, "Python not built with IPv6 support")
+@requireSocket("AF_INET6", "SOCK_DGRAM")
+@unittest.skipUnless(thread, 'Threading required for this test.')
+class SendmsgUDP6Test(SendmsgConnectionlessTests, SendrecvmsgUDP6TestBase):
+    pass
+
+@requireAttrs(socket.socket, "recvmsg")
+@unittest.skipUnless(socket.has_ipv6, "Python not built with IPv6 support")
+@requireSocket("AF_INET6", "SOCK_DGRAM")
+@unittest.skipUnless(thread, 'Threading required for this test.')
+class RecvmsgUDP6Test(RecvmsgTests, SendrecvmsgUDP6TestBase):
+    pass
+
+@requireAttrs(socket.socket, "recvmsg_into")
+@unittest.skipUnless(socket.has_ipv6, "Python not built with IPv6 support")
+@requireSocket("AF_INET6", "SOCK_DGRAM")
+@unittest.skipUnless(thread, 'Threading required for this test.')
+class RecvmsgIntoUDP6Test(RecvmsgIntoTests, SendrecvmsgUDP6TestBase):
+    pass
+
+@requireAttrs(socket.socket, "recvmsg")
+@unittest.skipUnless(socket.has_ipv6, "Python not built with IPv6 support")
+@requireAttrs(socket, "IPPROTO_IPV6")
+@requireSocket("AF_INET6", "SOCK_DGRAM")
+@unittest.skipUnless(thread, 'Threading required for this test.')
+class RecvmsgRFC3542AncillaryUDP6Test(RFC3542AncillaryTest,
+                                      SendrecvmsgUDP6TestBase):
+    pass
+
+@requireAttrs(socket.socket, "recvmsg_into")
+@unittest.skipUnless(socket.has_ipv6, "Python not built with IPv6 support")
+@requireAttrs(socket, "IPPROTO_IPV6")
+@requireSocket("AF_INET6", "SOCK_DGRAM")
+@unittest.skipUnless(thread, 'Threading required for this test.')
+class RecvmsgIntoRFC3542AncillaryUDP6Test(RecvmsgIntoMixin,
+                                          RFC3542AncillaryTest,
+                                          SendrecvmsgUDP6TestBase):
+    pass
+
+
+class SendrecvmsgTCPTestBase(SendrecvmsgConnectedBase,
+                             ConnectedStreamTestMixin, TCPTestBase):
+    pass
+
+@requireAttrs(socket.socket, "sendmsg")
+@unittest.skipUnless(thread, 'Threading required for this test.')
+class SendmsgTCPTest(SendmsgStreamTests, SendrecvmsgTCPTestBase):
+    pass
+
+@requireAttrs(socket.socket, "recvmsg")
+@unittest.skipUnless(thread, 'Threading required for this test.')
+class RecvmsgTCPTest(RecvmsgTests, RecvmsgGenericStreamTests,
+                     SendrecvmsgTCPTestBase):
+    pass
+
+@requireAttrs(socket.socket, "recvmsg_into")
+@unittest.skipUnless(thread, 'Threading required for this test.')
+class RecvmsgIntoTCPTest(RecvmsgIntoTests, RecvmsgGenericStreamTests,
+                         SendrecvmsgTCPTestBase):
+    pass
+
+
+class SendrecvmsgSCTPStreamTestBase(SendrecvmsgSCTPFlagsBase,
+                                    SendrecvmsgConnectedBase,
+                                    ConnectedStreamTestMixin, SCTPStreamBase):
+    pass
+
+@requireAttrs(socket.socket, "sendmsg")
+@requireSocket("AF_INET", "SOCK_STREAM", "IPPROTO_SCTP")
+@unittest.skipUnless(thread, 'Threading required for this test.')
+class SendmsgSCTPStreamTest(SendmsgStreamTests, SendrecvmsgSCTPStreamTestBase):
+    pass
+
+@requireAttrs(socket.socket, "recvmsg")
+@requireSocket("AF_INET", "SOCK_STREAM", "IPPROTO_SCTP")
+@unittest.skipUnless(thread, 'Threading required for this test.')
+class RecvmsgSCTPStreamTest(RecvmsgTests, RecvmsgGenericStreamTests,
+                            SendrecvmsgSCTPStreamTestBase):
+    pass
+
+@requireAttrs(socket.socket, "recvmsg_into")
+@requireSocket("AF_INET", "SOCK_STREAM", "IPPROTO_SCTP")
+@unittest.skipUnless(thread, 'Threading required for this test.')
+class RecvmsgIntoSCTPStreamTest(RecvmsgIntoTests, RecvmsgGenericStreamTests,
+                                SendrecvmsgSCTPStreamTestBase):
+    pass
+
+
+class SendrecvmsgUnixStreamTestBase(SendrecvmsgConnectedBase,
+                                    ConnectedStreamTestMixin, UnixStreamBase):
+    pass
+
+@requireAttrs(socket.socket, "sendmsg")
+@requireAttrs(socket, "AF_UNIX")
+@unittest.skipUnless(thread, 'Threading required for this test.')
+class SendmsgUnixStreamTest(SendmsgStreamTests, SendrecvmsgUnixStreamTestBase):
+    pass
+
+@requireAttrs(socket.socket, "recvmsg")
+@requireAttrs(socket, "AF_UNIX")
+@unittest.skipUnless(thread, 'Threading required for this test.')
+class RecvmsgUnixStreamTest(RecvmsgTests, RecvmsgGenericStreamTests,
+                            SendrecvmsgUnixStreamTestBase):
+    pass
+
+@requireAttrs(socket.socket, "recvmsg_into")
+@requireAttrs(socket, "AF_UNIX")
+@unittest.skipUnless(thread, 'Threading required for this test.')
+class RecvmsgIntoUnixStreamTest(RecvmsgIntoTests, RecvmsgGenericStreamTests,
+                                SendrecvmsgUnixStreamTestBase):
+    pass
+
+@requireAttrs(socket.socket, "sendmsg", "recvmsg")
+@requireAttrs(socket, "AF_UNIX", "SOL_SOCKET", "SCM_RIGHTS")
+@unittest.skipUnless(thread, 'Threading required for this test.')
+class RecvmsgSCMRightsStreamTest(SCMRightsTest, SendrecvmsgUnixStreamTestBase):
+    pass
+
+@requireAttrs(socket.socket, "sendmsg", "recvmsg_into")
+@requireAttrs(socket, "AF_UNIX", "SOL_SOCKET", "SCM_RIGHTS")
+@unittest.skipUnless(thread, 'Threading required for this test.')
+class RecvmsgIntoSCMRightsStreamTest(RecvmsgIntoMixin, SCMRightsTest,
+                                     SendrecvmsgUnixStreamTestBase):
+    pass
+
+
+# Test interrupting the interruptible send/receive methods with a
+# signal when a timeout is set.  These tests avoid having multiple
+# threads alive during the test so that the OS cannot deliver the
+# signal to the wrong one.
+
