客戶端連接服務器的回傳值是10035，于是我這樣寫：
res=connect(sockCli,(SOCKADDR*)&addrSer,sizeof(SOCKADDR));
if(res==10035 || res==0){
MessageBox(NULL,"客戶端連接服務器成功","警告",0);}
else
MessageBox(NULL,"客戶端連接服務器失敗","通知",0);
break;
可結果是客戶端彈出兩次訊息框，第一次是 客戶端連接服務器成功，第二次是 客戶端連接服務器失敗，為什么會這樣呢？
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connectThe Windows Sockets connect function establishes a connection to a specifed socket.
int connect (
SOCKET s,
const struct sockaddr FAR* name,
int namelen
);
Parameters
s
[in] A descriptor identifying an unconnected socket.
name
[in] The name of the socket to connect to.
namelen
[in] The length of the name parameter.
Remarks
The connect function is used to create a connection to the specified destination. If the socket, s, is unbound, unique values are assigned to the local association by the system, and the socket is marked as bound.
For connection-oriented sockets (for example, type SOCK_STREAM), an active connection is initiated to the foreign host using name (an address in the name space of the socket; for a detailed description, please see bind and SOCKADDR). When the socket call completes successfully, the socket is ready to send/receive data. If the address member of the structure specified by the name parameter is all zeroes, connect will return the error WSAEADDRNOTAVAIL. Any attempt to re-connect an active connection will fail with the error code WSAEISCONN.
For connection-oriented, nonblocking sockets, it is often not possible to complete the connection immediately. In such a case, this function returns the error WSAEWOULDBLOCK. However, the operation proceeds. When the success or failure outcome becomes known, it may be reported in one of several ways depending on how the client registers for notification. If the client uses the select function, success is reported in the writefds set and failure is reported in the exceptfds set. If the client uses the functions WSAAsyncSelect or WSAEventSelect, 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
For a connectionless socket (for example, type SOCK_DGRAM), the operation performed by connect is merely to establish a default destination address that will be used on subsequent send/WSASend and recv/WSARecv calls. Any datagrams received from an address other than the destination address specified will be discarded. If the address member of the structure specifed by name is all zeroes, the socket will be "dis-connected." Then, the default remote address will be indeterminate, so send/WSASend and recv/WSARecv calls will return the error code WSAENOTCONN. However, sendto/WSASendTo and recvfrom/WSARecvFrom can still be used. The default destination can be changed by simply calling connect again, even if the socket is already connected. Any datagrams queued for receipt are discarded if name is different from the previous connect.
For connectionless sockets, name can indicate any valid address, including a broadcast address. However, to connect to a broadcast address, a socket must use setsockopt to enable the SO_BROADCAST option. Otherwise, connect will fail with the error code WSAEACCES.
When a connection between sockets is broken, the sockets should be discarded and recreated. When a problem develops on a connected socket, the application must discard and recreate the needed sockets in order to return to a stable point.
Windows CE: Windows CE does not support the WSAEINTR error value.
For IrSocket implementation:
The Af_irda.h file must be explicitly included.
If WSAENETDOWN is returned, an existing IrDA connection was detected at the media access level.
If WSAEADDRINUSE is returned, active connections to a device with a different address exist.
If WSAEINPROGRESS is returned, IAS name resolution failed because another IAS query is in progress. Retry the operation at one second intervals.
If WSAEISCONN is returned, the socket is already connected or an exclusive/multiplexed mode change failed.
IrSockets implements the connect function with addresses of the form sockaddr_irda. Typically, a client application will create a socket with the socket function, scan the immediate vicinity for IrDA devices with the IRLMP_ENUMDEVICES socket option, choose a device from the returned list, form an address, and call connect. There is no difference in blocking and non-blocking semantics.
Return Values
If no error occurs, connect returns zero. Otherwise, it returns SOCKET_ERROR, and a specific error code can be retrieved by calling WSAGetLastError.
On a blocking socket, the return value indicates success or failure of the connection attempt.
With a nonblocking socket, the connection attempt cannot be completed immediately. In this case, connect will return SOCKET_ERROR, and WSAGetLastError will return WSAEWOULDBLOCK. In this case, there are three different steps you can take:
Use the select function to determine the completion of the connection request by checking to see if the socket is writeable.
If the application is using WSAAsyncSelect to indicate interest in connection events, then the application will receive an FD_CONNECT notification indicating that the connect operation is complete (successfully or not).
