INFO: Win32 Replacement for the hmemcpy Function

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The hmemcpy function is no longer available in Windows 32-bit development systems. This article describes the Win32 Replacement for HMEMCPY function.


Excerpted from "Hardcore Visual Basic" by Bruce McKinney, published by Microsoft Press:

CopyMemory: A Strange and Terrible Saga

Experienced Basic API programmers have come to know, if not love, the hmemcpy function. When C's weak data typing and Basic's strong data typing meet in the Windows API, hmemcpy is frequently called on to mediate. The h in the name indicates that hmemcpy can handle huge memory (greater than 64K bytes), but Basic programmers rarely need it for such large chunks. Unfortunately, if you look for hmemcpy in the Win32 documentation, you'll come up with nothing - not even a note saying that the function is obsolete.

But you might happen to run across the Win32 CopyMemory function, which has exactly the same arguments and in fact looks like the same procedure. The h has disappeared because all memory in 32-bit mode is huge. If you write a Declare statement for CopyMemory, however, giving KERNEL32.DLL as the most likely library, you'll get nothing but an error indicating that no such function exists. In fact, you can search all the 32-bit DLLs with the DumpBin utility, but you won't find any containing CopyMemory.

But a careful search of Win32 C include files turns up the following in WINBASE.H:
   #define CopyMemory RtlCopyMemory
   #define MoveMemory RtlMoveMemory
   #define ZeroMemory RtlZeroMemory

This C equivalent of an alias indicates that CopyMemory is another name for a function called RtlCopyMemory. Don't ask why; just check for RtlCopyMemory in KERNEL32.DLL. Again, nothing. A little more sleuthing in Win32 include files reveals the reason. WINNT.H contains something like this:
   #define RtlCopyMemory(dst, src, len) memcpy(dst, src, len)

In other words, RtlCopyMemory is an alias for the C memcpy function, but you can't use memcpy or any other C library function from Basic. If it's not exported from a DLL, you can't call it.

But KERNEL32.DLL does contain an entry for RtlMoveMemory. If you check the Win32 documentation, you'll see that MoveMemory does the same thing as CopyMemory except that it handles overlapped memory differently. I can't imagine a situation in which a Basic programmer would be copying overlapped memory. No reason not to use MoveMemory instead. Because CopyMemory is more intelligible than hmemcpy, I alias this name for both 16-bit and 32-bit versions:
   #If Win32 Then
      Declare Sub CopyMemory Lib "KERNEL32" Alias "RtlMoveMemory" ( _
         hpvDest As Any, hpvSource As Any, ByVal cbCopy As Long)
      Declare Sub CopyMemory Lib "KERNEL" Alias "hmemcpy" ( _
         hpvDest As Any, hpvSource As Any, ByVal cbCopy As Long)
   #End If

You must still be careful about what you pass to these functions. There are a lot of issues with ByVal versus ByRef depending on whether you're passing strings, UDTs, pointers, and so on.


Article ID: 129947 - Last Review: November 18, 2003 - Revision: 2.0
  • Microsoft Visual Basic 4.0 Standard Edition
  • Microsoft Visual Basic 4.0 Professional Edition
  • Microsoft Visual Basic 4.0 32-Bit Enterprise Edition
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