WSPJoinLeaf function

The WSPJoinLeaf function joins a leaf node into a multipoint session, exchanges connect data, and specifies needed quality of service based on the supplied flow specifications.


  _In_  SOCKET                s,
  _In_  const struct sockaddr *name,
  _In_  int                   namelen,
  _In_  LPWSABUF              lpCallerData,
  _Out_ LPWSABUF              lpCalleeData,
  _In_  LPQOS                 lpSQOS,
  _In_  LPQOS                 lpGQOS,
  _In_  DWORD                 dwFlags,
  _Out_ LPINT                 lpErrno


s [in]

Descriptor identifying a multipoint socket.

name [in]

Name of the peer to which the socket in the sockaddr structure is to be joined.

namelen [in]

Length of the name, in bytes.

lpCallerData [in]

Pointer to the user data that is to be transferred to the peer during multipoint session establishment.

lpCalleeData [out]

Pointer to the user data that is to be transferred back from the peer during multipoint session establishment.

lpSQOS [in]

Pointer to the flow specifications for socket s, one for each direction.

lpGQOS [in]


dwFlags [in]

Flags to indicate that the socket is acting as a sender, receiver, or both.

lpErrno [out]

Pointer to the error code.

Return value

If no error occurs, WSPJoinLeaf returns a value of type SOCKET that is a descriptor for the newly created multipoint socket. Otherwise, a value of INVALID_SOCKET is returned, and a specific error code is available in lpErrno.

On a blocking socket, the return value indicates success or failure of the join operation.

With a nonblocking socket, successful initiation of a join operation is indicated by a return value of a valid socket descriptor. Subsequently, an FD_CONNECT indication is given when the join operation completes, either successfully or otherwise. The error code associated with the FD_CONNECT indicates the success or failure of the WSPJoinLeaf.

Also, until the multipoint session join attempt completes all subsequent calls to WSPJoinLeaf on the same socket will fail with the error code WSAEALREADY. After the WSPJoinLeaf completes successfully a subsequent attempt will usually fail with the error code WSAEISCONN. An exception to the WSAEISCONN rule occurs for a c_root socket that allows root-initiated joins. In such a case another join may be initiated after a prior WSPJoinLeaf completes.

If the return error code indicates the multipoint session join attempt failed (that is, WSAECONNREFUSED, WSAENETUNREACH, WSAETIMEDOUT) the Windows Sockets SPI client can call WSPJoinLeaf again for the same socket.

Error codeMeaning

The network subsystem has failed.


Socket's local address is already in use and the socket was not marked to allow address reuse with SO_REUSEADDR. This error usually occurs at the time of bind, but could be delayed until this function if the bind was to a partially wild-card address (involving ADDR_ANY) and if a specific address needs to be "committed" at the time of this function.


(Blocking) call was canceled through WSPCancelBlockingCall.


Blocking Windows Sockets call is in progress, or the service provider is still processing a callback function.


Nonblocking WSPJoinLeaf call is in progress on the specified socket.


Remote address is not a valid address (for example, ADDR_ANY).


Addresses in the specified family cannot be used with this socket.


The attempt to join was forcefully rejected.


The name or the namelen parameter is not a valid part of the user address space, the namelen parameter is too small, the buffer length for lpCalleeData, lpSQOS, and lpGQOS are too small, or the buffer length for lpCallerData is too large.


Socket is already member of the multipoint session.


The network cannot be reached from this host at this time.


No buffer space is available. The socket cannot be joined.


The descriptor is not a socket.


Flow specifications specified in lpSQOS cannot be satisfied.


The lpCallerData augment is not supported by the service provider.


An attempt to join timed out without establishing a multipoint session.




This function is used to join a leaf node to a multipoint session, and to perform a number of other ancillary operations that occur at session join time as well. If the socket, s, is unbound, unique values are assigned to the local association by the system, and the socket is marked as bound.

WSPJoinLeaf has the same parameters and semantics as WSPConnect except that it returns a socket descriptor (as in WSPAccept), and it has an additional dwFlags parameter. Only multipoint sockets created using WSPSocket with appropriate multipoint flags set can be used for input parameter s in this function. If the socket is in the nonblocking mode, the returned socket descriptor will not be usable until after a corresponding FD_CONNECT indication on the original socket s has been received, except that closesocket can be invoked on this new socket descriptor to cancel a pending join operation. A root node in a multipoint session can call WSPJoinLeaf one or more times in order to add a number of leaf nodes, however at most one multipoint connection request can be outstanding at a time. Refer to Protocol-Independent Multicast and Multipoint in the SPI for additional information.

