Include Files

The original include file for use with Windows Sockets 1.1 was the Winsock.h header file. To ease porting existing source code based on Berkeley UNIX sockets to Windows sockets, Windows Sockets development kits for Winsock 1.1 were encouraged to be supplied with several include files with the same names as standard UNIX include files (the sys/socket.h and arpa/inet.h header files, for example). However, these similarly-name Winsock header files merely contained a directive to include the Winsock2.h header file.

When Windows Sockets 2 was released, the primary include file for use with Windows Sockets was renamed to Winsock2.h. The older original Winsock.h header file for Winsock 1.1 was also retained for compatibility with older applications. The development of Winsock 1.1 compatible applications has been deprecated since Windows 2000 was released. All applications should now use use the include Winsock2.h directive in Winsock application source files.

The Winsock2.h header file contains most of the Winsock functions, structures, and definitions. The Ws2tcpip.h header file contains definitions introduced in the WinSock 2 Protocol-Specific Annex document for TCP/IP that includes newer functions and structures used to retrieve IP addresses. These include the getaddrinfo and getnameinfo family of functions that provide name resolution for both IPv4 or IPv6 addresses. The Ws2tcpip.h header file is only needed if these IP-agnostic naming functions are required by the application.

The Mswsock.h header file contains definitions for Microsoft-specific extensions to the Windows Sockets 2 (TransmitFile, AcceptEx, and ConnectEx, for example). The Mswsock.h header file is not normally needed unless these Microsoft-specific extensions are used by the application.

The Winsock2.h header file internally includes core elements from the Windows.h header file, so there is not usually an #include line for the Windows.h header file in Winsock applications. If an #include line is needed for the Windows.h header file, this should be preceded with the #define WIN32_LEAN_AND_MEAN macro. For historical reasons, the Windows.h header defaults to including the Winsock.h header file for Windows Sockets 1.1. The declarations in the Winsock.h header file will conflict with the declarations in the Winsock2.h header file required by Windows Sockets 2. The WIN32_LEAN_AND_MEAN macro prevents the Winsock.h from being included by the Windows.h header. An example illustrating this is shown below.

#include <winsock2.h>
#include <ws2tcpip.h>
#include <stdio.h>

#pragma comment(lib, "Ws2_32.lib")

int main() {
  return 0;

Related topics

Creating a Basic Winsock Application
Getting Started With Winsock
Porting Socket Applications to Winsock
Winsock Programming Considerations