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CAsyncSocket Class

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The latest version of this topic can be found at CAsyncSocket Class.

Represents a Windows Socket — an endpoint of network communication.

Syntax

class CAsyncSocket : public CObject

Members

Public Constructors

Name Description
CAsyncSocket::CAsyncSocket Constructs a CAsyncSocket object.

Public Methods

Name Description
CAsyncSocket::Accept Accepts a connection on the socket.
CAsyncSocket::AsyncSelect Requests event notification for the socket.
CAsyncSocket::Attach Attaches a socket handle to a CAsyncSocket object.
CAsyncSocket::Bind Associates a local address with the socket.
CAsyncSocket::Close Closes the socket.
CAsyncSocket::Connect Establishes a connection to a peer socket.
CAsyncSocket::Create Creates a socket.
CAsyncSocket::Detach Detaches a socket handle from a CAsyncSocket object.
CAsyncSocket::FromHandle Returns a pointer to a CAsyncSocket object, given a socket handle.
CAsyncSocket::GetLastError Gets the error status for the last operation that failed.
CAsyncSocket::GetPeerName Gets the address of the peer socket to which the socket is connected.
CAsyncSocket::GetPeerNameEx Gets the address of the peer socket to which the socket is connected (handles IPv6 addresses).
CAsyncSocket::GetSockName Gets the local name for a socket.
CAsyncSocket::GetSockNameEx Gets the local name for a socket (handles IPv6 addresses).
CAsyncSocket::GetSockOpt Retrieves a socket option.
CAsyncSocket::IOCtl Controls the mode of the socket.
CAsyncSocket::Listen Establishes a socket to listen for incoming connection requests.
CAsyncSocket::Receive Receives data from the socket.
CAsyncSocket::ReceiveFrom Receives a datagram and stores the source address.
CAsyncSocket::ReceiveFromEx Receives a datagram and stores the source address (handles IPv6 addresses).
CAsyncSocket::Send Sends data to a connected socket.
CAsyncSocket::SendTo Sends data to a specific destination.
CAsyncSocket::SendToEx Sends data to a specific destination (handles IPv6 addresses).
CAsyncSocket::SetSockOpt Sets a socket option.
CAsyncSocket::ShutDown Disables Send and/or Receive calls on the socket.
CASyncSocket::Socket Allocates a socket handle.

Protected Methods

Name Description
CAsyncSocket::OnAccept Notifies a listening socket that it can accept pending connection requests by calling Accept.
CAsyncSocket::OnClose Notifies a socket that the socket connected to it has closed.
CAsyncSocket::OnConnect Notifies a connecting socket that the connection attempt is complete, whether successfully or in error.
CAsyncSocket::OnOutOfBandData Notifies a receiving socket that there is out-of-band data to be read on the socket, usually an urgent message.
CAsyncSocket::OnReceive Notifies a listening socket that there is data to be retrieved by calling Receive.
CAsyncSocket::OnSend Notifies a socket that it can send data by calling Send.

Public Operators

Name Description
CAsyncSocket::operator = Assigns a new value to a CAsyncSocket object.
CAsyncSocket::operator SOCKET Use this operator to retrieve the SOCKET handle of the CAsyncSocket object.

Public Data Members

Name Description
CAsyncSocket::m_hSocket Indicates the SOCKET handle attached to this CAsyncSocket object.

Remarks

Class CAsyncSocket encapsulates the Windows Socket Functions API, providing an object-oriented abstraction for programmers who want to use Windows Sockets in conjunction with MFC.

This class is based on the assumption that you understand network communications. You are responsible for handling blocking, byte-order differences, and conversions between Unicode and multibyte character set (MBCS) strings. If you want a more convenient interface that manages these issues for you, see class CSocket.

To use a CAsyncSocket object, call its constructor, then call the Create function to create the underlying socket handle (type SOCKET), except on accepted sockets. For a server socket call the Listen member function, and for a client socket call the Connect member function. The server socket should call the Accept function upon receiving a connection request. Use the remaining CAsyncSocket functions to carry out communications between sockets. Upon completion, destroy the CAsyncSocket object if it was created on the heap; the destructor automatically calls the Close function. The SOCKET data type is described in the article Windows Sockets: Background.

Note

When using MFC sockets in secondary threads in a statically linked MFC application, you must call AfxSocketInit in each thread that uses sockets to initialize the socket libraries. By default, AfxSocketInit is called only in the primary thread.

For more information, see Windows Sockets: Using Class CAsyncSocket and related articles., as well as Windows Sockets 2 API.

Inheritance Hierarchy

CObject

CAsyncSocket

Requirements

Header: afxsock.h

CAsyncSocket::Accept

Call this member function to accept a connection on a socket.

virtual BOOL Accept(
    CAsyncSocket& rConnectedSocket,  
    SOCKADDR* lpSockAddr = NULL,  
    int* lpSockAddrLen = NULL);

Parameters

rConnectedSocket
A reference identifying a new socket that is available for connection.

lpSockAddr
A pointer to a SOCKADDR structure that receives the address of the connecting socket, as known on the network. The exact format of the lpSockAddr argument is determined by the address family established when the socket was created. If lpSockAddr and/or lpSockAddrLen are equal to NULL, then no information about the remote address of the accepted socket is returned.

lpSockAddrLen
A pointer to the length of the address in lpSockAddr in bytes. The lpSockAddrLen is a value-result parameter: it should initially contain the amount of space pointed to by lpSockAddr; on return it will contain the actual length (in bytes) of the address returned.

Return Value

Nonzero if the function is successful; otherwise 0, and a specific error code can be retrieved by calling GetLastError. The following errors apply to this member function:

  • WSANOTINITIALISED A successful AfxSocketInit must occur before using this API.

  • WSAENETDOWN The Windows Sockets implementation detected that the network subsystem failed.

  • WSAEFAULT The lpSockAddrLen argument is too small (less than the size of a SOCKADDR structure).

  • WSAEINPROGRESS A blocking Windows Sockets call is in progress.

  • WSAEINVAL Listen was not invoked prior to accept.

  • WSAEMFILE The queue is empty upon entry to accept and there are no descriptors available.

  • WSAENOBUFS No buffer space is available.

  • WSAENOTSOCK The descriptor is not a socket.

  • WSAEOPNOTSUPP The referenced socket is not a type that supports connection-oriented service.

  • WSAEWOULDBLOCK The socket is marked as nonblocking and no connections are present to be accepted.

Remarks

This routine extracts the first connection in the queue of pending connections, creates a new socket with the same properties as this socket, and attaches it to rConnectedSocket. If no pending connections are present on the queue, Accept returns zero and GetLastError returns an error. The accepted socket ( rConnectedSocket) cannot be used to accept more connections. The original socket remains open and listening.

The argument lpSockAddr is a result parameter that is filled in with the address of the connecting socket, as known to the communications layer. Accept is used with connection-based socket types such as SOCK_STREAM.

CAsyncSocket::AsyncSelect

Call this member function to request event notification for a socket.

BOOL AsyncSelect(long lEvent = FD_READ | FD_WRITE | FD_OOB | FD_ACCEPT | FD_CONNECT | FD_CLOSE);

Parameters

lEvent
A bitmask which specifies a combination of network events in which the application is interested.

  • FD_READ Want to receive notification of readiness for reading.

  • FD_WRITE Want to receive notification when data is available to be read.

  • FD_OOB Want to receive notification of the arrival of out-of-band data.

  • FD_ACCEPT Want to receive notification of incoming connections.

  • FD_CONNECT Want to receive notification of connection results.

  • FD_CLOSE Want to receive notification when a socket has been closed by a peer.

Return Value

Nonzero if the function is successful; otherwise 0, and a specific error code can be retrieved by calling GetLastError. The following errors apply to this member function:

  • WSANOTINITIALISED A successful AfxSocketInit must occur before using this API.

  • WSAENETDOWN The Windows Sockets implementation detected that the network subsystem failed.

  • WSAEINVAL Indicates that one of the specified parameters was invalid.

  • WSAEINPROGRESS A blocking Windows Sockets operation is in progress.

Remarks

This function is used to specify which MFC callback notification functions will be called for the socket. AsyncSelect automatically sets this socket to nonblocking mode. For more information, see the article Windows Sockets: Socket Notifications.

CAsyncSocket::Attach

Call this member function to attach the hSocket handle to an CAsyncSocket object.

BOOL Attach(
    SOCKET hSocket, long lEvent = FD_READ | FD_WRITE | FD_OOB | FD_ACCEPT | FD_CONNECT | FD_CLOSE);

Parameters

hSocket
Contains a handle to a socket.

lEvent
A bitmask which specifies a combination of network events in which the application is interested.

  • FD_READ Want to receive notification of readiness for reading.

  • FD_WRITE Want to receive notification when data is available to be read.

  • FD_OOB Want to receive notification of the arrival of out-of-band data.

  • FD_ACCEPT Want to receive notification of incoming connections.

  • FD_CONNECT Want to receive notification of connection results.

  • FD_CLOSE Want to receive notification when a socket has been closed by a peer.

Return Value

Nonzero if the function is successful.