+class InterruptedTimeoutBase(unittest.TestCase):
+    # Base class for interrupted send/receive tests.  Installs an
+    # empty handler for SIGALRM and removes it on teardown, along with
+    # any scheduled alarms.
+
+    def setUp(self):
+        super().setUp()
+        orig_alrm_handler = signal.signal(signal.SIGALRM,
+                                          lambda signum, frame: None)
+        self.addCleanup(signal.signal, signal.SIGALRM, orig_alrm_handler)
+        self.addCleanup(self.setAlarm, 0)
+
+    # Timeout for socket operations
+    timeout = 4.0
+
+    # Provide setAlarm() method to schedule delivery of SIGALRM after
+    # given number of seconds, or cancel it if zero, and an
+    # appropriate time value to use.  Use setitimer() if available.
+    if hasattr(signal, "setitimer"):
+        alarm_time = 0.05
+
+        def setAlarm(self, seconds):
+            signal.setitimer(signal.ITIMER_REAL, seconds)
+    else:
+        # Old systems may deliver the alarm up to one second early
+        alarm_time = 2
+
+        def setAlarm(self, seconds):
+            signal.alarm(seconds)
+
+
+# Require siginterrupt() in order to ensure that system calls are
+# interrupted by default.
+@requireAttrs(signal, "siginterrupt")
+@unittest.skipUnless(hasattr(signal, "alarm") or hasattr(signal, "setitimer"),
+                     "Don't have signal.alarm or signal.setitimer")
+class InterruptedRecvTimeoutTest(InterruptedTimeoutBase, UDPTestBase):
+    # Test interrupting the recv*() methods with signals when a
+    # timeout is set.
+
+    def setUp(self):
+        super().setUp()
+        self.serv.settimeout(self.timeout)
+
+    def checkInterruptedRecv(self, func, *args, **kwargs):
+        # Check that func(*args, **kwargs) raises socket.error with an
+        # errno of EINTR when interrupted by a signal.
+        self.setAlarm(self.alarm_time)
+        with self.assertRaises(socket.error) as cm:
+            func(*args, **kwargs)
+        self.assertNotIsInstance(cm.exception, socket.timeout)
+        self.assertEqual(cm.exception.errno, errno.EINTR)
+
+    def testInterruptedRecvTimeout(self):
+        self.checkInterruptedRecv(self.serv.recv, 1024)
+
+    def testInterruptedRecvIntoTimeout(self):
+        self.checkInterruptedRecv(self.serv.recv_into, bytearray(1024))
+
+    def testInterruptedRecvfromTimeout(self):
+        self.checkInterruptedRecv(self.serv.recvfrom, 1024)
+
+    def testInterruptedRecvfromIntoTimeout(self):
+        self.checkInterruptedRecv(self.serv.recvfrom_into, bytearray(1024))
+
+    @requireAttrs(socket.socket, "recvmsg")
+    def testInterruptedRecvmsgTimeout(self):
+        self.checkInterruptedRecv(self.serv.recvmsg, 1024)
+
+    @requireAttrs(socket.socket, "recvmsg_into")
+    def testInterruptedRecvmsgIntoTimeout(self):
+        self.checkInterruptedRecv(self.serv.recvmsg_into, [bytearray(1024)])
+
+
+# Require siginterrupt() in order to ensure that system calls are
+# interrupted by default.
+@requireAttrs(signal, "siginterrupt")
+@unittest.skipUnless(hasattr(signal, "alarm") or hasattr(signal, "setitimer"),
+                     "Don't have signal.alarm or signal.setitimer")
+@unittest.skipUnless(thread, 'Threading required for this test.')
+class InterruptedSendTimeoutTest(InterruptedTimeoutBase,
+                                 ThreadSafeCleanupTestCase,
+                                 SocketListeningTestMixin, TCPTestBase):
+    # Test interrupting the interruptible send*() methods with signals
+    # when a timeout is set.
+
+    def setUp(self):
+        super().setUp()
+        self.serv_conn = self.newSocket()
+        self.addCleanup(self.serv_conn.close)
+        # Use a thread to complete the connection, but wait for it to
+        # terminate before running the test, so that there is only one
+        # thread to accept the signal.
+        cli_thread = threading.Thread(target=self.doConnect)
+        cli_thread.start()
+        self.cli_conn, addr = self.serv.accept()
+        self.addCleanup(self.cli_conn.close)
+        cli_thread.join()
+        self.serv_conn.settimeout(self.timeout)
+
+    def doConnect(self):
+        self.serv_conn.connect(self.serv_addr)
+
+    def checkInterruptedSend(self, func, *args, **kwargs):
+        # Check that func(*args, **kwargs), run in a loop, raises
+        # socket.error with an errno of EINTR when interrupted by a
+        # signal.
+        with self.assertRaises(socket.error) as cm:
+            while True:
+                self.setAlarm(self.alarm_time)
+                func(*args, **kwargs)
+        self.assertNotIsInstance(cm.exception, socket.timeout)
+        self.assertEqual(cm.exception.errno, errno.EINTR)
+
+    def testInterruptedSendTimeout(self):
+        self.checkInterruptedSend(self.serv_conn.send, b"a"*512)
+
+    def testInterruptedSendtoTimeout(self):
+        # Passing an actual address here as Python's wrapper for
+        # sendto() doesn't allow passing a zero-length one; POSIX
+        # requires that the address is ignored since the socket is
+        # connection-mode, however.
+        self.checkInterruptedSend(self.serv_conn.sendto, b"a"*512,
+                                  self.serv_addr)
+
+    @requireAttrs(socket.socket, "sendmsg")
+    def testInterruptedSendmsgTimeout(self):
+        self.checkInterruptedSend(self.serv_conn.sendmsg, [b"a"*512])
+
+
 @unittest.skipUnless(thread, 'Threading required for this test.')
 class TCPCloserTest(ThreadedTCPSocketTest):
 
