How do I call SetWindowLong() in the 64-bit versions of Windows?

Question

In the header file WinUser.h, there is a part in which the constants of the second parameter of SetWindowLong() are defined.

// ...

#define GWL_WNDPROC         (-4)
#define GWL_HINSTANCE       (-6)
#define GWL_HWNDPARENT      (-8)
#define GWL_STYLE           (-16)
#define GWL_EXSTYLE         (-20)
#define GWL_USERDATA        (-21)
#define GWL_ID              (-12)

#ifdef _WIN64

#undef GWL_WNDPROC
#undef GWL_HINSTANCE
#undef GWL_HWNDPARENT
#undef GWL_USERDATA

#endif /* _WIN64 */

#define GWLP_WNDPROC        (-4)
#define GWLP_HINSTANCE      (-6)
#define GWLP_HWNDPARENT     (-8)
#define GWLP_USERDATA       (-21)
#define GWLP_ID             (-12)

// ...

But they are right after undefined if _WIN64 is defined; and it is defined in my 64-bit system.

As you see, there is also a GWLP_*** set of constants, but they are not documented in the page of SetWindowLong().

Why are these constants undefined in x64 systems?
What is the alternative way of calling SetWindowLong() in x64 systems?

My system:
OS: Windows 7 Ultimate x64 SP1
IDE: Visual Studio 2012 Ultimate Update 3

Right at the top of the documentation is *Note This function has been superseded by the SetWindowLongPtr function. To write code that is compatible with both 32-bit and 64-bit versions of Windows, use the SetWindowLongPtr function.* — chris, Aug 12 '13 at 02:11
@chir I can't find that note in the documentation page I gave in my question. — hkBattousai, Aug 12 '13 at 08:53
I guess the mobile version of the documentation doesn't include the note. — chris, Aug 14 '13 at 08:48

score 51 · Accepted Answer · answered Aug 12 '13 at 02:09

51

Some of the window data values (the ones that refer to "pointer sized" objects like the window procedure, for example) need to be 64 bit in an x64 build. The old SetWindowLong() and GetWindowLong() functions are limited to DWORD sized (32 bit) values for backwards compatibility, and Microsoft have introduced new versions, SetWindowLongPtr() and GetWindowLongPtr() that allow you to work with pointer-sized values (32 bit in a 32 bit build, and 64 bit in a 64 bit build).

These days it is recommended that you always use SetWindowLongPtr() and the GWLP_xxx constants, whether you are building for 32 or 64 bit, but in a 64 bit build you need to use the new functions and so the defines are #undefined to cause build errors that force you to fix your code.

answered Aug 12 '13 at 02:09

Jonathan Potter

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I hit this issue and this answer helped me. Thank you! – Damian Feb 12 '19 at 06:14
Is it harmless to use GetWindowLong/SetWindowLong on GWL_STYLE and GWL_EXSTYLE since those never exceed 32 bits of data? – William Jan 01 '20 at 06:33
@William Yes that's fine. – Jonathan Potter Jan 01 '20 at 07:39

score 2 · Answer 2 · answered Sep 23 '20 at 07:07

    As specified in <WinUser.h>
    //If config is _WIN64 then use new versioned macro

    #define GWLP_WNDPROC        (-4)
    #define GWLP_HINSTANCE      (-6)
    #define GWLP_HWNDPARENT     (-8)
    #define GWLP_USERDATA       (-21)
    #define GWLP_ID             (-12)

    //else for _WIN32

    #undef GWL_WNDPROC
    #undef GWL_HINSTANCE
    #undef GWL_HWNDPARENT
    #undef GWL_USERDATA

KeyC0de · Answer 3 · 2021-12-04T01:51:50.893

2

Also make sure you're NOT defining the following:

#define NOWINOFFSETS

which disables GWL_*, GCL_*, ie. GetWindowLongPtr and family, as well as associated routines.

edited Dec 04 '21 at 01:51

answered Jan 06 '21 at 20:28

KeyC0de

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How do I call SetWindowLong() in the 64-bit versions of Windows?

3 Answers3

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