Remarks

The SOCKET handle is stored in the object's m_hSocket data member.

CAsyncSocket::Bind

Call this member function to associate a local address with the socket.

BOOL Bind(
    UINT nSocketPort,  
    LPCTSTR lpszSocketAddress = NULL);

 
BOOL Bind (
    const SOCKADDR* lpSockAddr,  
    int nSockAddrLen);

Parameters

nSocketPort
The port identifying the socket application.

lpszSocketAddress
The network address, a dotted number such as "128.56.22.8". Passing the NULL string for this parameter indicates the CAsyncSocket instance should listen for client activity on all network interfaces.

lpSockAddr
A pointer to a SOCKADDR structure that contains the address to assign to this socket.

nSockAddrLen
The length of the address in lpSockAddr in bytes.

Return Value

Nonzero if the function is successful; otherwise 0, and a specific error code can be retrieved by calling GetLastError. The following errors apply to this member function:

  • WSANOTINITIALISED A successful AfxSocketInit must occur before using this API.

  • WSAENETDOWN The Windows Sockets implementation detected that the network subsystem failed.

  • WSAEADDRINUSE The specified address is already in use. (See the SO_REUSEADDR socket option under SetSockOpt.)

  • WSAEFAULT The nSockAddrLen argument is too small (less than the size of a SOCKADDR structure).

  • WSAEINPROGRESS A blocking Windows Sockets call is in progress.

  • WSAEAFNOSUPPORT The specified address family is not supported by this port.

  • WSAEINVAL The socket is already bound to an address.

  • WSAENOBUFS Not enough buffers available, too many connections.

  • WSAENOTSOCK The descriptor is not a socket.

Remarks

This routine is used on an unconnected datagram or stream socket, before subsequent Connect or Listen calls. Before it can accept connection requests, a listening server socket must select a port number and make it known to Windows Sockets by calling Bind. Bind establishes the local association (host address/port number) of the socket by assigning a local name to an unnamed socket.

CAsyncSocket::CAsyncSocket

Constructs a blank socket object.

CAsyncSocket();

Remarks

After constructing the object, you must call its Create member function to create the SOCKET data structure and bind its address. (On the server side of a Windows Sockets communication, when the listening socket creates a socket to use in the Accept call, you do not call Create for that socket.)

CAsyncSocket::Close

Closes the socket.

virtual void Close();

Remarks

This function releases the socket descriptor so that further references to it will fail with the error WSAENOTSOCK. If this is the last reference to the underlying socket, the associated naming information and queued data are discarded. The socket object's destructor calls Close for you.

For CAsyncSocket, but not for CSocket, the semantics of Close are affected by the socket options SO_LINGER and SO_DONTLINGER. For further information, see member function GetSockOpt.

CAsyncSocket::Connect

Call this member function to establish a connection to an unconnected stream or datagram socket.

BOOL Connect(
    LPCTSTR lpszHostAddress,  
    UINT nHostPort);

 
BOOL Connect(
    const SOCKADDR* lpSockAddr,  
    int nSockAddrLen);

Parameters

lpszHostAddress
The network address of the socket to which this object is connected: a machine name such as "ftp.microsoft.com", or a dotted number such as "128.56.22.8".

nHostPort
The port identifying the socket application.

lpSockAddr
A pointer to a SOCKADDR structure that contains the address of the connected socket.

nSockAddrLen
The length of the address in lpSockAddr in bytes.

Return Value

Nonzero if the function is successful; otherwise 0, and a specific error code can be retrieved by calling GetLastError. If this indicates an error code of WSAEWOULDBLOCK, and your application is using the overridable callbacks, your application will receive an OnConnect message when the connect operation is complete. The following errors apply to this member function:

  • WSANOTINITIALISED A successful AfxSocketInit must occur before using this API.

  • WSAENETDOWN The Windows Sockets implementation detected that the network subsystem failed.

  • WSAEADDRINUSE The specified address is already in use.

  • WSAEINPROGRESS A blocking Windows Sockets call is in progress.

  • WSAEADDRNOTAVAIL The specified address is not available from the local machine.

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

  • WSAECONNREFUSED The attempt to connect was rejected.

  • WSAEDESTADDRREQ A destination address is required.

  • WSAEFAULT The nSockAddrLen argument is incorrect.

  • WSAEINVAL Invalid host address.

  • WSAEISCONN The socket is already connected.

  • WSAEMFILE No more file descriptors are available.

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

  • WSAENOBUFS No buffer space is available. The socket cannot be connected.

  • WSAENOTSOCK The descriptor is not a socket.

  • WSAETIMEDOUT Attempt to connect timed out without establishing a connection.

  • WSAEWOULDBLOCK The socket is marked as nonblocking and the connection cannot be completed immediately.

Remarks

If the socket is unbound, unique values are assigned to the local association by the system, and the socket is marked as bound. Note that if the address field of the name structure is all zeroes, Connect will return zero. To get extended error information, call the GetLastError member function.

For stream sockets (type SOCK_STREAM), an active connection is initiated to the foreign host. When the socket call completes successfully, the socket is ready to send/receive data.

For a datagram socket (type SOCK_DGRAM), a default destination is set, which will be used on subsequent Send and Receive calls.

CAsyncSocket::Create

Call the Create member function after constructing a socket object to create the Windows socket and attach it.

BOOL Create(
    UINT nSocketPort = 0,  
    int nSocketType = SOCK_STREAM,  
    long lEvent = FD_READ | FD_WRITE | FD_OOB | FD_ACCEPT | FD_CONNECT | FD_CLOSE,  
    LPCTSTR lpszSocketAddress = NULL);

Parameters

nSocketPort
A well-known port to be used with the socket, or 0 if you want Windows Sockets to select a port.

nSocketType
SOCK_STREAM or SOCK_DGRAM.

lEvent
A bitmask which specifies a combination of network events in which the application is interested.

  • FD_READ Want to receive notification of readiness for reading.

  • FD_WRITE Want to receive notification of readiness for writing.

  • FD_OOB Want to receive notification of the arrival of out-of-band data.

  • FD_ACCEPT Want to receive notification of incoming connections.

  • FD_CONNECT Want to receive notification of completed connection.

  • FD_CLOSE Want to receive notification of socket closure.

lpszSockAddress
A pointer to a string containing the network address of the connected socket, a dotted number such as "128.56.22.8".Passing the NULL string for this parameter indicates the CAsyncSocket instance should listen for client activity on all network interfaces.

Return Value

Nonzero if the function is successful; otherwise 0, and a specific error code can be retrieved by calling GetLastError. The following errors apply to this member function:

  • WSANOTINITIALISED A successful AfxSocketInit must occur before using this API.

  • WSAENETDOWN The Windows Sockets implementation detected that the network subsystem failed.

  • WSAEAFNOSUPPORT The specified address family is not supported.

  • WSAEINPROGRESS A blocking Windows Sockets operation is in progress.

  • WSAEMFILE No more file descriptors are available.

  • WSAENOBUFS No buffer space is available. The socket cannot be created.

  • WSAEPROTONOSUPPORT The specified port is not supported.

  • WSAEPROTOTYPE The specified port is the wrong type for this socket.

  • WSAESOCKTNOSUPPORT The specified socket type is not supported in this address family.

Remarks

Create calls Socket and if successful, it calls Bind to bind the socket to the specified address. The following socket types are supported:

  • SOCK_STREAM Provides sequenced, reliable, full-duplex, connection-based byte streams. Uses the Transmission Control Protocol (TCP) for the Internet address family.

  • SOCK_DGRAM Supports datagrams, which are connectionless, unreliable packets of a fixed (typically small) maximum length. Uses the User Datagram Protocol (UDP) for the Internet address family.

    Note

    The Accept member function takes a reference to a new, empty CSocket object as its parameter. You must construct this object before you call Accept. Keep in mind that if this socket object goes out of scope, the connection closes. Do not call Create for this new socket object.

Important

Create is not thread-safe. If you are calling it in a multi-threaded environment where it could be invoked simultaneously by different threads, be sure to protect each call with a mutex or other synchronization lock.

For more information about stream and datagram sockets, see the articles Windows Sockets: Background and Windows Sockets: Ports and Socket Addresses and Windows Sockets 2 API.

CAsyncSocket::Detach

Call this member function to detach the SOCKET handle in the m_hSocket data member from the CAsyncSocket object and set m_hSocket to NULL.

SOCKET Detach();

CAsyncSocket::FromHandle

Returns a pointer to a CAsyncSocket object.

static CAsyncSocket* PASCAL FromHandle(SOCKET hSocket);

Parameters

hSocket
Contains a handle to a socket.

Return Value

A pointer to an CAsyncSocket object, or NULL if there is no CAsyncSocket object attached to hSocket.

Remarks

When given a SOCKET handle, if a CAsyncSocket object is not attached to the handle, the member function returns NULL.

CAsyncSocket::GetLastError

Call this member function to get the error status for the last operation that failed.

static int PASCAL GetLastError();

Return Value

The return value indicates the error code for the last Windows Sockets API routine performed by this thread.

