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; } WSADATA, * LPWSADATA; 

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 (see Chapter 2). 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.

Table 1-1 lists the versions of Winsock that the various Microsoft Windows platforms support. What's important to remember is the difference between major versions. WINSOCK 1.x does not support many of the advanced Winsock features detailed in this section.

Table 1-1 Supported Winsock Versions

Platform	Winsock Version
Windows 95	1.1 (2.2)
Windows 98	2.2
Windows Me	2.2
Windows NT 4.0	2.2
Windows 2000	2.2
Windows XP	2.2
Windows CE	1.1

Note that even though a platform supports Winsock 2, you do not have to request the latest version. That is, if you want to write an application that is supported on a majority of platforms, you should write it to the Winsock 1.1 specification. This application will run perfectly well on Windows NT 4.0 because all Winsock 1.1 calls are mapped through the Winsock 2 DLL. Also, if a newer version of the Winsock library becomes available for a platform that you use, it is often in your best interest to upgrade. These new versions contain bug fixes, and your old code should run without a problem—at least theoretically. In some cases, the Winsock stack's behavior is different from what the specification defines. As a result, many programmers write their applications according to the behavior of the particular platform they are targeting instead of the specification.

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.