If the application is using WSAEventSelect to indicate interest in connection events, then the associated event object will be signaled indicating that the connect operation is complete (successfully or not).
Until the connection attempt completes on a nonblocking socket, all subsequent calls to connect on the same socket will fail with the error code WSAEALREADY, and WSAEISCONN when the connection completes successfully. Due to ambiguities in version 1.1 of the Windows Sockets specification, error codes returned from connect while a connection is already pending may vary among implementations. As a result, it is not recommended that applications use multiple calls to connect to detect connection completion. If they do, they must be prepared to handle WSAEINVAL and WSAEWOULDBLOCK error values the same way that they handle WSAEALREADY, to assure robust execution.
If the error code returned indicates the connection attempt failed (that is, WSAECONNREFUSED, WSAENETUNREACH, WSAETIMEDOUT) the application can call connect again for the same socket.
Error Codes
WSANOTINITIALISED A successful WSAStartup must occur before using this function.
WSAENETDOWN The network subsystem has failed.
WSAEADDRINUSE The 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 when executing 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.
WSAEINTR The (blocking) Windows Socket 1.1 call was canceled through WSACancelBlockingCall.
WSAEINPROGRESS A blocking Windows Sockets 1.1 call is in progress, or the service provider is still processing a callback function.
WSAEALREADY A nonblocking connect call is in progress on the specified socket.
Note In order to preserve backward compatibility, this error is reported as WSAEINVAL to Windows Sockets 1.1 applications that link to either WINSOCK.DLL or WSOCK32.DLL.
WSAEADDRNOTAVAIL The remote address is not a valid address (such as ADDR_ANY).
WSAEAFNOSUPPORT Addresses in the specified family cannot be used with this socket.
WSAECONNREFUSED The attempt to connect was forcefully rejected.
WSAEFAULT The name or the namelen parameter is not a valid part of the user address space, the namelen parameter is too small, or the name parameter contains incorrect address format for the associated address family.
WSAEINVAL The parameter s is a listening socket, or the destination address specified is not consistent with that of the constrained group the socket belongs to.
WSAEISCONN The socket is already connected (connection-oriented sockets only).
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.
WSAEACCES Attempt to connect datagram socket to broadcast address failed because setsockopt option SO_BROADCAST is not enabled.
QuickInfo
Windows NT: Yes
Windows: Yes
Windows CE: Use version 1.0 and later.
Header: Declared in winsock2.h.
Import Library: Link with ws2_32.lib.
See Also
accept, bind, getsockname, select, socket, WSAAsyncSelect, WSAConnect
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CONNECTSection: POSIX Programmer's Manual (P)
Updated: 2003
--------------------------------------------------------------------------------
NAME
connect - connect a socket
SYNOPSIS
#include <sys/socket.h>
int connect(int socket, const struct sockaddr *address,
socklen_t address_len);
DESCRIPTION
The connect() function shall attempt to make a connection on a socket. The function takes the following arguments:
socket
Specifies the file descriptor associated with the socket.
address
Points to a sockaddr structure containing the peer address. The length and format of the address depend on the address family of the socket.
address_len
Specifies the length of the sockaddr structure pointed to by the address argument.
If the socket has not already been bound to a local address, connect() shall bind it to an address which, unless the socket's address family is AF_UNIX, is an unused local address.
If the initiating socket is not connection-mode, then connect() shall set the socket's peer address, and no connection is made. For SOCK_DGRAM sockets, the peer address identifies where all datagrams are sent on subsequent send() functions, and limits the remote sender for subsequent recv() functions. If address is a null address for the protocol, the socket's peer address shall be reset.
If the initiating socket is connection-mode, then connect() shall attempt to establish a connection to the address specified by the address argument. If the connection cannot be established immediately and O_NONBLOCK is not set for the file descriptor for the socket, connect() shall block for up to an unspecified timeout interval until the connection is established. If the timeout interval expires before the connection is established, connect() shall fail and the connection attempt shall be aborted. If connect() is interrupted by a signal that is caught while blocked waiting to establish a connection, connect() shall fail and set errno to [EINTR], but the connection request shall not be aborted, and the connection shall be established asynchronously.
If the connection cannot be established immediately and O_NONBLOCK is set for the file descriptor for the socket, connect() shall fail and set errno to [EINPROGRESS], but the connection request shall not be aborted, and the connection shall be established asynchronously. Subsequent calls to connect() for the same socket, before the connection is established, shall fail and set errno to [EALREADY].
