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CALL32.TXT
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CALL32.DLL: 32-bit DLL calling library for Visual Basic
by Peter Golde
This program is placed in the public domain.
Please feel free to redistribute as you wish.
No guarantees are made as to its suitability or
usefulness, and no support can be provided.
1. Summary
----------
CALL32.DLL is a DLL that can be used for calling routines in 32-bit
DLLs on Windows NT. It cannot be used on Windows 3.1, Win32s, Chicago,
or other operating systems. Using it, a Visual Basic program, running
in the Win16 subsystem, can declare and call functions in any 32-bit
DLL (including, but not limited to, the system DLLs). CALL32.DLL works
on both the x86 and MIPS versions on NT. It has not been tested
on Alpha or other versions, but should work.
2. Usage
--------
To call a function in a 32-bit DLL, follow the following steps. Declare
the "Declare32" function as follows (all one one line):
Declare Function Declare32 Lib "call32.dll" (ByVal Func$,
ByVal Library$, ByVal Args$) As Long
Next, declare the function you wish to call. Declare it in the ordinary
fashion, with the following exceptions:
- Use a library name of "call32.dll"
- Use an Alias of "Call32"
- Add an additional argument at the end, of type ByVal Long
For example, if you are calling the function:
GetWindowText(HWND hwnd, LPSTR lpsz, int cch)
declare it as follows (remember that ints and all handles are 32 bits,
so use a Long):
Declare Function GetWindowText Lib "call32.dll" Alias "Call32"
(ByVal hwnd As Long, ByVal lpsz As String,
ByVal cch As Long, ByVal id As Long) As Long
In the initialization section of your application, you declare the
actual library and name of the function you want to call with
the Declare32 function. Pass it the name of the function, the
library, and a string describing the argument types. Each letter
in the string declares the type of one argument, and should be
either "i" for a 32 bit integer or handle type, "p" for any
pointer type, or "w" for an HWND parameter you want to pass
a 16 bit HWND to and have be automatically converted to a 32 bit
HWND. The return value of Declare32 should be saved away in
a global variable to be passed as the last parameter to the
function you declared earlier. So, continue the example, you
would call:
idGetWindowText = Declare32("GetWindowText", "user32", "wpi")
(As a side note, this would be more properly declared as
"GetWindowTextA", since this is the real exported name. However,
Declare32 will automatically add an "A" to the end of a
function name if necessary).
To call the function, you would just call:
cbCopy = GetWindowText(hwnd, sz, cb, idGetWindowText)
3. Data Types and Handles
-------------------------
It is important to use the correct data types when calling DLL
functions. There are two important points to pay attention
to when using CALL32.DLL. First, only 32 bit integers can
be passed to a DLL procedures. Since virtually all 32 bit
functions take int, UINT, LONG, DWORD, or HANDLE parameters,
which are all 32 bits, this is not a major restriction. However,
you must remember to always declare functions arguments are
Long, and not Integer.
Secondly, 16 bit handles and 32 bit handles are not interchangable.
For example, a 16 bit bitmap handle that you get from calling
a 16 bit DLL or from the VB environment cannot be passed to
a 32 bit function expecting a bitmap handle. Similarly, a
32 bit handle gotten from a 32 bit function cannot be passed to a
16 bit DLL. The only exception is window handles (HWND). If
you declare a function parameter with the "w" letter in the
argument description string passed to Declare32, the corresponding
parameter will be automatically converted from a 16 bit HWND to
a 32 bit HWND when the call is made. You must still declare the
argument as a LONG. This is convenient, for example, when passing
the value returned by the "hWnd" property of a control to a
32 bit DLL function. Only windows created by your application
can be translated.
Summary of data types:
C data type Type specified in Declare Character for Declare32
int, UINT ByVal Long i
LONG, DWORD ByVal Long i
HANDLE ByVal Long i
WORD, short not supported
HWND ByVal Long w (i for no 16->32
translation)
LPSTR ByVal String p
LPLONG, LPDWORD,
LPUINT, int FAR * Long p
LPWORD Integer p
4. Note on Declare32 function names
-----------------------------------
Declare32 will automatically try three different names for
the function name you pass in. First, it uses the exact
name you pass in. If that function name isn't found,
it converts the name to the stdcall decorated name convention,
by adding an underscore at the beginning, and adding "@nn" at
the end, where "nn" is the number of bytes of arguments. If
that name isn't found, it adds an "A" to the end of the original
name to try the Win32 ANSI function calling convention.
5. Run-time Error Summary
-------------------------
The following run-time errors can be generated by CALL32.DLL
30001 Can't load DLL: "|" (error=|)
The DLL name passed to Declare32 was not the name of a
valid 32 bit DLL. The Win32 error code is specified at the
end of the error message, this can help determine why
the DLL didn't load.
30002 Can't find specified function
The function name passed to Declare32 could not be found
in the DLL.
30003 Invalid parameter definition string
The parameter definition string passed to Declare32 had
an invalid character in it, or was too long (32 parameters
is the limit).
30004 Not running on Windows NT
The program is not running in the Windows16 subsystem of
Windows NT.
30005 Invalid window handle
The 16 bit window handle passed as a parameter declared
with the 'w' character was not a valid 16 bit window handle,
or refers to a window from a different process.
6. Change History
------------------
1.00 Original Version
(8/31/93)
1.01 Better error message when DLL can't be loaded
(9/27/93) Stdcall name decoration support
Source code available
Sample Bezier program from Adam Rauch