Remarks

When a particular member function indicates that an error has occurred, GetLastError should be called to retrieve the appropriate error code. See the individual member function descriptions for a list of applicable error codes.

For more information about the error codes, see Windows Sockets 2 API.

CAsyncSocket::GetPeerName

Call this member function to get the address of the peer socket to which this socket is connected.

BOOL GetPeerName(
    CString& rPeerAddress,  
    UINT& rPeerPort);

 
BOOL GetPeerName(
    SOCKADDR* lpSockAddr,  
    int* lpSockAddrLen);

Parameters

rPeerAddress
Reference to a CString object that receives a dotted number IP address.

rPeerPort
Reference to a UINT that stores a port.

lpSockAddr
A pointer to the SOCKADDR structure that receives the name of the peer socket.

lpSockAddrLen
A pointer to the length of the address in lpSockAddr in bytes. On return, the lpSockAddrLen argument contains the actual size of lpSockAddr returned in bytes.

Return Value

Nonzero if the function is successful; otherwise 0, and a specific error code can be retrieved by calling GetLastError. The following errors apply to this member function:

  • WSANOTINITIALISED A successful AfxSocketInit must occur before using this API.

  • WSAENETDOWN The Windows Sockets implementation detected that the network subsystem failed.

  • WSAEFAULT The lpSockAddrLen argument is not large enough.

  • WSAEINPROGRESS A blocking Windows Sockets call is in progress.

  • WSAENOTCONN The socket is not connected.

  • WSAENOTSOCK The descriptor is not a socket.

Remarks

To handle IPv6 addresses, use CAsyncSocket::GetPeerNameEx.

CAsyncSocket::GetPeerNameEx

Call this member function to get the address of the peer socket to which this socket is connected (handles IPv6 addresses).

BOOL GetPeerNameEx(
    CString& rPeerAddress,  
    UINT& rPeerPort);

Parameters

rPeerAddress
Reference to a CString object that receives a dotted number IP address.

rPeerPort
Reference to a UINT that stores a port.

Return Value

Nonzero if the function is successful; otherwise 0, and a specific error code can be retrieved by calling GetLastError. The following errors apply to this member function:

  • WSANOTINITIALISED A successful AfxSocketInit must occur before using this API.

  • WSAENETDOWN The Windows Sockets implementation detected that the network subsystem failed.

  • WSAEFAULT The lpSockAddrLen argument is not large enough.

  • WSAEINPROGRESS A blocking Windows Sockets call is in progress.

  • WSAENOTCONN The socket is not connected.

  • WSAENOTSOCK The descriptor is not a socket.

Remarks

This function is the same as CAsyncSocket::GetPeerName except that it handles IPv6 addresses as well as older protocols.

CAsyncSocket::GetSockName

Call this member function to get the local name for a socket.

BOOL GetSockName(
    CString& rSocketAddress,  
    UINT& rSocketPort);

 
BOOL GetSockName(
    SOCKADDR* lpSockAddr,  
    int* lpSockAddrLen);

Parameters

rSocketAddress
Reference to a CString object that receives a dotted number IP address.

rSocketPort
Reference to a UINT that stores a port.

lpSockAddr
A pointer to a SOCKADDR structure that receives the address of the socket.

lpSockAddrLen
A pointer to the length of the address in lpSockAddr in bytes.

Return Value

Nonzero if the function is successful; otherwise 0, and a specific error code can be retrieved by calling GetLastError. The following errors apply to this member function:

  • WSANOTINITIALISED A successful AfxSocketInit must occur before using this API.

  • WSAENETDOWN The Windows Sockets implementation detected that the network subsystem failed.

  • WSAEFAULT The lpSockAddrLen argument is not large enough.

  • WSAEINPROGRESS A blocking Windows Sockets operation is in progress.

  • WSAENOTSOCK The descriptor is not a socket.

  • WSAEINVAL The socket has not been bound to an address with Bind.

Remarks

This call is especially useful when a Connect call has been made without doing a Bind first; this call provides the only means by which you can determine the local association which has been set by the system.

To handle IPv6 addresses, use CAsyncSocket::GetSockNameEx

CAsyncSocket::GetSockNameEx

Call this member function to get the local name for a socket (handles IPv6 addresses).

BOOL GetSockNameEx(
    CString& rSocketAddress,  
    UINT& rSocketPort);

Parameters

rSocketAddress
Reference to a CString object that receives a dotted number IP address.

rSocketPort
Reference to a UINT that stores a port.

Return Value

Nonzero if the function is successful; otherwise 0, and a specific error code can be retrieved by calling GetLastError. The following errors apply to this member function:

  • WSANOTINITIALISED A successful AfxSocketInit must occur before using this API.

  • WSAENETDOWN The Windows Sockets implementation detected that the network subsystem failed.

  • WSAEFAULT The lpSockAddrLen argument is not large enough.

  • WSAEINPROGRESS A blocking Windows Sockets operation is in progress.

  • WSAENOTSOCK The descriptor is not a socket.

  • WSAEINVAL The socket has not been bound to an address with Bind.

Remarks

This call is the same as CAsyncSocket::GetSockName except that it handles IPv6 addresses as well as older protocols.

This call is especially useful when a Connect call has been made without doing a Bind first; this call provides the only means by which you can determine the local association which has been set by the system.

CAsyncSocket::GetSockOpt

Call this member function to retrieve a socket option.

BOOL GetSockOpt(
    int nOptionName,  
    void* lpOptionValue,  
    int* lpOptionLen,  
    int nLevel = SOL_SOCKET);

Parameters

nOptionName
The socket option for which the value is to be retrieved.

lpOptionValue
A pointer to the buffer in which the value for the requested option is to be returned. The value associated with the selected option is returned in the buffer lpOptionValue. The integer pointed to by lpOptionLen should originally contain the size of this buffer in bytes; and on return, it will be set to the size of the value returned. For SO_LINGER, this will be the size of a LINGER structure; for all other options it will be the size of a BOOL or an int, depending on the option. See the list of options and their sizes in the Remarks section.

lpOptionLen
A pointer to the size of the lpOptionValue buffer in bytes.

nLevel
The level at which the option is defined; the only supported levels are SOL_SOCKET and IPPROTO_TCP.

Return Value

Nonzero if the function is successful; otherwise 0, and a specific error code can be retrieved by calling GetLastError. If an option was never set with SetSockOpt, then GetSockOpt returns the default value for the option. The following errors apply to this member function:

  • WSANOTINITIALISED A successful AfxSocketInit must occur before using this API.

  • WSAENETDOWN The Windows Sockets implementation detected that the network subsystem failed.

  • WSAEFAULT The lpOptionLen argument was invalid.

  • WSAEINPROGRESS A blocking Windows Sockets operation is in progress.

  • WSAENOPROTOOPT The option is unknown or unsupported. In particular, SO_BROADCAST is not supported on sockets of type SOCK_STREAM, while SO_ACCEPTCONN, SO_DONTLINGER, SO_KEEPALIVE, SO_LINGER, and SO_OOBINLINE are not supported on sockets of type SOCK_DGRAM.

  • WSAENOTSOCK The descriptor is not a socket.

Remarks

GetSockOpt retrieves the current value for a socket option associated with a socket of any type, in any state, and stores the result in lpOptionValue. Options affect socket operations, such as the routing of packets, out-of-band data transfer, and so on.

The following options are supported for GetSockOpt. The Type identifies the type of data addressed by lpOptionValue. The TCP_NODELAY option uses level IPPROTO_TCP; all other options use level SOL_SOCKET.

Value Type Meaning
SO_ACCEPTCONN BOOL Socket is listening.
SO_BROADCAST BOOL Socket is configured for the transmission of broadcast messages.
SO_DEBUG BOOL Debugging is enabled.
SO_DONTLINGER BOOL If true, the SO_LINGER option is disabled.
SO_DONTROUTE BOOL Routing is disabled.
SO_ERROR int Retrieve error status and clear.
SO_KEEPALIVE BOOL Keep-alives are being sent.
SO_LINGER struct LINGER Returns the current linger options.
SO_OOBINLINE BOOL Out-of-band data is being received in the normal data stream.
SO_RCVBUF int Buffer size for receives.
SO_REUSEADDR BOOL The socket can be bound to an address which is already in use.
SO_SNDBUF int Buffer size for sends.
SO_TYPE int The type of the socket (for example, SOCK_STREAM).
TCP_NODELAY BOOL Disables the Nagle algorithm for send coalescing.

Berkeley Software Distribution (BSD) options not supported for GetSockOpt are:

Value Type Meaning
SO_RCVLOWAT int Receive low water mark.
SO_RCVTIMEO int Receive timeout.
SO_SNDLOWAT int Send low water mark.
SO_SNDTIMEO int Send timeout.
IP_OPTIONS Get options in IP header.
TCP_MAXSEG int Get TCP maximum segment size.

Calling GetSockOpt with an unsupported option will result in an error code of WSAENOPROTOOPT being returned from GetLastError.

CAsyncSocket::IOCtl

Call this member function to control the mode of a socket.

BOOL IOCtl(
    long lCommand,  
    DWORD* lpArgument);

Parameters

lCommand
The command to perform on the socket.

lpArgument
A pointer to a parameter for lCommand.

