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Visual Basic 6 Black Book:Connecting To The Windows API And Visual C++

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Visual Basic 6 Black Book

(Publisher: The Coriolis Group)

Author(s): Steven Holzner

ISBN: 1576102831

Publication Date: 08/01/98

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Now we can call PlaySound directly when the user clicks a command button, Command1 ; here, we pass it a

value of &H20000 to indicate that we’re reading the sound from a file and ignore the function’s return value this

way:

Private Sub Command1_Click()

retVal = PlaySound("c:\windows\media\Tada.wav", 0&, &H20000)

End Sub

And that’s it—now we’re playing sounds with a Windows API function.

Allocating Memory And Storing Data

One reason programmers use Windows API calls is to work with a lot of memory, and you can use the

GlobalAlloc (allocate memory), GlobalLock (lock the memory and get a pointer to it), GlobalUnlock (unlock the

memory), and GlobalFree (deallocate the memory) functions for that. We’ll take a look at the first two of these

functions in this topic, and the last two in the following topic. We’ll also see how to copy data into and out of our

newly allocated memory with the MoveMemory function.

Here’s how you use GlobalAlloc to allocate memory; this function returns a non-zero handle to the memory if

successful:

Function GlobalAlloc Lib "kernel32" (ByVal wFlags As Long, ByVal

dwBytes As Long) As Long

You set the flags you want to use in wFlags, and the number of memory bytes you want in dwBytes. Here are the

possible flags to use with GlobalAlloc:

• GMEM_FIXED—&H0

• GMEM_MOVEABLE—&H2

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Visual Basic 6 Black Book:Connecting To The Windows API And Visual C++

• GMEM_NOCOMPACT—&H10

• GMEM_NODISCARD—&H20

• GMEM_ZEROINIT—&H40

• GMEM_MODIFY—&H80

• GMEM_DISCARDABLE—&H100

• GMEM_NOT_BANKED—&H1000

• GMEM_SHARE—&H2000

• GMEM_DDESHARE—&H2000

• GMEM_NOTIFY—&H4000

• GMEM_LOWER—GMEM_NOT_BANKED

• GMEM_VALID_FLAGS—&H7F72

• GMEM_INVALID_HANDLE—&H8000

To get a pointer to the memory and so put it to use, you use GlobalLock, passing it the memory handle you got

from GlobalAlloc. GlobalLock returns a non-zero pointer to the memory if successful:

Function GlobalLock Lib "kernel32" (ByVal hMem As Long) As Long

Besides GlobalAlloc and GlobalLock, you can move data into the memory you’ve allocated with MoveMemory:

Sub MoveMemory Lib "kernel32" Alias "RtlMoveMemory" (ByVal dest As _

Any, ByVal src As Any, ByVal length As Long)

Here are what the arguments to MoveMemory mean:

• dest—Pointer to the destination buffer

• src—Pointer to the source buffer

• length—Number of bytes to move

Let’s see an example. Here, we’ll store a string of text that the user types into a text box in memory; in the next

topic in this chapter, we’ll read that string back. This example will give us a good general overview of working

with memory and memory buffers.

We start by setting up a 40-character-long buffer for the string to store in the form’s (General) declarations section:

Const DataLength = 40

Dim outbuffer As String * DataLength

...

We also declare the memory handle and pointer we’ll use:

Const DataLength = 40

Dim outbuffer As String * DataLength

Dim hMemory As Long

Dim hMemoryPointer As Long

...

Finally, we declare the functions we’ll use, GlobalAlloc, GlobalLock, and MoveMemory, as well as the memory

flag we’ll use, GMEM_MOVEABLE, which means that Windows can move the memory we are using if it needs

to as part of its memory-handling operations:

Const DataLength = 40

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Visual Basic 6 Black Book:Connecting To The Windows API And Visual C++

Dim outbuffer As String * DataLength

Dim hMemory As Long

Dim hMemoryPointer As Long

Private Declare Function GlobalAlloc Lib "kernel32" (ByVal wFlags As_

Long, ByVal dwBytes As Long) As Long

rivate Declare Function GlobalLock Lib "kernel32" (ByVal hMem As Long) _

As Long

Private Declare Sub MoveMemory Lib "kernel32" Alias "RtlMoveMemory" _

(ByVal dest As Any, ByVal src As Any, ByVal length As Long)

Const GMEM_MOVEABLE = &H2

When the user clicks a command button, Command1, we will allocate and lock the memory, and store the text

string now in Text1 in it. We start by storing the text from the text box in the buffer we’ve named outbuffer:

Private Sub Command1_Click()

outbuffer = Text1.Text

...