@@ -2077,6 +4172,31 @@ def test_main():
     if isTipcAvailable():
         tests.append(TIPCTest)
         tests.append(TIPCThreadableTest)
+    tests.extend([
+        CmsgMacroTests,
+        SendmsgUDPTest,
+        RecvmsgUDPTest,
+        RecvmsgIntoUDPTest,
+        SendmsgUDP6Test,
+        RecvmsgUDP6Test,
+        RecvmsgRFC3542AncillaryUDP6Test,
+        RecvmsgIntoRFC3542AncillaryUDP6Test,
+        RecvmsgIntoUDP6Test,
+        SendmsgTCPTest,
+        RecvmsgTCPTest,
+        RecvmsgIntoTCPTest,
+        SendmsgSCTPStreamTest,
+        RecvmsgSCTPStreamTest,
+        RecvmsgIntoSCTPStreamTest,
+        SendmsgUnixStreamTest,
+        RecvmsgUnixStreamTest,
+        RecvmsgIntoUnixStreamTest,
+        RecvmsgSCMRightsStreamTest,
+        RecvmsgIntoSCMRightsStreamTest,
+        # These are slow when setitimer() is not available
+        InterruptedRecvTimeoutTest,
+        InterruptedSendTimeoutTest,
+    ])
 
     thread_info = support.threading_setup()
     support.run_unittest(*tests)
diff --git a/Lib/test/test_ssl.py b/Lib/test/test_ssl.py
index f3f0c54..1d26c11 100644
--- a/Lib/test/test_ssl.py
+++ b/Lib/test/test_ssl.py
@@ -186,8 +186,11 @@ class BasicSocketTests(unittest.TestCase):
         self.assertRaises(socket.error, ss.recv_into, bytearray(b'x'))
         self.assertRaises(socket.error, ss.recvfrom, 1)
         self.assertRaises(socket.error, ss.recvfrom_into, bytearray(b'x'), 1)
+        self.assertRaises(socket.error, ss.recvmsg, 1)
+        self.assertRaises(socket.error, ss.recvmsg_into, [bytearray(b'x')])
         self.assertRaises(socket.error, ss.send, b'x')
         self.assertRaises(socket.error, ss.sendto, b'x', ('0.0.0.0', 0))
+        self.assertRaises(socket.error, ss.sendmsg, [b'x'])
 
     def test_timeout(self):
         # Issue #8524: when creating an SSL socket, the timeout of the
@@ -1520,17 +1523,30 @@ else:
                     count, addr = s.recvfrom_into(b)
                     return b[:count]
 
+                def _recvmsg(*args, **kwargs):
+                    return s.recvmsg(*args, **kwargs)[0]
+
+                def _recvmsg_into(bufsize, *args, **kwargs):
+                    b = bytearray(bufsize)
+                    return bytes(b[:s.recvmsg_into([b], *args, **kwargs)[0]])
+
+                def _sendmsg(msg, *args, **kwargs):
+                    return s.sendmsg([msg])
+
                 # (name, method, whether to expect success, *args)
                 send_methods = [
                     ('send', s.send, True, []),
                     ('sendto', s.sendto, False, ["some.address"]),
+                    ('sendmsg', _sendmsg, False, []),
                     ('sendall', s.sendall, True, []),
                 ]
                 recv_methods = [
                     ('recv', s.recv, True, []),
                     ('recvfrom', s.recvfrom, False, ["some.address"]),
+                    ('recvmsg', _recvmsg, False, [100]),
                     ('recv_into', _recv_into, True, []),
                     ('recvfrom_into', _recvfrom_into, False, []),
+                    ('recvmsg_into', _recvmsg_into, False, [100]),
                 ]
                 data_prefix = "PREFIX_"
 
diff --git a/Misc/NEWS b/Misc/NEWS
index a2943a7..67e8218 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -265,6 +265,10 @@ Core and Builtins
 Library
 -------
 
+- Issue #6560: The sendmsg/recvmsg API is now exposed by the socket module
+  when provided by the underlying platform, supporting processing of
+  ancillary data in pure Python code.
+
 - Issue #12326: On Linux, sys.platform doesn't contain the major version
   anymore. It is now always 'linux', instead of 'linux2' or 'linux3' depending
   on the Linux version used to build Python.
diff --git a/Modules/socketmodule.c b/Modules/socketmodule.c
index 8de84b7..75cde79 100644
--- a/Modules/socketmodule.c
+++ b/Modules/socketmodule.c
@@ -263,6 +263,7 @@ if_indextoname(index) -- return the corresponding interface name\n\
 #ifdef HAVE_NET_IF_H
 #include <net/if.h>
 #endif
+#include <unistd.h>
 
 /* Generic socket object definitions and includes */
 #define PySocket_BUILDING_SOCKET
@@ -469,6 +470,17 @@ static PyTypeObject sock_type;
 #include <sys/poll.h>
 #endif
 
+/* Largest value to try to store in a socklen_t (used when handling
+   ancillary data).  POSIX requires socklen_t to hold at least
+   (2**31)-1 and recommends against storing larger values, but
+   socklen_t was originally int in the BSD interface, so to be on the
+   safe side we use the smaller of (2**31)-1 and INT_MAX. */
+#if INT_MAX > 0x7fffffff
+#define SOCKLEN_T_LIMIT 0x7fffffff
+#else
+#define SOCKLEN_T_LIMIT INT_MAX
+#endif
+
 #ifdef Py_SOCKET_FD_CAN_BE_GE_FD_SETSIZE
 /* Platform can select file descriptors beyond FD_SETSIZE */
 #define IS_SELECTABLE(s) 1
@@ -1678,6 +1690,117 @@ getsockaddrlen(PySocketSockObject *s, socklen_t *len_ret)
 }
 
 
+/* Support functions for the sendmsg() and recvmsg[_into]() methods.
+   Currently, these methods are only compiled if the RFC 2292/3542
+   CMSG_LEN() macro is available.  Older systems seem to have used
+   sizeof(struct cmsghdr) + (length) where CMSG_LEN() is used now, so
+   it may be possible to define CMSG_LEN() that way if it's not
+   provided.  Some architectures might need extra padding after the
+   cmsghdr, however, and CMSG_LEN() would have to take account of
+   this. */
+#ifdef CMSG_LEN
+/* If length is in range, set *result to CMSG_LEN(length) and return
+   true; otherwise, return false. */
+static int
+get_CMSG_LEN(size_t length, size_t *result)
+{
+    size_t tmp;
+
+    if (length > (SOCKLEN_T_LIMIT - CMSG_LEN(0)))
+        return 0;
+    tmp = CMSG_LEN(length);
+    if (tmp > SOCKLEN_T_LIMIT || tmp < length)
+        return 0;
+    *result = tmp;
+    return 1;
+}
+
+#ifdef CMSG_SPACE
+/* If length is in range, set *result to CMSG_SPACE(length) and return
+   true; otherwise, return false. */
+static int
+get_CMSG_SPACE(size_t length, size_t *result)
+{
+    size_t tmp;
+
+    /* Use CMSG_SPACE(1) here in order to take account of the padding
+       necessary before *and* after the data. */
+    if (length > (SOCKLEN_T_LIMIT - CMSG_SPACE(1)))
+        return 0;
+    tmp = CMSG_SPACE(length);
+    if (tmp > SOCKLEN_T_LIMIT || tmp < length)
+        return 0;
+    *result = tmp;
+    return 1;
+}
+#endif
+
+/* Return true iff msg->msg_controllen is valid, cmsgh is a valid
+   pointer in msg->msg_control with at least "space" bytes after it,
+   and its cmsg_len member inside the buffer. */
+static int
+cmsg_min_space(struct msghdr *msg, struct cmsghdr *cmsgh, size_t space)
+{
+    size_t cmsg_offset;
+    static const size_t cmsg_len_end = (offsetof(struct cmsghdr, cmsg_len) +
+                                        sizeof(cmsgh->cmsg_len));
+
+    if (cmsgh == NULL || msg->msg_control == NULL || msg->msg_controllen < 0)
+        return 0;
+    if (space < cmsg_len_end)
+        space = cmsg_len_end;
+    cmsg_offset = (char *)cmsgh - (char *)msg->msg_control;
+    return (cmsg_offset <= (size_t)-1 - space &&
+            cmsg_offset + space <= msg->msg_controllen);
+}
+
+/* If pointer CMSG_DATA(cmsgh) is in buffer msg->msg_control, set
+   *space to number of bytes following it in the buffer and return
+   true; otherwise, return false.  Assumes cmsgh, msg->msg_control and
+   msg->msg_controllen are valid. */
+static int
+get_cmsg_data_space(struct msghdr *msg, struct cmsghdr *cmsgh, size_t *space)
+{
+    size_t data_offset;
+    char *data_ptr;
+
+    if ((data_ptr = (char *)CMSG_DATA(cmsgh)) == NULL)
+        return 0;
+    data_offset = data_ptr - (char *)msg->msg_control;
+    if (data_offset > msg->msg_controllen)
+        return 0;
+    *space = msg->msg_controllen - data_offset;
+    return 1;
+}
+
+/* If cmsgh is invalid or not contained in the buffer pointed to by
+   msg->msg_control, return -1.  If cmsgh is valid and its associated
+   data is entirely contained in the buffer, set *data_len to the
+   length of the associated data and return 0.  If only part of the
+   associated data is contained in the buffer but cmsgh is otherwise
+   valid, set *data_len to the length contained in the buffer and
+   return 1. */
+static int
+get_cmsg_data_len(struct msghdr *msg, struct cmsghdr *cmsgh, size_t *data_len)
+{
+    size_t space, cmsg_data_len;
+
+    if (!cmsg_min_space(msg, cmsgh, CMSG_LEN(0)) ||
+        cmsgh->cmsg_len < CMSG_LEN(0))
+        return -1;
+    cmsg_data_len = cmsgh->cmsg_len - CMSG_LEN(0);
+    if (!get_cmsg_data_space(msg, cmsgh, &space))
+        return -1;
+    if (space >= cmsg_data_len) {
+        *data_len = cmsg_data_len;
+        return 0;
+    }
+    *data_len = space;
+    return 1;
+}
+#endif    /* CMSG_LEN */
+
+
 /* s._accept() -> (fd, address) */
 