Return Value

Nonzero if the function is successful; otherwise 0, and a specific error code can be retrieved by calling GetLastError. The following errors apply to this member function:

  • WSANOTINITIALISED A successful AfxSocketInit must occur before using this API.

  • WSAENETDOWN The Windows Sockets implementation detected that the network subsystem failed.

  • WSAEINVAL lCommand is not a valid command, or lpArgument is not an acceptable parameter for lCommand, or the command is not applicable to the type of socket supplied.

  • WSAEINPROGRESS A blocking Windows Sockets operation is in progress.

  • WSAENOTSOCK The descriptor is not a socket.

Remarks

This routine can be used on any socket in any state. It is used to get or retrieve operating parameters associated with the socket, independent of the protocol and communications subsystem. The following commands are supported:

  • FIONBIO Enable or disable nonblocking mode on the socket. The lpArgument parameter points at a DWORD, which is nonzero if nonblocking mode is to be enabled and zero if it is to be disabled. If AsyncSelect has been issued on a socket, then any attempt to use IOCtl to set the socket back to blocking mode will fail with WSAEINVAL. To set the socket back to blocking mode and prevent the WSAEINVAL error, an application must first disable AsyncSelect by calling AsyncSelect with the lEvent parameter equal to 0, then call IOCtl.

  • FIONREAD Determine the maximum number of bytes that can be read with one Receive call from this socket. The lpArgument parameter points at a DWORD in which IOCtl stores the result. If this socket is of type SOCK_STREAM, FIONREAD returns the total amount of data which can be read in a single Receive; this is normally the same as the total amount of data queued on the socket. If this socket is of type SOCK_DGRAM, FIONREAD returns the size of the first datagram queued on the socket.

  • SIOCATMARK Determine whether all out-of-band data has been read. This applies only to a socket of type SOCK_STREAM which has been configured for in-line reception of any out-of-band data ( SO_OOBINLINE). If no out-of-band data is waiting to be read, the operation returns nonzero. Otherwise it returns 0, and the next Receive or ReceiveFrom performed on the socket will retrieve some or all of the data preceding the "mark"; the application should use the SIOCATMARK operation to determine whether any data remains. If there is any normal data preceding the "urgent" (out-of-band) data, it will be received in order. (Note that a Receive or ReceiveFrom will never mix out-of-band and normal data in the same call.) The lpArgument parameter points at a DWORD in which IOCtl stores the result.

This function is a subset of ioctl() as used in Berkeley sockets. In particular, there is no command which is equivalent to FIOASYNC, while SIOCATMARK is the only socket-level command which is supported.

CAsyncSocket::Listen

Call this member function to listen for incoming connection requests.

BOOL Listen(int nConnectionBacklog = 5);

Parameters

nConnectionBacklog
The maximum length to which the queue of pending connections can grow. Valid range is from 1 to 5.

Return Value

Nonzero if the function is successful; otherwise 0, and a specific error code can be retrieved by calling GetLastError. The following errors apply to this member function:

  • WSANOTINITIALISED A successful AfxSocketInit must occur before using this API.

  • WSAENETDOWN The Windows Sockets implementation detected that the network subsystem failed.

  • WSAEADDRINUSE An attempt has been made to listen on an address in use.

  • WSAEINPROGRESS A blocking Windows Sockets operation is in progress.

  • WSAEINVAL The socket has not been bound with Bind or is already connected.

  • WSAEISCONN The socket is already connected.

  • WSAEMFILE No more file descriptors are available.

  • WSAENOBUFS No buffer space is available.

  • WSAENOTSOCK The descriptor is not a socket.

  • WSAEOPNOTSUPP The referenced socket is not of a type that supports the Listen operation.

Remarks

To accept connections, the socket is first created with Create, a backlog for incoming connections is specified with Listen, and then the connections are accepted with Accept. Listen applies only to sockets that support connections, that is, those of type SOCK_STREAM. This socket is put into "passive" mode where incoming connections are acknowledged and queued pending acceptance by the process.

This function is typically used by servers (or any application that wants to accept connections) that could have more than one connection request at a time: if a connection request arrives with the queue full, the client will receive an error with an indication of WSAECONNREFUSED.

Listen attempts to continue to function rationally when there are no available ports (descriptors). It will accept connections until the queue is emptied. If ports become available, a later call to Listen or Accept will refill the queue to the current or most recent "backlog," if possible, and resume listening for incoming connections.

CAsyncSocket::m_hSocket

Contains the SOCKET handle for the socket encapsulated by this CAsyncSocket object.

SOCKET m_hSocket;

CAsyncSocket::OnAccept

Called by the framework to notify a listening socket that it can accept pending connection requests by calling the Accept member function.

virtual void OnAccept(int nErrorCode);

Parameters

nErrorCode
The most recent error on a socket. The following error codes applies to the OnAccept member function:

  • 0 The function executed successfully.

  • WSAENETDOWN The Windows Sockets implementation detected that the network subsystem failed.

Remarks

For more information, see Windows Sockets: Socket Notifications.

CAsyncSocket::OnClose

Called by the framework to notify this socket that the connected socket is closed by its process.

virtual void OnClose(int nErrorCode);

Parameters

nErrorCode
The most recent error on a socket. The following error codes apply to the OnClose member function:

  • 0 The function executed successfully.

  • WSAENETDOWN The Windows Sockets implementation detected that the network subsystem failed.

  • WSAECONNRESET The connection was reset by the remote side.

  • WSAECONNABORTED The connection was aborted due to timeout or other failure.

Remarks

For more information, see Windows Sockets: Socket Notifications.

CAsyncSocket::OnConnect

Called by the framework to notify this connecting socket that its connection attempt is completed, whether successfully or in error.

virtual void OnConnect(int nErrorCode);

Parameters

nErrorCode
The most recent error on a socket. The following error codes apply to the OnConnect member function:

  • 0 The function executed successfully.

  • WSAEADDRINUSE The specified address is already in use.

  • WSAEADDRNOTAVAIL The specified address is not available from the local machine.

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

  • WSAECONNREFUSED The attempt to connect was forcefully rejected.

  • WSAEDESTADDRREQ A destination address is required.

  • WSAEFAULT The lpSockAddrLen argument is incorrect.

  • WSAEINVAL The socket is already bound to an address.

  • WSAEISCONN The socket is already connected.

  • WSAEMFILE No more file descriptors are available.

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

  • WSAENOBUFS No buffer space is available. The socket cannot be connected.

  • WSAENOTCONN The socket is not connected.

  • WSAENOTSOCK The descriptor is a file, not a socket.

  • WSAETIMEDOUT The attempt to connect timed out without establishing a connection.

Remarks

Note

In CSocket, the OnConnect notification function is never called. For connections, you simply call Connect, which will return when the connection is completed (either successfully or in error). How connection notifications are handled is an MFC implementation detail.

For more information, see Windows Sockets: Socket Notifications.

Example

void CMyAsyncSocket::OnConnect(int nErrorCode)   // CMyAsyncSocket is 
                                                 // derived from CAsyncSocket
{
   if (0 != nErrorCode)
   {
      switch(nErrorCode)
      {
         case WSAEADDRINUSE: 
            AfxMessageBox(_T("The specified address is already in use.\n"));
            break;
         case WSAEADDRNOTAVAIL: 
            AfxMessageBox(_T("The specified address is not available from ")
            _T("the local machine.\n"));
            break;
         case WSAEAFNOSUPPORT: 
            AfxMessageBox(_T("Addresses in the specified family cannot be ")
            _T("used with this socket.\n"));
            break;
         case WSAECONNREFUSED: 
            AfxMessageBox(_T("The attempt to connect was forcefully rejected.\n"));
            break;
         case WSAEDESTADDRREQ: 
            AfxMessageBox(_T("A destination address is required.\n"));
            break;
         case WSAEFAULT: 
            AfxMessageBox(_T("The lpSockAddrLen argument is incorrect.\n"));
            break;
         case WSAEINVAL: 
            AfxMessageBox(_T("The socket is already bound to an address.\n"));
            break;
         case WSAEISCONN: 
            AfxMessageBox(_T("The socket is already connected.\n"));
            break;
         case WSAEMFILE: 
            AfxMessageBox(_T("No more file descriptors are available.\n"));
            break;
         case WSAENETUNREACH: 
            AfxMessageBox(_T("The network cannot be reached from this host ")
            _T("at this time.\n"));
            break;
         case WSAENOBUFS: 
            AfxMessageBox(_T("No buffer space is available. The socket ")
               _T("cannot be connected.\n"));
            break;
         case WSAENOTCONN: 
            AfxMessageBox(_T("The socket is not connected.\n"));
            break;
         case WSAENOTSOCK: 
            AfxMessageBox(_T("The descriptor is a file, not a socket.\n"));
            break;
         case WSAETIMEDOUT: 
            AfxMessageBox(_T("The attempt to connect timed out without ")
               _T("establishing a connection. \n"));
            break;
         default:
            TCHAR szError[256];
            _stprintf_s(szError, _T("OnConnect error: %d"), nErrorCode);
            AfxMessageBox(szError);
            break;
      }
      AfxMessageBox(_T("Please close the application"));
   }
   CAsyncSocket::OnConnect(nErrorCode);
}

CAsyncSocket::OnOutOfBandData

Called by the framework to notify the receiving socket that the sending socket has out-of-band data to send.

virtual void OnOutOfBandData(int nErrorCode);

Parameters

nErrorCode
The most recent error on a socket. The following error codes apply to the OnOutOfBandData member function:

  • 0 The function executed successfully.