For nonblocking sockets it is often not possible to complete the connection immediately. In such a case, this function returns an as-yet unusable socket descriptor and the operation proceeds. There is no error code such as WSAEWOULDBLOCK in this case, since the function has effectively returned a "successful start" indication. When the final outcome success or failure becomes known, it may be reported through WSPAsyncSelect or WSPEventSelect depending on how the client registers for notification on the original socket s. In either case, the notification is announced with FD_CONNECT and the error code associated with the FD_CONNECT indicates either success or a specific reason for failure. Note that WSPSelect cannot be used to detect completion notification for WSPJoinLeaf.

The socket descriptor returned by WSPJoinLeaf is different depending on whether the input socket descriptor, s, is a c_root or a c_leaf. When used with a c_root socket, the name parameter designates a particular leaf node to be added and the returned socket descriptor is a c_leaf socket corresponding to the newly added leaf node. (As is described in section Descriptor Allocation, when new socket descriptors are allocated IFS providers must call WPUModifyIFSHandle and non-IFS providers must call WPUCreateSocketHandle). The newly created socket has the same properties as s including asynchronous events registered with WSPAsyncSelect or with WSPEventSelect. It is not intended to be used for exchange of multipoint data, but rather is used to receive network event indications (for example, FD_CLOSE) for the connection that exists to the particular c_leaf. Some multipoint implementations can also allow this socket to be used for "side chats" between the root and an individual leaf node. An FD_CLOSE indication will be received for this socket if the corresponding leaf node calls WSPCloseSocket to drop out of the multipoint session. Symmetrically, invoking WSPCloseSocket on the c_leaf socket returned from WSPJoinLeaf will cause the socket in the corresponding leaf node to get FD_CLOSE notification.

When WSPJoinLeaf is invoked with a c_leaf socket, the name parameter contains the address of the root node (for a rooted control scheme) or an existing multipoint session (nonrooted control scheme), and the returned socket descriptor is the same as the input socket descriptor. In other words, a new socket descriptor is not allocated. In a rooted control scheme, the root application would put its c_root socket in the listening mode by calling WSPListen. The standard FD_ACCEPT notification will be delivered when the leaf node requests to join itself to the multipoint session. The root application uses the usual WSPAccept functions to admit the new leaf node. The value returned from WSPAccept is also a c_leaf socket descriptor just like those returned from WSPJoinLeaf. To accommodate multipoint schemes that allow both root-initiated and leaf-initiated joins, it is acceptable for a c_root socket that is already in listening mode to be used as an input to WSPJoinLeaf.

The Windows Sockets SPI client is responsible for allocating any memory space pointed to directly or indirectly by any of the parameters it specifies.

The lpCallerData is a value parameter that contains any user data that is to be sent along with the multipoint session join request. If lpCallerData is NULL, no user data will be passed to the peer. The lpCalleeData is a result parameter that will contain any user data passed back from the peer as part of the multipoint session establishment. lpCalleeData->len initially contains the length of the buffer allocated by the Windows Sockets SPI client and pointed to by lpCalleeData->buf. lpCalleeData->len will be set to zero if no user data has been passed back. The lpCalleeData information will be valid when the multipoint join operation is complete. For blocking sockets, this will be when the WSPJoinLeaf function returns. For nonblocking sockets, this will be after the FD_CONNECT notification has occurred on the original socket s. If lpCalleeData is NULL, no user data will be passed back. The exact format of the user data is specific to the address family to which the socket belongs and/or the applications involved.

At multipoint session establishment time, a Windows Sockets SPI client can use the lpSQOS parameters to override any previous QoS specification made for the socket through WSPIoctl with the SIO_SET_QOS opcode.

lpSQOS specifies the flow specifications for socket s, one for each direction, followed by any additional provider-specific parameters. If either the associated transport provider in general or the specific type of socket in particular cannot honor the QoS request, an error will be returned as indicated below. The sending or receiving flow specification values will be ignored, respectively, for any unidirectional sockets. If no provider-specific parameters are supplied, the buf and len members of lpSQOS->ProviderSpecific should be set to NULL and zero, respectively. A NULL value for lpSQOS indicates no application supplied quality of service.

The dwFlags parameter is used to indicate whether the socket will be acting only as a sender (JL_SENDER_ONLY), only as a receiver (JL_RECEIVER_ONLY), or both (JL_BOTH).

Note  When connected sockets break (that is, become closed for whatever reason), they should be discarded and recreated. It is safest to assume that when things go awry for any reason on a connected socket, the Windows Sockets SPI client must discard and recreate the needed sockets in order to return to a stable point.


Minimum supported client

Windows 2000 Professional [desktop apps only]

Minimum supported server

Windows 2000 Server [desktop apps only]



See also