boost::asio, Visual Studio 2005 and Windows 2000

As we upgraded our compiler to 2005 earlier this year, I converted one project from Visual Studio 6 to the 2005 version now. It is a service-application and I struggled with it because it did not start. Then I ran the .exe from Explorer instead of the Services-management console to see that it required the (POSIX) function "freeaddrinfo", which is not present in Windows 2000's ws2_32.dll. This is a serious problem, because we want to support Windows 2000 Server edition for that software, especially since we run that version ourself! So I digged into it and found out about this:

Lost backwards-compatibility

According to this Microsoft-Blog the old versions of Visual Studio (read: 6 and earlier) defined getaddrinfo, freeaddrinfo and others as macros that called in-line functions that were declared in the now-legacy wspiapi.h header. When including Winsock2.h nowadays with VS 2003 (.NET) and higher, you don't include the wspiapi.h header anymore and use the usual function-declarations from ws2tcpip.h instead. Those functions now are resolved at run-time to be loaded from ws2_32.dll. Windows 2000's version of this dll does not provide these functions, because at that time they did not yet exist and were expanded to in-line functions instead.

The problem with asio

And here comes the trouble with boost::asio: boost::asio does not let you include the old Winsock-headers when compiling for a Windows-target greater than NT-version 5.1 (this means XP). It will #error out with the message "WinSock.h has already been included".

Resolution

To circumvent this, you need to define _WIN32_WINNT to something lower than 0x0501. 0x0500 is quite resonable for this.