  • WSAENETDOWN The Windows Sockets implementation detected that the network subsystem failed.

Remarks

Out-of-band data is a logically independent channel that is associated with each pair of connected sockets of type SOCK_STREAM. The channel is generally used to send urgent data.

MFC supports out-of-band data, but users of class CAsyncSocket are discouraged from using it. The easier way is to create a second socket for passing such data. For more information about out-of-band data, see Windows Sockets: Socket Notifications.

CAsyncSocket::OnReceive

Called by the framework to notify this socket that there is data in the buffer that can be retrieved by calling the Receive member function.

virtual void OnReceive(int nErrorCode);

Parameters

nErrorCode
The most recent error on a socket. The following error codes apply to the OnReceive member function:

  • 0 The function executed successfully.

  • WSAENETDOWN The Windows Sockets implementation detected that the network subsystem failed.

Remarks

For more information, see Windows Sockets: Socket Notifications.

Example

void CMyAsyncSocket::OnReceive(int nErrorCode)   // CMyAsyncSocket is 
                                                // derived from CAsyncSocket
{
   static int i = 0;

   i++;

   TCHAR buff[4096];
   int nRead;
   nRead = Receive(buff, 4096); 
   
   switch (nRead)
   {
      case 0:
        Close();
        break;
      case SOCKET_ERROR:
        if (GetLastError() != WSAEWOULDBLOCK) 
        {
          AfxMessageBox (_T("Error occurred"));
          Close();
        }
        break;
      default:
        buff[nRead] = _T('\0'); //terminate the string
        CString szTemp(buff);
        m_strRecv += szTemp;   // m_strRecv is a CString declared 
                        // in CMyAsyncSocket
        if (szTemp.CompareNoCase(_T("bye")) == 0)
        {
           ShutDown();
           s_eventDone.SetEvent();
        }
   }
   CAsyncSocket::OnReceive(nErrorCode);
}

CAsyncSocket::OnSend

Called by the framework to notify the socket that it can now send data by calling the Send member function.

virtual void OnSend(int nErrorCode);

Parameters

nErrorCode
The most recent error on a socket. The following error codes apply to the OnSend member function:

  • 0 The function executed successfully.

  • WSAENETDOWN The Windows Sockets implementation detected that the network subsystem failed.

Remarks

For more information, see Windows Sockets: Socket Notifications.

Example

// CMyAsyncSocket is derived from CAsyncSocket and defines the 
// following variables:
//    CString  m_sendBuffer;   //for async send
//    int      m_nBytesSent;
//    int      m_nBytesBufferSize;
void CMyAsyncSocket::OnSend(int nErrorCode)
{
   while (m_nBytesSent < m_nBytesBufferSize)
   {
      int dwBytes;

      if ((dwBytes = Send((LPCTSTR)m_sendBuffer + m_nBytesSent, 
         m_nBytesBufferSize - m_nBytesSent)) == SOCKET_ERROR)
      {
         if (GetLastError() == WSAEWOULDBLOCK)
       {
          break;
       }
         else
         {
            TCHAR szError[256];
            _stprintf_s(szError, _T("Server Socket failed to send: %d"), 
               GetLastError());
            Close();
            AfxMessageBox (szError);
         }
      }
      else
      {
         m_nBytesSent += dwBytes;
      }
   }

   if (m_nBytesSent == m_nBytesBufferSize)
   {
      m_nBytesSent = m_nBytesBufferSize = 0;
      m_sendBuffer = _T("");
   }

   CAsyncSocket::OnSend(nErrorCode);
}

CAsyncSocket::operator =

Assigns a new value to a CAsyncSocket object.

void operator=(const CAsyncSocket& rSrc);

Parameters

rSrc
A reference to an existing CAsyncSocket object.

Remarks

Call this function to copy an existing CAsyncSocket object to another CAsyncSocket object.

CAsyncSocket::operator SOCKET

Use this operator to retrieve the SOCKET handle of the CAsyncSocket object.

operator SOCKET() const;

Return Value

If successful, the handle of the SOCKET object; otherwise, NULL.

Remarks

You can use the handle to call Windows APIs directly.

CAsyncSocket::Receive

Call this member function to receive data from a socket.

virtual int Receive(
    void* lpBuf,  
    int nBufLen,  
    int nFlags = 0);

Parameters

lpBuf
A buffer for the incoming data.

nBufLen
The length of lpBuf in bytes.

nFlags
Specifies the way in which the call is made. The semantics of this function are determined by the socket options and the nFlags parameter. The latter is constructed by combining any of the following values with the C++ OR operator:

  • MSG_PEEK Peek at the incoming data. The data is copied into the buffer but is not removed from the input queue.

  • MSG_OOB Process out-of-band data.

Return Value

If no error occurs, Receive returns the number of bytes received. If the connection has been closed, it returns 0. Otherwise, a value of SOCKET_ERROR is returned, and a specific error code can be retrieved by calling GetLastError. The following errors apply to this member function:

  • WSANOTINITIALISED A successful AfxSocketInit must occur before using this API.

  • WSAENETDOWN The Windows Sockets implementation detected that the network subsystem failed.

  • WSAENOTCONN The socket is not connected.

  • WSAEINPROGRESS A blocking Windows Sockets operation is in progress.

  • WSAENOTSOCK The descriptor is not a socket.

  • WSAEOPNOTSUPP MSG_OOB was specified, but the socket is not of type SOCK_STREAM.

  • WSAESHUTDOWN The socket has been shut down; it is not possible to call Receive on a socket after ShutDown has been invoked with nHow set to 0 or 2.

  • WSAEWOULDBLOCK The socket is marked as nonblocking and the Receive operation would block.

  • WSAEMSGSIZE The datagram was too large to fit into the specified buffer and was truncated.

  • WSAEINVAL The socket has not been bound with Bind.

  • WSAECONNABORTED The virtual circuit was aborted due to timeout or other failure.

  • WSAECONNRESET The virtual circuit was reset by the remote side.

Remarks

This function is used for connected stream or datagram sockets and is used to read incoming data.

For sockets of type SOCK_STREAM, as much information as is currently available up to the size of the buffer supplied is returned. If the socket has been configured for in-line reception of out-of-band data (socket option SO_OOBINLINE) and out-of-band data is unread, only out-of-band data will be returned. The application can use the IOCtlSIOCATMARK option or OnOutOfBandData to determine whether any more out-of-band data remains to be read.

For datagram sockets, data is extracted from the first enqueued datagram, up to the size of the buffer supplied. If the datagram is larger than the buffer supplied, the buffer is filled with the first part of the datagram, the excess data is lost, and Receive returns a value of SOCKET_ERROR with the error code set to WSAEMSGSIZE. If no incoming data is available at the socket, a value of SOCKET_ERROR is returned with the error code set to WSAEWOULDBLOCK. The OnReceive callback function can be used to determine when more data arrives.

If the socket is of type SOCK_STREAM and the remote side has shut down the connection gracefully, a Receive will complete immediately with 0 bytes received. If the connection has been reset, a Receive will fail with the error WSAECONNRESET.

Receive should be called only once for each time CAsyncSocket::OnReceive is called.

Example

See the example for CAsyncSocket::OnReceive.

CAsyncSocket::ReceiveFrom

Call this member function to receive a datagram and store the source address in the SOCKADDR structure or in rSocketAddress.

int ReceiveFrom(
    void* lpBuf,  
    int nBufLen,  
    CString& rSocketAddress,  
    UINT& rSocketPort,  
    int nFlags = 0);

 
int ReceiveFrom(
    void* lpBuf,  
    int nBufLen,  
    SOCKADDR* lpSockAddr,  
    int* lpSockAddrLen,  
    int nFlags = 0);

Parameters

lpBuf
A buffer for the incoming data.

nBufLen
The length of lpBuf in bytes.

rSocketAddress
Reference to a CString object that receives a dotted number IP address.

rSocketPort
Reference to a UINT that stores a port.

lpSockAddr
A pointer to a SOCKADDR structure that holds the source address upon return.

lpSockAddrLen
A pointer to the length of the source address in lpSockAddr in bytes.

nFlags
Specifies the way in which the call is made. The semantics of this function are determined by the socket options and the nFlags parameter. The latter is constructed by combining any of the following values with the C++ OR operator:

  • MSG_PEEK Peek at the incoming data. The data is copied into the buffer but is not removed from the input queue.

  • MSG_OOB Process out-of-band data.

