WSAStartup function

【WSAStartup function】

Parameters

wVersionRequested [in]: The highest version of Windows Sockets specification that the caller can use. The high-order byte specifies the minor version number; the low-order byte specifies the major version number.
lpWSAData [out]: A pointer to the WSADATA data structure that is to receive details of the Windows Sockets implementation.

Return value

If successful, the WSAStartup function returns zero. Otherwise, it returns one of the error codes listed below.

The WSAStartup function directly returns the extended error code in the return value for this function. A call to the WSAGetLastError function is not needed and should not be used.

Error code	Meaning
WSA SYS NOTREADY	The underlying network subsystem is not ready for network communication.
WSA VER NOT SUPPORTED	The version of Windows Sockets support requested is not provided by this particular Windows Sockets implementation.
WSA E IN PROGRESS	A blocking Windows Sockets 1.1 operation is in progress.
WSA E PROC LIM	A limit on the number of tasks supported by the Windows Sockets implementation has been reached.
WSA E FAULT	The lpWSAData parameter is not a valid pointer.

Remarks

The WSAStartup function must be the first Windows Sockets function called by an application or DLL. It allows an application or DLL to specify the version of Windows Sockets required and retrieve details of the specific Windows Sockets implementation. The application or DLL can only issue further Windows Sockets functions after successfully calling WSAStartup.

In order to support various Windows Sockets implementations and applications that can have functional differences from the latest version of Windows Sockets specification, a negotiation takes place in WSAStartup. The caller of WSAStartup passes in the wVersionRequested parameter the highest version of the Windows Sockets specification that the application supports. The Winsock DLL indicates the highest version of the Windows Sockets specification that it can support in its response. The Winsock DLL also replies with version of the Windows Sockets specification that it expects the caller to use.

wVersionRequested指明程序支持的最高版本，winsock返回其支持的最高版本，以及返回winsock期望程序使用的版本。

When an application or DLL calls the WSAStartup function, the Winsock DLL examines the version of the Windows Sockets specification requested by the application passed in the wVersionRequested parameter. If the version requested by the application is equal to or higher than the lowest version supported by the Winsock DLL, the call succeeds and the Winsock DLL returns detailed information in the WSADATA structure pointed to by thelpWSAData parameter. The wHighVersion member of the WSADATA structure indicates the highest version of the Windows Sockets specification that the Winsock DLL supports. The wVersion member of the WSADATA structure indicates the version of the Windows Sockets specification that the Winsock DLL expects the caller to use.

The current version of the Windows Sockets specification is version 2.2. The current Winsock DLL, Ws2_32.dll, supports applications that request any of the following versions of Windows Sockets specification:

To get full access to the new syntax of a higher version of the Windows Sockets specification, the application must negotiate for this higher version. In this case, the wVersionRequested parameter should be set to request version 2.2. The application must also fully conform to that higher version of the Windows Socket specification, such as compiling against the appropriate header file, linking with a new library, or other special cases. The Winsock2.h header file for Winsock 2 support is included with the Microsoft Windows Software Development Kit (SDK).

2015年的今天，所有PC已全面支持winsock2.2。

It is legal and possible for an application or DLL written to use a lower version of the Windows Sockets specification that is supported by the Winsock DLL to successfully negotiate this lower version using the WSAStartup function. For example, an application can request version 1.1 in the wVersionRequested parameter passed to the WSAStartup function on a platform with the Winsock 2.2 DLL. In this case, the application should only rely on features that fit within the version requested. New Ioctl codes, new behavior of existing functions, and new functions should not be used. The version negotiation provided by the WSAStartup was primarily used to allow older Winsock 1.1 applications developed for Windows 95 and Windows NT 3.51 and earlier to run with the same behavior on later versions of Windows. The Winsock.h header file for Winsock 1.1 support is included with the Windows SDK.

This negotiation in the WSAStartup function allows both the application or DLL that uses Windows Sockets and the Winsock DLL to support a range of Windows Sockets versions. An application or DLL can use the Winsock DLL if there is any overlap in the version ranges. Detailed information on the Windows Sockets implementation is provided in the WSADATA structure returned by the WSAStartup function.

Once an application or DLL has made a successful WSAStartup call, it can proceed to make other Windows Sockets calls as needed. When it has finished using the services of the Winsock DLL, the application must call WSACleanup to allow the Winsock DLL to free internal Winsock resources used by the application.

An application can call WSAStartup more than once if it needs to obtain the WSADATA structure information more than once. On each such call, the application can specify any version number supported by the Winsock DLL.

The WSAStartup function typically leads to protocol-specific helper DLLs being loaded. As a result, the WSAStartup function should not be called from the DllMain function in a application DLL. This can potentially cause deadlocks. For more information, please see the DLL Main Function.

不要在Dll Main中调用WASSartup。

An application must call the WSACleanup function for every successful time the WSAStartup function is called. This means, for example, that if an application calls WSAStartup three times, it must call WSACleanup three times. The first two calls to WSACleanup do nothing except decrement an internal counter; the final WSACleanup call for the task does all necessary resource deallocation for the task.

Startup、Cleanup一一对应。

Note An application can call the WSAGetLastError function to determine the extended error code for other Windows sockets functions as is normally done in Windows Sockets even if the WSAStartup function fails or the WSAStartup function was not called to properly initialize Windows Sockets before calling a Windows Sockets function. The WSAGetLastError function is one of the only functions in the Winsock 2.2 DLL that can be called in the case of a WSAStartup failure.

参考：https://msdn.microsoft.com/zh-cn/subscriptions/downloads/ms742213(v=vs.85).aspx