 static PyObject *
@@ -2631,6 +2754,333 @@ PyDoc_STRVAR(recvfrom_into_doc,
 Like recv_into(buffer[, nbytes[, flags]]) but also return the sender's address info.");
 
 
+/* The sendmsg() and recvmsg[_into]() methods require a working
+   CMSG_LEN().  See the comment near get_CMSG_LEN(). */
+#ifdef CMSG_LEN
+/*
+ * Call recvmsg() with the supplied iovec structures, flags, and
+ * ancillary data buffer size (controllen).  Returns the tuple return
+ * value for recvmsg() or recvmsg_into(), with the first item provided
+ * by the supplied makeval() function.  makeval() will be called with
+ * the length read and makeval_data as arguments, and must return a
+ * new reference (which will be decrefed if there is a subsequent
+ * error).  On error, closes any file descriptors received via
+ * SCM_RIGHTS.
+ */
+static PyObject *
+sock_recvmsg_guts(PySocketSockObject *s, struct iovec *iov, int iovlen,
+                  int flags, Py_ssize_t controllen,
+                  PyObject *(*makeval)(ssize_t, void *), void *makeval_data)
+{
+    ssize_t bytes_received = -1;
+    int timeout;
+    sock_addr_t addrbuf;
+    socklen_t addrbuflen;
+    static const struct msghdr msg_blank;
+    struct msghdr msg;
+    PyObject *cmsg_list = NULL, *retval = NULL;
+    void *controlbuf = NULL;
+    struct cmsghdr *cmsgh;
+    size_t cmsgdatalen = 0;
+    int cmsg_status;
+
+    /* XXX: POSIX says that msg_name and msg_namelen "shall be
+       ignored" when the socket is connected (Linux fills them in
+       anyway for AF_UNIX sockets at least).  Normally msg_namelen
+       seems to be set to 0 if there's no address, but try to
+       initialize msg_name to something that won't be mistaken for a
+       real address if that doesn't happen. */
+    if (!getsockaddrlen(s, &addrbuflen))
+        return NULL;
+    memset(&addrbuf, 0, addrbuflen);
+    SAS2SA(&addrbuf)->sa_family = AF_UNSPEC;
+
+    if (controllen < 0 || controllen > SOCKLEN_T_LIMIT) {
+        PyErr_SetString(PyExc_ValueError,
+                        "invalid ancillary data buffer length");
+        return NULL;
+    }
+    if (controllen > 0 && (controlbuf = PyMem_Malloc(controllen)) == NULL)
+        return PyErr_NoMemory();
+
+    /* Make the system call. */
+    if (!IS_SELECTABLE(s)) {
+        select_error();
+        goto finally;
+    }
+
+    BEGIN_SELECT_LOOP(s)
+    Py_BEGIN_ALLOW_THREADS;
+    msg = msg_blank;    /* Set all members to 0 or NULL */
+    msg.msg_name = SAS2SA(&addrbuf);
+    msg.msg_namelen = addrbuflen;
+    msg.msg_iov = iov;
+    msg.msg_iovlen = iovlen;
+    msg.msg_control = controlbuf;
+    msg.msg_controllen = controllen;
+    timeout = internal_select_ex(s, 0, interval);
+    if (!timeout)
+        bytes_received = recvmsg(s->sock_fd, &msg, flags);
+    Py_END_ALLOW_THREADS;
+    if (timeout == 1) {
+        PyErr_SetString(socket_timeout, "timed out");
+        goto finally;
+    }
+    END_SELECT_LOOP(s)
+
+    if (bytes_received < 0) {
+        s->errorhandler();
+        goto finally;
+    }
+
+    /* Make list of (level, type, data) tuples from control messages. */
+    if ((cmsg_list = PyList_New(0)) == NULL)
+        goto err_closefds;
+    /* Check for empty ancillary data as old CMSG_FIRSTHDR()
+       implementations didn't do so. */
+    for (cmsgh = ((msg.msg_controllen > 0) ? CMSG_FIRSTHDR(&msg) : NULL);
+         cmsgh != NULL; cmsgh = CMSG_NXTHDR(&msg, cmsgh)) {
+        PyObject *bytes, *tuple;
+        int tmp;
+
+        cmsg_status = get_cmsg_data_len(&msg, cmsgh, &cmsgdatalen);
+        if (cmsg_status != 0) {
+            if (PyErr_WarnEx(PyExc_RuntimeWarning,
+                             "received malformed or improperly-truncated "
+                             "ancillary data", 1) == -1)
+                goto err_closefds;
+        }
+        if (cmsg_status < 0)
+            break;
+        if (cmsgdatalen > PY_SSIZE_T_MAX) {
+            PyErr_SetString(socket_error, "control message too long");
+            goto err_closefds;
+        }
+
+        bytes = PyBytes_FromStringAndSize((char *)CMSG_DATA(cmsgh),
+                                          cmsgdatalen);
+        tuple = Py_BuildValue("iiN", (int)cmsgh->cmsg_level,
+                              (int)cmsgh->cmsg_type, bytes);
+        if (tuple == NULL)
+            goto err_closefds;
+        tmp = PyList_Append(cmsg_list, tuple);
+        Py_DECREF(tuple);
+        if (tmp != 0)
+            goto err_closefds;
+
+        if (cmsg_status != 0)
+            break;
+    }
+
+    retval = Py_BuildValue("NOiN",
+                           (*makeval)(bytes_received, makeval_data),
+                           cmsg_list,
+                           (int)msg.msg_flags,
+                           makesockaddr(s->sock_fd, SAS2SA(&addrbuf),
+                                        ((msg.msg_namelen > addrbuflen) ?
+                                         addrbuflen : msg.msg_namelen),
+                                        s->sock_proto));
+    if (retval == NULL)
+        goto err_closefds;
+
+finally:
+    Py_XDECREF(cmsg_list);
+    PyMem_Free(controlbuf);
+    return retval;
+
+err_closefds:
+#ifdef SCM_RIGHTS
+    /* Close all descriptors coming from SCM_RIGHTS, so they don't leak. */
+    for (cmsgh = ((msg.msg_controllen > 0) ? CMSG_FIRSTHDR(&msg) : NULL);
+         cmsgh != NULL; cmsgh = CMSG_NXTHDR(&msg, cmsgh)) {
+        cmsg_status = get_cmsg_data_len(&msg, cmsgh, &cmsgdatalen);
+        if (cmsg_status < 0)
+            break;
+        if (cmsgh->cmsg_level == SOL_SOCKET &&
+            cmsgh->cmsg_type == SCM_RIGHTS) {
+            size_t numfds;
+            int *fdp;
+
+            numfds = cmsgdatalen / sizeof(int);
+            fdp = (int *)CMSG_DATA(cmsgh);
+            while (numfds-- > 0)
+                close(*fdp++);
+        }
+        if (cmsg_status != 0)
+            break;
+    }
+#endif /* SCM_RIGHTS */
+    goto finally;
+}
+
+
+static PyObject *
+makeval_recvmsg(ssize_t received, void *data)
+{
+    PyObject **buf = data;
+
+    if (received < PyBytes_GET_SIZE(*buf))
+        _PyBytes_Resize(buf, received);
+    Py_XINCREF(*buf);
+    return *buf;
+}
+
+/* s.recvmsg(bufsize[, ancbufsize[, flags]]) method */
+
+static PyObject *
+sock_recvmsg(PySocketSockObject *s, PyObject *args)
+{
+    Py_ssize_t bufsize, ancbufsize = 0;
+    int flags = 0;
+    struct iovec iov;
+    PyObject *buf = NULL, *retval = NULL;
+