Return Value

If no error occurs, ReceiveFrom returns the number of bytes received. If the connection has been closed, it returns 0. Otherwise, a value of SOCKET_ERROR is returned, and a specific error code can be retrieved by calling GetLastError. The following errors apply to this member function:

  • WSANOTINITIALISED A successful AfxSocketInit must occur before using this API.

  • WSAENETDOWN The Windows Sockets implementation detected that the network subsystem failed.

  • WSAEFAULT The lpSockAddrLen argument was invalid: the lpSockAddr buffer was too small to accommodate the peer address.

  • WSAEINPROGRESS A blocking Windows Sockets operation is in progress.

  • WSAEINVAL The socket has not been bound with Bind.

  • WSAENOTCONN The socket is not connected ( SOCK_STREAM only).

  • WSAENOTSOCK The descriptor is not a socket.

  • WSAEOPNOTSUPP MSG_OOB was specified, but the socket is not of type SOCK_STREAM.

  • WSAESHUTDOWN The socket has been shut down; it is not possible to call ReceiveFrom on a socket after ShutDown has been invoked with nHow set to 0 or 2.

  • WSAEWOULDBLOCK The socket is marked as nonblocking and the ReceiveFrom operation would block.

  • WSAEMSGSIZE The datagram was too large to fit into the specified buffer and was truncated.

  • WSAECONNABORTED The virtual circuit was aborted due to timeout or other failure.

  • WSAECONNRESET The virtual circuit was reset by the remote side.

Remarks

This function is used to read incoming data on a (possibly connected) socket and capture the address from which the data was sent.

To handle IPv6 addresses, use CAsyncSocket::ReceiveFromEx.

For sockets of type SOCK_STREAM, as much information as is currently available up to the size of the buffer supplied is returned. If the socket has been configured for in-line reception of out-of-band data (socket option SO_OOBINLINE) and out-of-band data is unread, only out-of-band data will be returned. The application can use the IOCtlSIOCATMARK option or OnOutOfBandData to determine whether any more out-of-band data remains to be read. The lpSockAddr and lpSockAddrLen parameters are ignored for SOCK_STREAM sockets.

For datagram sockets, data is extracted from the first enqueued datagram, up to the size of the buffer supplied. If the datagram is larger than the buffer supplied, the buffer is filled with the first part of the message, the excess data is lost, and ReceiveFrom returns a value of SOCKET_ERROR with the error code set to WSAEMSGSIZE.

If lpSockAddr is nonzero, and the socket is of type SOCK_DGRAM, the network address of the socket which sent the data is copied to the corresponding SOCKADDR structure. The value pointed to by lpSockAddrLen is initialized to the size of this structure, and is modified on return to indicate the actual size of the address stored there. If no incoming data is available at the socket, the ReceiveFrom call waits for data to arrive unless the socket is nonblocking. In this case, a value of SOCKET_ERROR is returned with the error code set to WSAEWOULDBLOCK. The OnReceive callback can be used to determine when more data arrives.

If the socket is of type SOCK_STREAM and the remote side has shut down the connection gracefully, a ReceiveFrom will complete immediately with 0 bytes received.

CAsyncSocket::ReceiveFromEx

Call this member function to receive a datagram and store the source address in the SOCKADDR structure or in rSocketAddress (handles IPv6 addresses).

int ReceiveFromEx(
    void* lpBuf,  
    int nBufLen,  
    CString& rSocketAddress,  
    UINT& rSocketPort,  
    int nFlags = 0);

Parameters

lpBuf
A buffer for the incoming data.

nBufLen
The length of lpBuf in bytes.

rSocketAddress
Reference to a CString object that receives a dotted number IP address.

rSocketPort
Reference to a UINT that stores a port.

nFlags
Specifies the way in which the call is made. The semantics of this function are determined by the socket options and the nFlags parameter. The latter is constructed by combining any of the following values with the C++ OR operator:

  • MSG_PEEK Peek at the incoming data. The data is copied into the buffer but is not removed from the input queue.

  • MSG_OOB Process out-of-band data.

Return Value

If no error occurs, ReceiveFromEx returns the number of bytes received. If the connection has been closed, it returns 0. Otherwise, a value of SOCKET_ERROR is returned, and a specific error code can be retrieved by calling GetLastError. The following errors apply to this member function:

  • WSANOTINITIALISED A successful AfxSocketInit must occur before using this API.

  • WSAENETDOWN The Windows Sockets implementation detected that the network subsystem failed.

  • WSAEFAULT The lpSockAddrLen argument was invalid: the lpSockAddr buffer was too small to accommodate the peer address.

  • WSAEINPROGRESS A blocking Windows Sockets operation is in progress.

  • WSAEINVAL The socket has not been bound with Bind.

  • WSAENOTCONN The socket is not connected ( SOCK_STREAM only).

  • WSAENOTSOCK The descriptor is not a socket.

  • WSAEOPNOTSUPP MSG_OOB was specified, but the socket is not of type SOCK_STREAM.

  • WSAESHUTDOWN The socket has been shut down; it is not possible to call ReceiveFromEx on a socket after ShutDown has been invoked with nHow set to 0 or 2.

  • WSAEWOULDBLOCK The socket is marked as nonblocking and the ReceiveFromEx operation would block.

  • WSAEMSGSIZE The datagram was too large to fit into the specified buffer and was truncated.

  • WSAECONNABORTED The virtual circuit was aborted due to timeout or other failure.

  • WSAECONNRESET The virtual circuit was reset by the remote side.

Remarks

This function is used to read incoming data on a (possibly connected) socket and capture the address from which the data was sent.

This function is the same as CAsyncSocket::ReceiveFrom except that it handles IPv6 addresses as well as older protocols.

For sockets of type SOCK_STREAM, as much information as is currently available up to the size of the buffer supplied is returned. If the socket has been configured for in-line reception of out-of-band data (socket option SO_OOBINLINE) and out-of-band data is unread, only out-of-band data will be returned. The application can use the IOCtlSIOCATMARK option or OnOutOfBandData to determine whether any more out-of-band data remains to be read. The lpSockAddr and lpSockAddrLen parameters are ignored for SOCK_STREAM sockets.

For datagram sockets, data is extracted from the first enqueued datagram, up to the size of the buffer supplied. If the datagram is larger than the buffer supplied, the buffer is filled with the first part of the message, the excess data is lost, and ReceiveFromEx returns a value of SOCKET_ERROR with the error code set to WSAEMSGSIZE.

If lpSockAddr is nonzero, and the socket is of type SOCK_DGRAM, the network address of the socket which sent the data is copied to the corresponding SOCKADDR structure. The value pointed to by lpSockAddrLen is initialized to the size of this structure, and is modified on return to indicate the actual size of the address stored there. If no incoming data is available at the socket, the ReceiveFromEx call waits for data to arrive unless the socket is nonblocking. In this case, a value of SOCKET_ERROR is returned with the error code set to WSAEWOULDBLOCK. The OnReceive callback can be used to determine when more data arrives.

If the socket is of type SOCK_STREAM and the remote side has shut down the connection gracefully, a ReceiveFromEx will complete immediately with 0 bytes received.

CAsyncSocket::Send

Call this member function to send data on a connected socket.

virtual int Send(
    const void* lpBuf,  
    int nBufLen,  
    int nFlags = 0);

Parameters

lpBuf
A buffer containing the data to be transmitted.

nBufLen
The length of the data in lpBuf in bytes.

nFlags
Specifies the way in which the call is made. The semantics of this function are determined by the socket options and the nFlags parameter. The latter is constructed by combining any of the following values with the C++ OR operator:

  • MSG_DONTROUTE Specifies that the data should not be subject to routing. A Windows Sockets supplier can choose to ignore this flag.

  • MSG_OOB Send out-of-band data ( SOCK_STREAM only).

Return Value

If no error occurs, Send returns the total number of characters sent. (Note that this can be less than the number indicated by nBufLen.) Otherwise, a value of SOCKET_ERROR is returned, and a specific error code can be retrieved by calling GetLastError. The following errors apply to this member function:

  • WSANOTINITIALISED A successful AfxSocketInit must occur before using this API.

  • WSAENETDOWN The Windows Sockets implementation detected that the network subsystem failed.

  • WSAEACCES The requested address is a broadcast address, but the appropriate flag was not set.

  • WSAEINPROGRESS A blocking Windows Sockets operation is in progress.

  • WSAEFAULT The lpBuf argument is not in a valid part of the user address space.

  • WSAENETRESET The connection must be reset because the Windows Sockets implementation dropped it.

  • WSAENOBUFS The Windows Sockets implementation reports a buffer deadlock.

  • WSAENOTCONN The socket is not connected.

  • WSAENOTSOCK The descriptor is not a socket.

  • WSAEOPNOTSUPP MSG_OOB was specified, but the socket is not of type SOCK_STREAM.

  • WSAESHUTDOWN The socket has been shut down; it is not possible to call Send on a socket after ShutDown has been invoked with nHow set to 1 or 2.

  • WSAEWOULDBLOCK The socket is marked as nonblocking and the requested operation would block.

  • WSAEMSGSIZE The socket is of type SOCK_DGRAM, and the datagram is larger than the maximum supported by the Windows Sockets implementation.

