An Intro to Windows Socket (Winsock2) Programming 2




Winsock Headers and Libraries


When developing new applications you should target the Winsock 2 specification by including WINSOCK2.H in your application. For compatibility with older Winsock applications and when developing on Windows CE platforms, WINSOCK.H is available. There is also an additional header file: MSWSOCK.H, which targets Microsoft-specific programming extensions that are normally used for developing high performance Winsock applications. When compiling your application with WINSOCK2.H, you should link with WS2_32.LIB library. When using WINSOCK.H (as on Windows CE) you should use WSOCK32.LIB. If you use extension APIs from MSWSOCK.H, you must also link with MSWSOCK.LIB (get the steps here  - VC++ .NET or here - VC++ 6). Once you have included the necessary header files and link environment, you are ready to begin coding your application, which requires initializing Winsock.




Initializing Winsock


Every Winsock application must load the appropriate version of the Winsock DLL. If you fail to load the Winsock library before calling a Winsock function, the function returns a SOCKET_ERROR; the error will be WSANOTINITIALISED. Loading the Winsock library is accomplished by calling the WSAStartup() function, which is defined as:


int WSAStartup(WORD wVersionRequested, LPWSADATA lpWSAData);


The wVersionRequested parameter is used to specify the version of the Winsock library you want to load. The high-order byte specifies the minor version of the requested Winsock library, while the low-order byte is the major version. You can use the handy macro MAKEWORD(x, y), in which x is the high byte and y is the low byte, to obtain the correct value for wVersionRequested. The lpWSAData parameter is a pointer to a LPWSADATA structure that WSAStartup() fills with information related to the version of the library it loads:


typedef struct WSAData


    WORD      wVersion;

    WORD      wHighVersion;

    char           szDescription[WSADESCRIPTION_LEN + 1];

    char           szSystemStatus[WSASYS_STATUS_LEN + 1];

    unsigned short iMaxSockets;

    unsigned short iMaxUdpDg;

    char FAR *     lpVendorInfo;



WSAStartup() sets the first field, wVersion, to the Winsock version you will be using. The wHighVersion parameter holds the highest version of the Winsock library available. Remember that in both of these fields, the high-order byte represents the Winsock minor version, and the low-order byte is the major version. The szDescription and szSystemStatus fields are set by the particular implementation of Winsock and aren't really useful. Do not use the next two fields, iMaxSockets and iMaxUdpDg. They are supposed to be the maximum number of concurrently open sockets and the maximum datagram size; however, to find the maximum datagram size you should query the protocol information through WSAEnumProtocols().

The maximum number of concurrent sockets isn't some magic number, it depends more on the physical resources available. Finally, the lpVendorInfo field is reserved for vendor-specific information regarding the implementation of Winsock. This field is not used on any Windows platforms. For the most part, when writing new applications you will load the latest version of the Winsock library currently available. Remember that if, for example, Winsock 3 is released, your application that loads version 2.2 should run as expected. If you request a Winsock version later than that which the platform supports, WSAStartup() will fail. Upon return, the wHighVersion of the WSADATA structure will be the latest version supported by the library on the current system. When your application is completely finished using the Winsock interface, you should call WSACleanup(), which allows Winsock to free up any resources allocated by Winsock and cancel any pending Winsock calls that your application made. WSACleanup() is defined as:



int WSACleanup(void);


Failure to call WSACleanup() when your application exits is not harmful because the operating system will free up resources automatically; however, your application will not be following the Winsock specification. Also, you should call WSACleanup() for each call that is made to WSAStartup().




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