When the connection has been established asynchronously, select() and poll() shall indicate that the file descriptor for the socket is ready for writing.
The socket in use may require the process to have appropriate privileges to use the connect() function.
RETURN VALUE
Upon successful completion, connect() shall return 0; otherwise, -1 shall be returned and errno set to indicate the error.
ERRORS
The connect() function shall fail if:
EADDRNOTAVAIL
The specified address is not available from the local machine.
EAFNOSUPPORT
The specified address is not a valid address for the address family of the specified socket.
EALREADY
A connection request is already in progress for the specified socket.
EBADF
The socket argument is not a valid file descriptor.
ECONNREFUSED
The target address was not listening for connections or refused the connection request.
EINPROGRESS
O_NONBLOCK is set for the file descriptor for the socket and the connection cannot be immediately established; the connection shall be established asynchronously.
EINTR
The attempt to establish a connection was interrupted by delivery of a signal that was caught; the connection shall be established asynchronously.
EISCONN
The specified socket is connection-mode and is already connected.
ENETUNREACH
No route to the network is present.
ENOTSOCK
The socket argument does not refer to a socket.
EPROTOTYPE
The specified address has a different type than the socket bound to the specified peer address.
ETIMEDOUT
The attempt to connect timed out before a connection was made.
If the address family of the socket is AF_UNIX, then connect() shall fail if:
EIO
An I/O error occurred while reading from or writing to the file system.
ELOOP
A loop exists in symbolic links encountered during resolution of the pathname in address.
ENAMETOOLONG
A component of a pathname exceeded {NAME_MAX} characters, or an entire pathname exceeded {PATH_MAX} characters.
ENOENT
A component of the pathname does not name an existing file or the pathname is an empty string.
ENOTDIR
A component of the path prefix of the pathname in address is not a directory.
The connect() function may fail if:
EACCES
Search permission is denied for a component of the path prefix; or write access to the named socket is denied.
EADDRINUSE
Attempt to establish a connection that uses addresses that are already in use.
ECONNRESET
Remote host reset the connection request.
EHOSTUNREACH
The destination host cannot be reached (probably because the host is down or a remote router cannot reach it).
EINVAL
The address_len argument is not a valid length for the address family; or invalid address family in the sockaddr structure.
ELOOP
More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the pathname in address.
ENAMETOOLONG
Pathname resolution of a symbolic link produced an intermediate result whose length exceeds {PATH_MAX}.
ENETDOWN
The local network interface used to reach the destination is down.
ENOBUFS
No buffer space is available.
EOPNOTSUPP
The socket is listening and cannot be connected.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
If connect() fails, the state of the socket is unspecified. Conforming applications should close the file descriptor and create a new socket before attempting to reconnect.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
accept() , bind() , close() , getsockname() , poll() , select() , send() , shutdown() , socket() , the Base Definitions volume of IEEE Std 1003.1-2001, <sys/socket.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .
--------------------------------------------------------------------------------
uj5u.com熱心網友回復：
可以通過select sock 檢查是否可寫。可寫說明鏈接成功fd_set Write, Err;
FD_ZERO(&Write);
FD_ZERO(&Err);
FD_SET(sock, &Write);
FD_SET(sock, &Err);
// check if the socket is ready
select(0,NULL,&Write,&Err,&Timeout);
if(FD_ISSET(sock, &Write))
{
return sock; //鏈接成功
}
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判斷連接是否成功建立：A. 如果connect 回傳0，表示連接成功（服務器和客戶端在同一臺機器上時就有可能發生這種情況）；
B. 呼叫select 來等待連接建立成功完成；
如果select 回傳0，則表示建立連接超時。我們回傳超時錯誤給用戶，同時關閉連接，以防止三路握手操作繼續進行下去。
如果select 回傳大于0的值，則需要檢查套介面描述符是否可寫，如果套介面描述符可寫，則我們可以通過呼叫getsockopt來得到套介面上待處理的錯誤（SO_ERROR）。如果連接建立成功，這個錯誤值將是0；如果建立連接時遇到錯誤，則這個值是連接錯誤所對應的errno值（比如：ECONNREFUSED,ETIMEDOUT等）。
uj5u.com熱心網友回復：
可能函式進去了兩次，第一次連接成功后，后面的鏈接會失敗，也就是會彈兩次彈框的原因uj5u.com熱心網友回復：
非阻塞，你用的是select嗎？轉載請註明出處，本文鏈接：https://www.uj5u.com/gongcheng/33420.html
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