  • WSAEINVAL The socket has not been bound with Bind.

  • WSAECONNABORTED The virtual circuit was aborted due to timeout or other failure.

  • WSAECONNRESET The virtual circuit was reset by the remote side.

Remarks

Send is used to write outgoing data on connected stream or datagram sockets. For datagram sockets, care must be taken not to exceed the maximum IP packet size of the underlying subnets, which is given by the iMaxUdpDg element in the WSADATA structure returned by AfxSocketInit. If the data is too long to pass atomically through the underlying protocol, the error WSAEMSGSIZE is returned via GetLastError, and no data is transmitted.

Note that for a datagram socket the successful completion of a Send does not indicate that the data was successfully delivered.

On CAsyncSocket objects of type SOCK_STREAM, the number of bytes written can be between 1 and the requested length, depending on buffer availability on both the local and foreign hosts.

Example

See the example for CAsyncSocket::OnSend.

CAsyncSocket::SendTo

Call this member function to send data to a specific destination.

int SendTo(
    const void* lpBuf,  
    int nBufLen,  
    UINT nHostPort,  
    LPCTSTR lpszHostAddress = NULL,  
    int nFlags = 0);

 
int SendTo(
    const void* lpBuf,  
    int nBufLen,  
    const SOCKADDR* lpSockAddr,  
    int nSockAddrLen,  
    int nFlags = 0);

Parameters

lpBuf
A buffer containing the data to be transmitted.

nBufLen
The length of the data in lpBuf in bytes.

nHostPort
The port identifying the socket application.

lpszHostAddress
The network address of the socket to which this object is connected: a machine name such as "ftp.microsoft.com," or a dotted number such as "128.56.22.8".

nFlags
Specifies the way in which the call is made. The semantics of this function are determined by the socket options and the nFlags parameter. The latter is constructed by combining any of the following values with the C++ OR operator:

  • MSG_DONTROUTE Specifies that the data should not be subject to routing. A Windows Sockets supplier can choose to ignore this flag.

  • MSG_OOB Send out-of-band data ( SOCK_STREAM only).

lpSockAddr
A pointer to a SOCKADDR structure that contains the address of the target socket.

nSockAddrLen
The length of the address in lpSockAddr in bytes.

Return Value

If no error occurs, SendTo returns the total number of characters sent. (Note that this can be less than the number indicated by nBufLen.) Otherwise, a value of SOCKET_ERROR is returned, and a specific error code can be retrieved by calling GetLastError. The following errors apply to this member function:

  • WSANOTINITIALISED A successful AfxSocketInit must occur before using this API.

  • WSAENETDOWN The Windows Sockets implementation detected that the network subsystem failed.

  • WSAEACCES The requested address is a broadcast address, but the appropriate flag was not set.

  • WSAEINPROGRESS A blocking Windows Sockets operation is in progress.

  • WSAEFAULT The lpBuf or lpSockAddr parameters are not part of the user address space, or the lpSockAddr argument is too small (less than the size of a SOCKADDR structure).

  • WSAEINVAL The host name is invalid.

  • WSAENETRESET The connection must be reset because the Windows Sockets implementation dropped it.

  • WSAENOBUFS The Windows Sockets implementation reports a buffer deadlock.

  • WSAENOTCONN The socket is not connected ( SOCK_STREAM only).

  • WSAENOTSOCK The descriptor is not a socket.

  • WSAEOPNOTSUPP MSG_OOB was specified, but the socket is not of type SOCK_STREAM.

  • WSAESHUTDOWN The socket has been shut down; it is not possible to call SendTo on a socket after ShutDown has been invoked with nHow set to 1 or 2.

  • WSAEWOULDBLOCK The socket is marked as nonblocking and the requested operation would block.

  • WSAEMSGSIZE The socket is of type SOCK_DGRAM, and the datagram is larger than the maximum supported by the Windows Sockets implementation.

  • WSAECONNABORTED The virtual circuit was aborted due to timeout or other failure.

  • WSAECONNRESET The virtual circuit was reset by the remote side.

  • WSAEADDRNOTAVAIL The specified address is not available from the local machine.

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

  • WSAEDESTADDRREQ A destination address is required.

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

Remarks

SendTo is used on datagram or stream sockets and is used to write outgoing data on a socket. For datagram sockets, care must be taken not to exceed the maximum IP packet size of the underlying subnets, which is given by the iMaxUdpDg element in the WSADATA structure filled out by AfxSocketInit. If the data is too long to pass atomically through the underlying protocol, the error WSAEMSGSIZE is returned, and no data is transmitted.

Note that the successful completion of a SendTo does not indicate that the data was successfully delivered.

SendTo is only used on a SOCK_DGRAM socket to send a datagram to a specific socket identified by the lpSockAddr parameter.

To send a broadcast (on a SOCK_DGRAM only), the address in the lpSockAddr parameter should be constructed using the special IP address INADDR_BROADCAST (defined in the Windows Sockets header file WINSOCK.H) together with the intended port number. Or, if the lpszHostAddress parameter is NULL, the socket is configured for broadcast. It is generally inadvisable for a broadcast datagram to exceed the size at which fragmentation can occur, which implies that the data portion of the datagram (excluding headers) should not exceed 512 bytes.

To handle IPv6 addresses, use CAsyncSocket::SendToEx.

CAsyncSocket::SendToEx

Call this member function to send data to a specific destination (handles IPv6 addresses).

int SendToEx(
    const void* lpBuf,  
    int nBufLen,  
    UINT nHostPort,  
    LPCTSTR lpszHostAddress = NULL,  
    int nFlags = 0);

Parameters

lpBuf
A buffer containing the data to be transmitted.

nBufLen
The length of the data in lpBuf in bytes.

nHostPort
The port identifying the socket application.

lpszHostAddress
The network address of the socket to which this object is connected: a machine name such as "ftp.microsoft.com," or a dotted number such as "128.56.22.8".

nFlags
Specifies the way in which the call is made. The semantics of this function are determined by the socket options and the nFlags parameter. The latter is constructed by combining any of the following values with the C++ OR operator:

  • MSG_DONTROUTE Specifies that the data should not be subject to routing. A Windows Sockets supplier can choose to ignore this flag.

  • MSG_OOB Send out-of-band data ( SOCK_STREAM only).

Return Value

If no error occurs, SendToEx returns the total number of characters sent. (Note that this can be less than the number indicated by nBufLen.) Otherwise, a value of SOCKET_ERROR is returned, and a specific error code can be retrieved by calling GetLastError. The following errors apply to this member function:

  • WSANOTINITIALISED A successful AfxSocketInit must occur before using this API.

  • WSAENETDOWN The Windows Sockets implementation detected that the network subsystem failed.

  • WSAEACCES The requested address is a broadcast address, but the appropriate flag was not set.

  • WSAEINPROGRESS A blocking Windows Sockets operation is in progress.

  • WSAEFAULT The lpBuf or lpSockAddr parameters are not part of the user address space, or the lpSockAddr argument is too small (less than the size of a SOCKADDR structure).

  • WSAEINVAL The host name is invalid.

  • WSAENETRESET The connection must be reset because the Windows Sockets implementation dropped it.

  • WSAENOBUFS The Windows Sockets implementation reports a buffer deadlock.

  • WSAENOTCONN The socket is not connected ( SOCK_STREAM only).

  • WSAENOTSOCK The descriptor is not a socket.

  • WSAEOPNOTSUPP MSG_OOB was specified, but the socket is not of type SOCK_STREAM.

  • WSAESHUTDOWN The socket has been shut down; it is not possible to call SendToEx on a socket after ShutDown has been invoked with nHow set to 1 or 2.

  • WSAEWOULDBLOCK The socket is marked as nonblocking and the requested operation would block.

  • WSAEMSGSIZE The socket is of type SOCK_DGRAM, and the datagram is larger than the maximum supported by the Windows Sockets implementation.

  • WSAECONNABORTED The virtual circuit was aborted due to timeout or other failure.

  • WSAECONNRESET The virtual circuit was reset by the remote side.

  • WSAEADDRNOTAVAIL The specified address is not available from the local machine.

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

  • WSAEDESTADDRREQ A destination address is required.

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

Remarks

This method is the same as CAsyncSocket::SendTo except that it handles IPv6 addresses as well as older protocols.

SendToEx is used on datagram or stream sockets and is used to write outgoing data on a socket. For datagram sockets, care must be taken not to exceed the maximum IP packet size of the underlying subnets, which is given by the iMaxUdpDg element in the WSADATA structure filled out by AfxSocketInit. If the data is too long to pass atomically through the underlying protocol, the error WSAEMSGSIZE is returned, and no data is transmitted.

Note that the successful completion of a SendToEx does not indicate that the data was successfully delivered.

SendToEx is only used on a SOCK_DGRAM socket to send a datagram to a specific socket identified by the lpSockAddr parameter.

To send a broadcast (on a SOCK_DGRAM only), the address in the lpSockAddr parameter should be constructed using the special IP address INADDR_BROADCAST (defined in the Windows Sockets header file WINSOCK.H) together with the intended port number. Or, if the lpszHostAddress parameter is NULL, the socket is configured for broadcast. It is generally inadvisable for a broadcast datagram to exceed the size at which fragmentation can occur, which implies that the data portion of the datagram (excluding headers) should not exceed 512 bytes.

