Thursday, November 22, 2007

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".


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


  1. The workaround for older winsock header problem wont work if you are using boost 1.39.0. They check for _WINSOCKAPI_ before _WIN32_WINNT and hence #error triggers much early.

    The only way to get away with this is to manually include winsoc2.h before including windows.h

  2. Hey, thanks for the info, Hiten!

  3. If i want to use visual studio windows Me or 98, what should i do?
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