Question

How does WSAStartup function initiates use of the Winsock DLL?

According to the documentation

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.

This function initializes WSADATA data structure, but in socket programming we don’t pass WSDATA to any function so how does the program comes to know about the Windows Sockets version and other details?

For example in this code

#include <stdio.h>
#include <winsock2.h>
#pragma comment(lib, "ws2_32")

void Run(int argc, char* argv[])
{
    char* host = argc < 2 ? "" : argv[1];
    struct hostent* entry = gethostbyname(host);

    if(entry)
    {
        struct in_addr* addr = (struct in_addr*) entry->h_addr;
        printf("IP Address: %s\n", inet_ntoa(*addr));
    }
    else
        printf("ERROR: Resolution failure.\n");
}

int main(int argc, char* argv[])
{
    WSADATA wsaData;

    if(WSAStartup(0x202, &wsaData) == 0)
    {
        Run(argc, argv);
        WSACleanup();
    }
    else
        printf("ERROR: Initialization failure.\n");
}

In this example I am initializing WSADATA data structure using WSAStartup() function and after wards I’m not passing wsaData anywhere.

So how does my program comes to know about wsaData details?

Thanks.

Answer 1

WSAStartup has two main purposes.

Firstly, it allows you to specify what version of WinSock you want to use (you are requesting 2.2 in your example). In the WSADATA that it populates, it will tell you what version it is offering you based on your request. It also fills in some other information which you are not required to look at if you aren’t interested. You never have to submit this WSADATA struct to WinSock again, because it is used purely to give you feedback on your WSAStartup request.

The second thing it does, is to set-up all the “behind the scenes stuff” that your app needs to use sockets. The WinSock DLL file is loaded into your process, and it has a whole lot of internal structures that need to be set-up for each process. These structures are hidden from you, but they are visible to each of the WinSock calls that you make.

Because these structures need to be set-up for each process that uses WinSock, each process must call WSAStartup to initialise the structures within its own memory space, and WSACleanup to tear them down again, when it is finished using sockets.