Windows System Programming Third Edition

346

CHAPTER 12 NETWORK PROGRAMMING WITH WINDOWS SOCKETS

In this chapter, the client/server system is used as a vehicle for demonstrating

Winsock, and, in the course of modifying the server, interesting new features are

added. In particular, DLL entry points (Chapter 5) and in-process DLL servers are

used for the first time. (These new features could hav

e been incorporated in the

initial Chapter 11 version, but doing so would have di stracted from the d evelop-

ment and understanding of the basic system architecture.) Finally, additional

examples show how to create reentrant thread-safe libraries.

Winsock, because of conformance to ind

ustry standards, has naming

conventions and programming characteristics somewhat different from the

Windows functions described so far. T he W insock API is not strictly a part of Win32/

64. Winsock also provides additional functions that are not part of the standard;

these functions are used only as absolutely required. Among other advantages,

programs will be more portable to other systems.

Windows Sockets

The Winsock API was developed as an extensi on of the Berkeley Sockets API into

the Windows environment, and Winsock is supported by all Windows systems.

Winsock’s benefits include the following.

•Porting of code already written for Berkeley Sockets is straightforward.

•W

indows systems easily integrate into TCP/IP networks, both IPv4 and the

emerging IPv6. IPv6, among other features, allows for longer IP addresses,

overcoming the 4-byte address limit of IPv4.

•Sockets can be used with Windows overlapped I/O (Chapter 14), which, among

other things, allows servers to scale when there is a large number of active cli

ents.

•Sockets can be treated as file

s for use with , ,

a nd, with some limitations, other Windows functions, just as UNIX allows

sockets to be used as file descriptors. This capability is convenient whenever

there is a need to use asynchronous I/O and I/O completion ports (Chapter 14).

•Extended, nonportable extensions are also available.

Winsock Initialization

The Winsock API is supported by a DLL ( ) that can be accessed by

linking

with your program. The DLL needs to be initialized with a

nonstandard, Winsock-specific function,

, which must be the first

Winsock function a program calls.

should be called when the pro-

gram no longer needs to use Winsock functionality. Note: The prefix

denotes

WINDOWS SOCKETS

347

“Windows Sockets asynchronous. . . .” The asynchronous capabilities will not be

used here because threads can and will be used where asynchronous operation is

requi red.

and , while always required, may be the only non-

standard functions you will use. A common practice is to use

statements

to test the

macro (normally defined at compile time by Visual C++) so that

the

functions are called only if you are building on Windows. This approach

assumes, of course, that the rest of your code is platform-independent.

Parameters

indicates the highest version of the Winsock DLL that you

need and can use. Version 1.1 is generally adequate and ensures the widest possi-

ble Windows interoperability. Nonetheless, Version 2.0 is ava i lable on all Windows

systems, including 9x, and is used in the examples. Versi

on 1.1 is obsolete.

The return value is nonzero if the DLL cannot support the version you wa nt.

The low byte of

specifies the major version; the high byte

specifies the minor version. The

macro is usually used; thus,

represents Version 2.0.

points to a structure that returns information on the

configuration of the DLL, including the highest version available. The Visual

Studio on-line help shows how to interpret the results.

can be used to get the error, but also

works, as does the

function developed in Chapter 2.

When a program has completed or no longer needs to use sockets, it should

call

so that , the sockets DLL, can free resources

allocated for this process.

Creating a Socket

Once the Winsock DLL has been initialized, you can use the standard (i.e., Berkeley

Sockets) functions to create sockets and connect for client/server or peer-to-peer

communication.

A Winsock

data type is a nalogous to the Windows and can

even be used with

and other Windows functions requiring a .