#ifdef _WIN32

When writing processor-specific code, the _M_IX86, _M_AMD64 and _M_IA64 can be used for conditional compilation — so far, so good. But sometimes code is not exactly processor-specific but rather specific to the natural machine word length (i.e. 32 bit or 64 bit). Fur such situations, there are defines, too — however there is a little catch: For ancient 16 bit code, there is _WIN16. For 64 bit, the WDK build environment defines _WIN64 by default. Given these two macros, it is tempting to conclude that _WIN32 should only be defined for 32 bit builds — however this is not the case. As it turns out, _WIN32 is always defined, both for 32 and 64 bit builds.

And yes, this behaviour is documented on MSDN, but it is stupid anyway.

However, where _WIN32 can be of use is when writing code targeting multiple platforms — as _WIN32 is always defined, it can be used as an indicator that you compile for Windows, regardless of the compiler used (another option is to use _MSC_VER, but that is compiler-specific).


3 Responses to “#ifdef _WIN32”

  1. 1 Roman Ryltsov May 2, 2011 at 1:04 pm

    Yeah it may look sick but perhaps it has quite some reasons behind the decision. _WIN32 is introduced quite some time ago and compared to 16-bit Windows it was a completely different world. Not so much difference now between 32-bit and 64-bit code and I am pretty sure they decided to keep _WIN32 defined for 64-bit code for compatibility and code migration reasons.

  2. 2 WndSks May 4, 2011 at 10:00 am

    It is not a big problem in practice since you often end up writing something like:

    #ifdef _WIN64

    For cross platform builds, the fact that _WIN32 is defined for 32 and 64 also comes in handy:

    #ifdef _WIN32

  3. 3 Somasundaram July 8, 2011 at 10:24 pm


    Is there a way we can avoid these #ifdef to make code compatible with both 32 bit and 64 bit?


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About me

Johannes Passing lives in Berlin, Germany and works as a Solutions Architect at Google Cloud.

While mostly focusing on Cloud-related stuff these days, Johannes still enjoys the occasional dose of Win32, COM, and NT kernel mode development.

He also is the author of cfix, a C/C++ unit testing framework for Win32 and NT kernel mode, Visual Assert, a Visual Studio Unit Testing-AddIn, and NTrace, a dynamic function boundary tracing toolkit for Windows NT/x86 kernel/user mode code.

Contact Johannes: jpassing (at) hotmail com

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