+    if (!PyArg_ParseTuple(args, "n|ni:recvmsg", &bufsize, &ancbufsize, &flags))
+        return NULL;
+
+    if (bufsize < 0) {
+        PyErr_SetString(PyExc_ValueError, "negative buffer size in recvmsg()");
+        return NULL;
+    }
+    if ((buf = PyBytes_FromStringAndSize(NULL, bufsize)) == NULL)
+        return NULL;
+    iov.iov_base = PyBytes_AS_STRING(buf);
+    iov.iov_len = bufsize;
+
+    /* Note that we're passing a pointer to *our pointer* to the bytes
+       object here (&buf); makeval_recvmsg() may incref the object, or
+       deallocate it and set our pointer to NULL. */
+    retval = sock_recvmsg_guts(s, &iov, 1, flags, ancbufsize,
+                               &makeval_recvmsg, &buf);
+    Py_XDECREF(buf);
+    return retval;
+}
+
+PyDoc_STRVAR(recvmsg_doc,
+"recvmsg(bufsize[, ancbufsize[, flags]]) -> (data, ancdata, msg_flags, address)\n\
+\n\
+Receive normal data (up to bufsize bytes) and ancillary data from the\n\
+socket.  The ancbufsize argument sets the size in bytes of the\n\
+internal buffer used to receive the ancillary data; it defaults to 0,\n\
+meaning that no ancillary data will be received.  Appropriate buffer\n\
+sizes for ancillary data can be calculated using CMSG_SPACE() or\n\
+CMSG_LEN(), and items which do not fit into the buffer might be\n\
+truncated or discarded.  The flags argument defaults to 0 and has the\n\
+same meaning as for recv().\n\
+\n\
+The return value is a 4-tuple: (data, ancdata, msg_flags, address).\n\
+The data item is a bytes object holding the non-ancillary data\n\
+received.  The ancdata item is a list of zero or more tuples\n\
+(cmsg_level, cmsg_type, cmsg_data) representing the ancillary data\n\
+(control messages) received: cmsg_level and cmsg_type are integers\n\
+specifying the protocol level and protocol-specific type respectively,\n\
+and cmsg_data is a bytes object holding the associated data.  The\n\
+msg_flags item is the bitwise OR of various flags indicating\n\
+conditions on the received message; see your system documentation for\n\
+details.  If the receiving socket is unconnected, address is the\n\
+address of the sending socket, if available; otherwise, its value is\n\
+unspecified.\n\
+\n\
+If recvmsg() raises an exception after the system call returns, it\n\
+will first attempt to close any file descriptors received via the\n\
+SCM_RIGHTS mechanism.");
+
+
+static PyObject *
+makeval_recvmsg_into(ssize_t received, void *data)
+{
+    return PyLong_FromSsize_t(received);
+}
+
+/* s.recvmsg_into(buffers[, ancbufsize[, flags]]) method */
+
+static PyObject *
+sock_recvmsg_into(PySocketSockObject *s, PyObject *args)
+{
+    Py_ssize_t ancbufsize = 0;
+    int flags = 0;
+    struct iovec *iovs = NULL;
+    Py_ssize_t i, nitems, nbufs = 0;
+    Py_buffer *bufs = NULL;
+    PyObject *buffers_arg, *fast, *retval = NULL;
+
+    if (!PyArg_ParseTuple(args, "O|ni:recvmsg_into",
+                          &buffers_arg, &ancbufsize, &flags))
+        return NULL;
+
+    if ((fast = PySequence_Fast(buffers_arg,
+                                "recvmsg_into() argument 1 must be an "
+                                "iterable")) == NULL)
+        return NULL;
+    nitems = PySequence_Fast_GET_SIZE(fast);
+    if (nitems > INT_MAX) {
+        PyErr_SetString(socket_error, "recvmsg_into() argument 1 is too long");
+        goto finally;
+    }
+
+    /* Fill in an iovec for each item, and save the Py_buffer
+       structs to release afterwards. */
+    if (nitems > 0 && ((iovs = PyMem_New(struct iovec, nitems)) == NULL ||
+                       (bufs = PyMem_New(Py_buffer, nitems)) == NULL)) {
+        PyErr_NoMemory();
+        goto finally;
+    }
+    for (; nbufs < nitems; nbufs++) {
+        if (!PyArg_Parse(PySequence_Fast_GET_ITEM(fast, nbufs),
+                         "w*;recvmsg_into() argument 1 must be an iterable "
+                         "of single-segment read-write buffers",
+                         &bufs[nbufs]))
+            goto finally;
+        iovs[nbufs].iov_base = bufs[nbufs].buf;
+        iovs[nbufs].iov_len = bufs[nbufs].len;
+    }
+
+    retval = sock_recvmsg_guts(s, iovs, nitems, flags, ancbufsize,
+                               &makeval_recvmsg_into, NULL);
+finally:
+    for (i = 0; i < nbufs; i++)
+        PyBuffer_Release(&bufs[i]);
+    PyMem_Free(bufs);
+    PyMem_Free(iovs);
+    Py_DECREF(fast);
+    return retval;
+}
+
+PyDoc_STRVAR(recvmsg_into_doc,
+"recvmsg_into(buffers[, ancbufsize[, flags]]) -> (nbytes, ancdata, msg_flags, address)\n\
+\n\
+Receive normal data and ancillary data from the socket, scattering the\n\
+non-ancillary data into a series of buffers.  The buffers argument\n\
+must be an iterable of objects that export writable buffers\n\
+(e.g. bytearray objects); these will be filled with successive chunks\n\
+of the non-ancillary data until it has all been written or there are\n\
+no more buffers.  The ancbufsize argument sets the size in bytes of\n\
+the internal buffer used to receive the ancillary data; it defaults to\n\
+0, meaning that no ancillary data will be received.  Appropriate\n\
+buffer sizes for ancillary data can be calculated using CMSG_SPACE()\n\
+or CMSG_LEN(), and items which do not fit into the buffer might be\n\
+truncated or discarded.  The flags argument defaults to 0 and has the\n\
+same meaning as for recv().\n\
+\n\
+The return value is a 4-tuple: (nbytes, ancdata, msg_flags, address).\n\
+The nbytes item is the total number of bytes of non-ancillary data\n\
+written into the buffers.  The ancdata item is a list of zero or more\n\
+tuples (cmsg_level, cmsg_type, cmsg_data) representing the ancillary\n\
+data (control messages) received: cmsg_level and cmsg_type are\n\
+integers specifying the protocol level and protocol-specific type\n\
+respectively, and cmsg_data is a bytes object holding the associated\n\
+data.  The msg_flags item is the bitwise OR of various flags\n\
+indicating conditions on the received message; see your system\n\
+documentation for details.  If the receiving socket is unconnected,\n\
+address is the address of the sending socket, if available; otherwise,\n\
+its value is unspecified.\n\
+\n\
+If recvmsg_into() raises an exception after the system call returns,\n\
+it will first attempt to close any file descriptors received via the\n\
+SCM_RIGHTS mechanism.");
+#endif    /* CMSG_LEN */
+
+
 /* s.send(data [,flags]) method */
 
