Windows Programming/Winsock

Winsock is the name of the library in Windows that handles the Berkely Socket API. Technically, this library is not part of the Win32 API, although there are some windows-specific issues that need to be considered when programming a Winsock application.

Making a Winsock Project

You can add Winsock to your programming project by including the <winsock2.h> header file. This header file is for the 32-bit version of the library. For the 16-bit version, include the file <winsock.h>.

winsock.dll is the 16-bit version of the library, and ws2_32.dll is the 32-bit version. You must instruct your linker to link to the appropriate version of the library.

Initializing Winsock

Before calling any of the Winsock routines, the library must first be initialized by calling the WSAStartup function. This function requires a pointer to the WSADATA structure. You do not need to initialize this structure, because the call to WSAStartup will fill in all the fields of the structure. You may, optionally, read the values from this structure, and use the results in your program. This is not necessary, however. WSAStartup also requires that you specify the version of winsock that you wish to use. The most current version of winsock is version 1.1, although the newest version of the library is version 2.0. To specify this parameter, pass the major and minor versions to the MAKEWORD macro. Here is an example:

WSADATA wd;
WSAStartup(MAKEWORD(2, 0), &wd);

Here is the definition of the WSADATA structure. From this data structure, you can determine some important system metrics, including the version of your library, the maximum number of simultaneous sockets available, etc.

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; 

The definition of WSAStartup is as follows:

int WSAStartup(WORD wVersionRequested, LPWSADATA lpWSAData);

This function returns zero on success, and will return non-zero on failure. These error codes can be handled, or the program can abort.

Exiting Winsock

After a program has completed, it must call WSACleanup, to unregister itself from the listing, and to free any resources in use by the library. WSACleanup takes no parameters, and returns zero on success. A non-zero return value signifies an error in the cleanup process.

Sockets as Handles

It has been said that unlike UNIX, Win32 does not allow sockets to be read/written using the file I/O functions. This is only partially true. Sockets may not be accessed using the standard-library functions such as fread, fwrite, fprintf, etc. However, if we cast our SOCKET structures to HANDLE structures, we can use the Win32 File I/O API to interface with the sockets. For instance, we can now use ReadFile and WriteFile to write to sockets, and any routines that we have written around these APIs can be used when writing to the network.

Under Win32, do not attempt to cast a SOCKET to a FILE type, and use the stdio.h file functions. This will result in some sort of error (most likely a bad error).

Advanced Win32 Sockets

Win32 has a full complement of socket functions, including bind, accept, socket, listen and recv. However, Win32 also provides a number of advanced function varieties that allow for advanced operation modes. For instance, using the advanced socket functions allow Overlapped I/O mode, asynchronous modes, events, etc. These functions can be explored on MSDN.