CAsyncSocket::SetSockOpt

Call this member function to set a socket option.

BOOL SetSockOpt(
    int nOptionName,  
    const void* lpOptionValue,  
    int nOptionLen,  
    int nLevel = SOL_SOCKET);

Parameters

nOptionName
The socket option for which the value is to be set.

lpOptionValue
A pointer to the buffer in which the value for the requested option is supplied.

nOptionLen
The size of the lpOptionValue buffer in bytes.

nLevel
The level at which the option is defined; the only supported levels are SOL_SOCKET and IPPROTO_TCP.

Return Value

Nonzero if the function is successful; otherwise 0, and a specific error code can be retrieved by calling GetLastError. The following errors apply to this member function:

  • WSANOTINITIALISED A successful AfxSocketInit must occur before using this API.

  • WSAENETDOWN The Windows Sockets implementation detected that the network subsystem failed.

  • WSAEFAULT lpOptionValue is not in a valid part of the process address space.

  • WSAEINPROGRESS A blocking Windows Sockets operation is in progress.

  • WSAEINVAL nLevel is not valid, or the information in lpOptionValue is not valid.

  • WSAENETRESET Connection has timed out when SO_KEEPALIVE is set.

  • WSAENOPROTOOPT The option is unknown or unsupported. In particular, SO_BROADCAST is not supported on sockets of type SOCK_STREAM, while SO_DONTLINGER, SO_KEEPALIVE, SO_LINGER, and SO_OOBINLINE are not supported on sockets of type SOCK_DGRAM.

  • WSAENOTCONN Connection has been reset when SO_KEEPALIVE is set.

  • WSAENOTSOCK The descriptor is not a socket.

Remarks

SetSockOpt sets the current value for a socket option associated with a socket of any type, in any state. Although options can exist at multiple protocol levels, this specification only defines options that exist at the uppermost "socket" level. Options affect socket operations, such as whether expedited data is received in the normal data stream, whether broadcast messages can be sent on the socket, and so on.

There are two types of socket options: Boolean options that enable or disable a feature or behavior, and options which require an integer value or structure. To enable a Boolean option, lpOptionValue points to a nonzero integer. To disable the option lpOptionValue points to an integer equal to zero. nOptionLen should be equal to sizeof(BOOL) for Boolean options. For other options, lpOptionValue points to the integer or structure that contains the desired value for the option, and nOptionLen is the length of the integer or structure.

SO_LINGER controls the action taken when unsent data is queued on a socket and the Close function is called to close the socket.

By default, a socket cannot be bound (see Bind) to a local address which is already in use. On occasion, however, it may be desirable to "reuse" an address in this way. Since every connection is uniquely identified by the combination of local and remote addresses, there is no problem with having two sockets bound to the same local address as long as the remote addresses are different.

To inform the Windows Sockets implementation that a Bind call on a socket should not be disallowed because the desired address is already in use by another socket, the application should set the SO_REUSEADDR socket option for the socket before issuing the Bind call. Note that the option is interpreted only at the time of the Bind call: it is therefore unnecessary (but harmless) to set the option on a socket which is not to be bound to an existing address, and setting or resetting the option after the Bind call has no effect on this or any other socket.

An application can request that the Windows Sockets implementation enable the use of "keep-alive" packets on Transmission Control Protocol (TCP) connections by turning on the SO_KEEPALIVE socket option. A Windows Sockets implementation need not support the use of keep-alives: if it does, the precise semantics are implementation-specific but should conform to section 4.2.3.6 of RFC 1122: "Requirements for Internet Hosts — Communication Layers." If a connection is dropped as the result of "keep-alives" the error code WSAENETRESET is returned to any calls in progress on the socket, and any subsequent calls will fail with WSAENOTCONN.

The TCP_NODELAY option disables the Nagle algorithm. The Nagle algorithm is used to reduce the number of small packets sent by a host by buffering unacknowledged send data until a full-size packet can be sent. However, for some applications this algorithm can impede performance, and TCP_NODELAY can be used to turn it off. Application writers should not set TCP_NODELAY unless the impact of doing so is well-understood and desired, since setting TCP_NODELAY can have a significant negative impact on network performance. TCP_NODELAY is the only supported socket option which uses level IPPROTO_TCP; all other options use level SOL_SOCKET.

Some implementations of Windows Sockets supply output debug information if the SO_DEBUG option is set by an application.

The following options are supported for SetSockOpt. The Type identifies the type of data addressed by lpOptionValue.

Value Type Meaning
SO_BROADCAST BOOL Allow transmission of broadcast messages on the socket.
SO_DEBUG BOOL Record debugging information.
SO_DONTLINGER BOOL Don't block Close waiting for unsent data to be sent. Setting this option is equivalent to setting SO_LINGER with l_onoff set to zero.
SO_DONTROUTE BOOL Don't route: send directly to interface.
SO_KEEPALIVE BOOL Send keep-alives.
SO_LINGER struct LINGER Linger on Close if unsent data is present.
SO_OOBINLINE BOOL Receive out-of-band data in the normal data stream.
SO_RCVBUF int Specify buffer size for receives.
SO_REUSEADDR BOOL Allow the socket to be bound to an address which is already in use. (See Bind.)
SO_SNDBUF int Specify buffer size for sends.
TCP_NODELAY BOOL Disables the Nagle algorithm for send coalescing.

Berkeley Software Distribution (BSD) options not supported for SetSockOpt are:

Value Type Meaning
SO_ACCEPTCONN BOOL Socket is listening
SO_ERROR int Get error status and clear.
SO_RCVLOWAT int Receive low water mark.
SO_RCVTIMEO int Receive timeout
SO_SNDLOWAT int Send low water mark.
SO_SNDTIMEO int Send timeout.
SO_TYPE int Type of the socket.
IP_OPTIONS Set options field in IP header.

CAsyncSocket::ShutDown

Call this member function to disable sends, receives, or both on the socket.

BOOL ShutDown(int nHow = sends);

Parameters

nHow
A flag that describes what types of operation will no longer be allowed, using the following enumerated values:

  • receives = 0

  • sends = 1

  • both = 2

Return Value

Nonzero if the function is successful; otherwise 0, and a specific error code can be retrieved by calling GetLastError. The following errors apply to this member function:

  • WSANOTINITIALISED A successful AfxSocketInit must occur before using this API.

  • WSAENETDOWN The Windows Sockets implementation detected that the network subsystem failed.

  • WSAEINVAL nHow is not valid.

  • WSAEINPROGRESS A blocking Windows Sockets operation is in progress.

  • WSAENOTCONN The socket is not connected ( SOCK_STREAM only).

  • WSAENOTSOCK The descriptor is not a socket.

Remarks

ShutDown is used on all types of sockets to disable reception, transmission, or both. If nHow is 0, subsequent receives on the socket will be disallowed. This has no effect on the lower protocol layers.

For Transmission Control Protocol (TCP), the TCP window is not changed and incoming data will be accepted (but not acknowledged) until the window is exhausted. For User Datagram Protocol (UDP), incoming datagrams are accepted and queued. In no case will an ICMP error packet be generated. If nHow is 1, subsequent sends are disallowed. For TCP sockets, a FIN will be sent. Setting nHow to 2 disables both sends and receives as described above.

Note that ShutDown does not close the socket, and resources attached to the socket will not be freed until Close is called. An application should not rely on being able to reuse a socket after it has been shut down. In particular, a Windows Sockets implementation is not required to support the use of Connect on such a socket.

Example

See the example for CAsyncSocket::OnReceive.

CASyncSocket::Socket

Allocates a socket handle.

BOOL Socket(
    int nSocketType = SOCK_STREAM,  
    long lEvent = FD_READ | FD_WRITE | FD_OOB | FD_ACCEPT | FD_CONNECT | FD_CLOSE,  
    int nProtocolType = 0,  
    int nAddressFormat = PF_INET);

Parameters

nSocketType
Specifies SOCK_STREAM or SOCK_DGRAM.

lEvent
A bitmask that specifies a combination of network events in which the application is interested.

  • FD_READ: Want to receive notification of readiness for reading.

  • FD_WRITE: Want to receive notification of readiness for writing.

  • FD_OOB: Want to receive notification of the arrival of out-of-band data.

  • FD_ACCEPT: Want to receive notification of incoming connections.

  • FD_CONNECT: Want to receive notification of completed connection.

  • FD_CLOSE: Want to receive notification of socket closure.

nProtocolType
Protocol to be used with the socket that is specific to the indicated address family.

nAddressFormat
Address family specification.

Return Value

Returns TRUE on success, FALSE on failure.

Remarks

This method allocates a socket handle. It does not call CAsyncSocket::Bind to bind the socket to a specified address, so you need to call Bind later to bind the socket to a specified address. You can use CAsyncSocket::SetSockOpt to set the socket option before it is bound.

See Also

CObject Class
Hierarchy Chart
CSocket Class
CSocketFile Class