 static PyObject *
@@ -2826,6 +3276,237 @@ Like send(data, flags) but allows specifying the destination address.\n\
 For IP sockets, the address is a pair (hostaddr, port).");
 
 
+/* The sendmsg() and recvmsg[_into]() methods require a working
+   CMSG_LEN().  See the comment near get_CMSG_LEN(). */
+#ifdef CMSG_LEN
+/* s.sendmsg(buffers[, ancdata[, flags[, address]]]) method */
+
+static PyObject *
+sock_sendmsg(PySocketSockObject *s, PyObject *args)
+{
+    Py_ssize_t i, ndataparts, ndatabufs = 0, ncmsgs, ncmsgbufs = 0;
+    Py_buffer *databufs = NULL;
+    struct iovec *iovs = NULL;
+    sock_addr_t addrbuf;
+    static const struct msghdr msg_blank;
+    struct msghdr msg;
+    struct cmsginfo {
+        int level;
+        int type;
+        Py_buffer data;
+    } *cmsgs = NULL;
+    void *controlbuf = NULL;
+    size_t controllen, controllen_last;
+    ssize_t bytes_sent = -1;
+    int addrlen, timeout, flags = 0;
+    PyObject *data_arg, *cmsg_arg = NULL, *addr_arg = NULL, *data_fast = NULL,
+        *cmsg_fast = NULL, *retval = NULL;
+
+    if (!PyArg_ParseTuple(args, "O|OiO:sendmsg",
+                          &data_arg, &cmsg_arg, &flags, &addr_arg))
+        return NULL;
+
+    msg = msg_blank;    /* Set all members to 0 or NULL */
+
+    /* Parse destination address. */
+    if (addr_arg != NULL && addr_arg != Py_None) {
+        if (!getsockaddrarg(s, addr_arg, SAS2SA(&addrbuf), &addrlen))
+            goto finally;
+        msg.msg_name = &addrbuf;
+        msg.msg_namelen = addrlen;
+    }
+
+    /* Fill in an iovec for each message part, and save the Py_buffer
+       structs to release afterwards. */
+    if ((data_fast = PySequence_Fast(data_arg,
+                                     "sendmsg() argument 1 must be an "
+                                     "iterable")) == NULL)
+        goto finally;
+    ndataparts = PySequence_Fast_GET_SIZE(data_fast);
+    if (ndataparts > INT_MAX) {
+        PyErr_SetString(socket_error, "sendmsg() argument 1 is too long");
+        goto finally;
+    }
+    msg.msg_iovlen = ndataparts;
+    if (ndataparts > 0 &&
+        ((msg.msg_iov = iovs = PyMem_New(struct iovec, ndataparts)) == NULL ||
+         (databufs = PyMem_New(Py_buffer, ndataparts)) == NULL)) {
+        PyErr_NoMemory();
+        goto finally;
+    }
+    for (; ndatabufs < ndataparts; ndatabufs++) {
+        if (!PyArg_Parse(PySequence_Fast_GET_ITEM(data_fast, ndatabufs),
+                         "y*;sendmsg() argument 1 must be an iterable of "
+                         "buffer-compatible objects",
+                         &databufs[ndatabufs]))
+            goto finally;
+        iovs[ndatabufs].iov_base = databufs[ndatabufs].buf;
+        iovs[ndatabufs].iov_len = databufs[ndatabufs].len;
+    }
+
+    if (cmsg_arg == NULL)
+        ncmsgs = 0;
+    else {
+        if ((cmsg_fast = PySequence_Fast(cmsg_arg,
+                                         "sendmsg() argument 2 must be an "
+                                         "iterable")) == NULL)
+            goto finally;
+        ncmsgs = PySequence_Fast_GET_SIZE(cmsg_fast);
+    }
+
+#ifndef CMSG_SPACE
+    if (ncmsgs > 1) {
+        PyErr_SetString(socket_error,
+                        "sending multiple control messages is not supported "
+                        "on this system");
+        goto finally;
+    }
+#endif
+    /* Save level, type and Py_buffer for each control message,
+       and calculate total size. */
+    if (ncmsgs > 0 && (cmsgs = PyMem_New(struct cmsginfo, ncmsgs)) == NULL) {
+        PyErr_NoMemory();
+        goto finally;
+    }
+    controllen = controllen_last = 0;
+    while (ncmsgbufs < ncmsgs) {
+        size_t bufsize, space;
+
+        if (!PyArg_Parse(PySequence_Fast_GET_ITEM(cmsg_fast, ncmsgbufs),
+                         "(iiy*):[sendmsg() ancillary data items]",
+                         &cmsgs[ncmsgbufs].level,
+                         &cmsgs[ncmsgbufs].type,
+                         &cmsgs[ncmsgbufs].data))
+            goto finally;
+        bufsize = cmsgs[ncmsgbufs++].data.len;
+
+#ifdef CMSG_SPACE
+        if (!get_CMSG_SPACE(bufsize, &space)) {
+#else
+        if (!get_CMSG_LEN(bufsize, &space)) {
+#endif
+            PyErr_SetString(socket_error, "ancillary data item too large");
+            goto finally;
+        }
+        controllen += space;
+        if (controllen > SOCKLEN_T_LIMIT || controllen < controllen_last) {
+            PyErr_SetString(socket_error, "too much ancillary data");
+            goto finally;
+        }
+        controllen_last = controllen;
+    }
+
+    /* Construct ancillary data block from control message info. */
+    if (ncmsgbufs > 0) {
+        struct cmsghdr *cmsgh = NULL;
+
+        if ((msg.msg_control = controlbuf =
