What happens to WPARAM, LPARAM, and LRESULT when they travel between 32-bit and 64-bit windows?

Raymond Chen

Raymond

The integral types WPARAM, LPARAM,
and LRESULT are 32 bits wide on 32-bit systems
and 64 bits wide on 64-bit systems.
What happens when a 32-bit process sends a message to a 64-bit window
or vice versa?

There’s really only one choice when
converting a 64-bit value to a 32-bit value: Truncation.
When a 64-bit process sends a message to a 32-bit window,
the 64-bit WPARAM and LPARAM values
are truncated to 32 bits.
Similarly, when a 64-bit window returns an LRESULT back
to a 32-bit sender, the value is truncated.

But converting a 32-bit value to a 64-bit value introduces a choice:
Do you zero-extend or sign-extend?

The answer is obvious if you remember

the history of WPARAM,
LPARAM, and LRESULT
,
or if you just look at the header file.

The WPARAM is zero-extended,
while LPARAM and LRESULT
are sign-extended.

If you remember that WPARAM used to be a WORD
and LPARAM and LRESULT used to be LONG,
then this follows from the fact that WORD is an unsigned type
(therefore zero-extended) and LONG is a signed type
(therefore sign-extended).

Even if you didn’t know that, you could look it up in the header file.

typedef UINT_PTR WPARAM;
typedef LONG_PTR LPARAM;
typedef LONG_PTR LRESULT;

UINT_PTR is an unsigned type (therefore zero-extended)
and
LONG_PTR is a signed type (therefore sign-extended).

Raymond Chen
Raymond Chen

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