Socket setup

Before we can use the socket API, we need to include the socket API header files. These files vary depending on whether we are using Berkeley sockets or Winsock. Additionally, Winsock requires initialization before use. It also requires that a cleanup function is called when we are finished. These initialization and cleanup steps are not used with Berkeley sockets.

We will use the C preprocessor to run the proper code on Windows compared to Berkeley socket systems. By using the preprocessor statement, #if defined(_WIN32), we can include code in our program that will only be compiled on Windows.

Here is a complete program that includes the needed socket API headers for each platform and properly initializes Winsock on Windows:

/*sock_init.c*/

#if defined(_WIN32)
#ifndef _WIN32_WINNT
#define _WIN32_WINNT 0x0600
#endif
#include <winsock2.h>
#include <ws2tcpip.h>
#pragma comment(lib, "ws2_32.lib")

#else
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <unistd.h>
#include <errno.h>

#endif

#include <stdio.h>

int main() {

#if defined(_WIN32)
    WSADATA d;
    if (WSAStartup(MAKEWORD(2, 2), &d)) {
        fprintf(stderr, "Failed to initialize.\n");
        return 1;
    }
#endif

    printf("Ready to use socket API.\n");

#if defined(_WIN32)
    WSACleanup();
#endif

    return 0;
}

The first part includes winsock.h and ws2tcpip.h on Windows. _WIN32_WINNT must be defined for the Winsock headers to provide all the functions we need. We also include the #pragma comment(lib,"ws2_32.lib") pragma statement. This tells the Microsoft Visual C compiler to link your program against the Winsock library, ws2_32.lib. If you're using MinGW as your compiler, then #pragma is ignored. In this case, you need to tell the compiler to link in ws2_32.lib on the command line using the -lws2_32 option.

If the program is not compiled on Windows, then the section after #else will compile. This section includes the various Berkeley socket API headers and other headers we need on these platforms.

In the main() function, we call WSAStartup() on Windows to initialize Winsock. The MAKEWORD macro allows us to request Winsock version 2.2. If our program is unable to initialize Winsock, it prints an error message and aborts.

When using Berkeley sockets, no special initialization is needed, and the socket API is always ready to use.

Before our program finishes, WSACleanup() is called if we're compiling for Winsock on Windows. This function allows the Windows operating system to do additional cleanup.

Compiling and running this program on Linux or macOS is done with the following command:

gcc sock_init.c -o sock_init
./sock_init

Compiling on Windows using MinGW can be done with the following command:

gcc sock_init.c -o sock_init.exe -lws2_32
sock_init.exe

Notice that the -lws2_32 flag is needed with MinGW to tell the compiler to link in the Winsock library, ws2_32.lib.

Now that we've done the necessary setup to begin using the socket APIs, let's take a closer look at what we will be using these sockets for.