+             PyMem_Malloc(controllen)) == NULL) {
+            PyErr_NoMemory();
+            goto finally;
+        }
+        msg.msg_controllen = controllen;
+
+        /* Need to zero out the buffer as a workaround for glibc's
+           CMSG_NXTHDR() implementation.  After getting the pointer to
+           the next header, it checks its (uninitialized) cmsg_len
+           member to see if the "message" fits in the buffer, and
+           returns NULL if it doesn't.  Zero-filling the buffer
+           ensures that that doesn't happen. */
+        memset(controlbuf, 0, controllen);
+
+        for (i = 0; i < ncmsgbufs; i++) {
+            size_t msg_len, data_len = cmsgs[i].data.len;
+            int enough_space = 0;
+
+            cmsgh = (i == 0) ? CMSG_FIRSTHDR(&msg) : CMSG_NXTHDR(&msg, cmsgh);
+            if (cmsgh == NULL) {
+                PyErr_Format(PyExc_RuntimeError,
+                             "unexpected NULL result from %s()",
+                             (i == 0) ? "CMSG_FIRSTHDR" : "CMSG_NXTHDR");
+                goto finally;
+            }
+            if (!get_CMSG_LEN(data_len, &msg_len)) {
+                PyErr_SetString(PyExc_RuntimeError,
+                                "item size out of range for CMSG_LEN()");
+                goto finally;
+            }
+            if (cmsg_min_space(&msg, cmsgh, msg_len)) {
+                size_t space;
+
+                cmsgh->cmsg_len = msg_len;
+                if (get_cmsg_data_space(&msg, cmsgh, &space))
+                    enough_space = (space >= data_len);
+            }
+            if (!enough_space) {
+                PyErr_SetString(PyExc_RuntimeError,
+                                "ancillary data does not fit in calculated "
+                                "space");
+                goto finally;
+            }
+            cmsgh->cmsg_level = cmsgs[i].level;
+            cmsgh->cmsg_type = cmsgs[i].type;
+            memcpy(CMSG_DATA(cmsgh), cmsgs[i].data.buf, data_len);
+        }
+    }
+
+    /* Make the system call. */
+    if (!IS_SELECTABLE(s)) {
+        select_error();
+        goto finally;
+    }
+
+    BEGIN_SELECT_LOOP(s)
+    Py_BEGIN_ALLOW_THREADS;
+    timeout = internal_select_ex(s, 1, interval);
+    if (!timeout)
+        bytes_sent = sendmsg(s->sock_fd, &msg, flags);
+    Py_END_ALLOW_THREADS;
+    if (timeout == 1) {
+        PyErr_SetString(socket_timeout, "timed out");
+        goto finally;
+    }
+    END_SELECT_LOOP(s)
+
+    if (bytes_sent < 0) {
+        s->errorhandler();
+        goto finally;
+    }
+    retval = PyLong_FromSsize_t(bytes_sent);
+
+finally:
+    PyMem_Free(controlbuf);
+    for (i = 0; i < ncmsgbufs; i++)
+        PyBuffer_Release(&cmsgs[i].data);
+    PyMem_Free(cmsgs);
+    Py_XDECREF(cmsg_fast);
+    for (i = 0; i < ndatabufs; i++)
+        PyBuffer_Release(&databufs[i]);
+    PyMem_Free(databufs);
+    PyMem_Free(iovs);
+    Py_XDECREF(data_fast);
+    return retval;
+}
+
+PyDoc_STRVAR(sendmsg_doc,
+"sendmsg(buffers[, ancdata[, flags[, address]]]) -> count\n\
+\n\
+Send normal and ancillary data to the socket, gathering the\n\
+non-ancillary data from a series of buffers and concatenating it into\n\
+a single message.  The buffers argument specifies the non-ancillary\n\
+data as an iterable of buffer-compatible objects (e.g. bytes objects).\n\
+The ancdata argument specifies the ancillary data (control messages)\n\
+as an iterable of zero or more tuples (cmsg_level, cmsg_type,\n\
+cmsg_data), where cmsg_level and cmsg_type are integers specifying the\n\
+protocol level and protocol-specific type respectively, and cmsg_data\n\
+is a buffer-compatible object holding the associated data.  The flags\n\
+argument defaults to 0 and has the same meaning as for send().  If\n\
+address is supplied and not None, it sets a destination address for\n\
+the message.  The return value is the number of bytes of non-ancillary\n\
+data sent.");
+#endif    /* CMSG_LEN */
+
+
 /* s.shutdown(how) method */
 
 static PyObject *
@@ -2952,6 +3633,14 @@ static PyMethodDef sock_methods[] = {
                       setsockopt_doc},
     {"shutdown",          (PyCFunction)sock_shutdown, METH_O,
                       shutdown_doc},
+#ifdef CMSG_LEN
+    {"recvmsg",           (PyCFunction)sock_recvmsg, METH_VARARGS,
+                      recvmsg_doc},
+    {"recvmsg_into",      (PyCFunction)sock_recvmsg_into, METH_VARARGS,
+                      recvmsg_into_doc,},
+    {"sendmsg",           (PyCFunction)sock_sendmsg, METH_VARARGS,
+                      sendmsg_doc},
+#endif
     {NULL,                      NULL}           /* sentinel */
 };
 
@@ -4377,6 +5066,68 @@ Returns the interface name corresponding to the interface index if_index.");
 #endif  /* HAVE_IF_NAMEINDEX */
 
 
+#ifdef CMSG_LEN
+/* Python interface to CMSG_LEN(length). */
+
+static PyObject *
+socket_CMSG_LEN(PyObject *self, PyObject *args)
+{
+    Py_ssize_t length;
+    size_t result;
+
+    if (!PyArg_ParseTuple(args, "n:CMSG_LEN", &length))
+        return NULL;
+    if (length < 0 || !get_CMSG_LEN(length, &result)) {
+        PyErr_Format(PyExc_OverflowError, "CMSG_LEN() argument out of range");
+        return NULL;
+    }
+    return PyLong_FromSize_t(result);
+}
+
+PyDoc_STRVAR(CMSG_LEN_doc,
+"CMSG_LEN(length) -> control message length\n\
+\n\
+Return the total length, without trailing padding, of an ancillary\n\
+data item with associated data of the given length.  This value can\n\
+often be used as the buffer size for recvmsg() to receive a single\n\
+item of ancillary data, but RFC 3542 requires portable applications to\n\
+use CMSG_SPACE() and thus include space for padding, even when the\n\
+item will be the last in the buffer.  Raises OverflowError if length\n\
+is outside the permissible range of values.");
+
+
+#ifdef CMSG_SPACE
+/* Python interface to CMSG_SPACE(length). */
+
+static PyObject *
+socket_CMSG_SPACE(PyObject *self, PyObject *args)
+{
+    Py_ssize_t length;
+    size_t result;
+
+    if (!PyArg_ParseTuple(args, "n:CMSG_SPACE", &length))
+        return NULL;
+    if (length < 0 || !get_CMSG_SPACE(length, &result)) {
+        PyErr_SetString(PyExc_OverflowError,
+                        "CMSG_SPACE() argument out of range");
+        return NULL;
+    }
+    return PyLong_FromSize_t(result);
+}
+
+PyDoc_STRVAR(CMSG_SPACE_doc,
+"CMSG_SPACE(length) -> buffer size\n\
+\n\
+Return the buffer size needed for recvmsg() to receive an ancillary\n\
+data item with associated data of the given length, along with any\n\
+trailing padding.  The buffer space needed to receive multiple items\n\
+is the sum of the CMSG_SPACE() values for their associated data\n\
+lengths.  Raises OverflowError if length is outside the permissible\n\
+range of values.");
+#endif    /* CMSG_SPACE */
+#endif    /* CMSG_LEN */
+
+
 /* List of functions exported by this module. */
 
 static PyMethodDef socket_methods[] = {
@@ -4440,6 +5191,14 @@ static PyMethodDef socket_methods[] = {
     {"if_indextoname", socket_if_indextoname,
      METH_O, if_indextoname_doc},
 #endif
+#ifdef CMSG_LEN
+    {"CMSG_LEN",                socket_CMSG_LEN,
+     METH_VARARGS, CMSG_LEN_doc},
+#ifdef CMSG_SPACE
+    {"CMSG_SPACE",              socket_CMSG_SPACE,
+     METH_VARARGS, CMSG_SPACE_doc},
+#endif
+#endif
     {NULL,                      NULL}            /* Sentinel */
 };
 
@@ -4927,6 +5686,15 @@ PyInit__socket(void)
 #ifdef  SO_SETFIB
     PyModule_AddIntConstant(m, "SO_SETFIB", SO_SETFIB);
 #endif
+#ifdef  SO_PASSCRED
+    PyModule_AddIntConstant(m, "SO_PASSCRED", SO_PASSCRED);
+#endif
+#ifdef  SO_PEERCRED
+    PyModule_AddIntConstant(m, "SO_PEERCRED", SO_PEERCRED);
+#endif
+#ifdef  LOCAL_PEERCRED
+    PyModule_AddIntConstant(m, "LOCAL_PEERCRED", LOCAL_PEERCRED);
+#endif
 
     /* Maximum number of connections for "listen" */
 #ifdef  SOMAXCONN
@@ -4935,6 +5703,17 @@ PyInit__socket(void)
     PyModule_AddIntConstant(m, "SOMAXCONN", 5); /* Common value */
 #endif
 
+    /* Ancilliary message types */
+#ifdef  SCM_RIGHTS
+    PyModule_AddIntConstant(m, "SCM_RIGHTS", SCM_RIGHTS);
+#endif
+#ifdef  SCM_CREDENTIALS
+    PyModule_AddIntConstant(m, "SCM_CREDENTIALS", SCM_CREDENTIALS);
+#endif
+#ifdef  SCM_CREDS
+    PyModule_AddIntConstant(m, "SCM_CREDS", SCM_CREDS);
+#endif
+
     /* Flags for send, recv */
 #ifdef  MSG_OOB
     PyModule_AddIntConstant(m, "MSG_OOB", MSG_OOB);
@@ -4966,6 +5745,33 @@ PyInit__socket(void)
 #ifdef  MSG_ETAG
     PyModule_AddIntConstant(m, "MSG_ETAG", MSG_ETAG);
 #endif
+#ifdef  MSG_NOSIGNAL
+    PyModule_AddIntConstant(m, "MSG_NOSIGNAL", MSG_NOSIGNAL);
+#endif
+#ifdef  MSG_NOTIFICATION
+    PyModule_AddIntConstant(m, "MSG_NOTIFICATION", MSG_NOTIFICATION);
+#endif
+#ifdef  MSG_CMSG_CLOEXEC
+    PyModule_AddIntConstant(m, "MSG_CMSG_CLOEXEC", MSG_CMSG_CLOEXEC);
+#endif
+#ifdef  MSG_ERRQUEUE
+    PyModule_AddIntConstant(m, "MSG_ERRQUEUE", MSG_ERRQUEUE);
+#endif
+#ifdef  MSG_CONFIRM
+    PyModule_AddIntConstant(m, "MSG_CONFIRM", MSG_CONFIRM);
+#endif
+#ifdef  MSG_MORE
+    PyModule_AddIntConstant(m, "MSG_MORE", MSG_MORE);
+#endif
+#ifdef  MSG_EOF
+    PyModule_AddIntConstant(m, "MSG_EOF", MSG_EOF);
+#endif
+#ifdef  MSG_BCAST
+    PyModule_AddIntConstant(m, "MSG_BCAST", MSG_BCAST);
+#endif
+#ifdef  MSG_MCAST
+    PyModule_AddIntConstant(m, "MSG_MCAST", MSG_MCAST);
+#endif
 
     /* Protocol level and numbers, usable for [gs]etsockopt */
 #ifdef  SOL_SOCKET
@@ -5105,6 +5911,9 @@ PyInit__socket(void)
 #ifdef  IPPROTO_VRRP
     PyModule_AddIntConstant(m, "IPPROTO_VRRP", IPPROTO_VRRP);
 #endif
+#ifdef  IPPROTO_SCTP
+    PyModule_AddIntConstant(m, "IPPROTO_SCTP", IPPROTO_SCTP);
+#endif
 #ifdef  IPPROTO_BIP
     PyModule_AddIntConstant(m, "IPPROTO_BIP", IPPROTO_BIP);
 #endif