Next, we use GlobalAlloc to allocate the memory we’ll use:

Private Sub Command1_Click()

outbuffer = Text1.Text

hMemory = GlobalAlloc(GMEM_MOVEABLE, DataLength)

...

Next, we pass the memory handle from GlobalAlloc to GlobalLock to get a pointer to the memory we’ve

allocated:

Private Sub Command1_Click()

outbuffer = Text1.Text

hMemory = GlobalAlloc(GMEM_MOVEABLE, DataLength)

hMemoryPointer = GlobalLock(hMemory)

...

Finally, we copy the data from our buffer to our newly allocated memory with MoveMemory (note that because

MoveMemory is a subroutine, we use the Call keyword instead of assigning a return value to a variable):

Private Sub Command1_Click()

outbuffer = Text1.Text

hMemory = GlobalAlloc(GMEM_MOVEABLE, DataLength)

hMemoryPointer = GlobalLock(hMemory)

Call MoveMemory(hMemoryPointer, outbuffer, DataLength)

End Sub

And that’s it—when the user clicks Command1, we copy the text string to our allocated memory.

We’ve stored data in allocated memory now—how do we read it back? We’ll take a look at that in the next topic.

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Visual Basic 6 Black Book:Connecting To The Windows API And Visual C++

Reading Data From Memory And Deallocating Memory

In the previous topic, we stored a text string from a text box, Text1, in memory when the user clicked a button,

Command1. In this topic, we’ll read the string back when the user clicks another button, Command2, and display

that string in a new text box, Text2. We’ll also free and deallocate the memory we’ve used.

We’ll use MoveMemory to read the data from memory:

Sub MoveMemory Lib "kernel32" Alias "RtlMoveMemory" (ByVal dest As _

Any, ByVal src As Any, ByVal length As Long)

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Visual Basic 6 Black Book:Connecting To The Windows API And Visual C++

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Visual Basic 6 Black Book

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ISBN: 1576102831

Publication Date: 08/01/98

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Here are what the arguments to MoveMemory mean:

• dest—Pointer to the destination buffer

• src—Pointer to the source buffer

• length—Number of bytes to move

To allocate memory, we used GlobalAlloc to get a handle to a memory area; to use that memory, we used

GlobalLock to get a pointer to the memory area. To unlock memory, you pass a pointer to that memory to

GlobalUnlock:

Function GlobalUnlock Lib "kernel32" (ByVal hMem As Long) As Long

To free memory, you pass a memory handle to GlobalFree:

Function GlobalFree Lib "kernel32" (ByVal hMem As Long) As Long

Let’s see this at work. Here, we add the declarations of the functions we’ll use to the program we developed in the

previous topic, as well as a buffer, inbuffer, to store the data we read:

Const DataLength = 40

Const GMEM_MOVEABLE = &H2

Dim outbuffer As String * DataLength

Dim inbuffer As String * DataLength

Dim memHandle As Long

Dim memPointer As Long

Private Declare Function GlobalAlloc Lib "kernel32" (ByVal wFlags As Long,_

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Visual Basic 6 Black Book:Connecting To The Windows API And Visual C++

ByVal dwBytes As Long) As Long

Private Declare Function GlobalLock Lib "kernel32" (ByVal hMem As Long) _

As Long

Private Declare Sub MoveMemory Lib "kernel32" Alias "RtlMoveMemory"

(ByVal dest As Any, ByVal src As Any, ByVal length As Long)

Private Declare Function GlobalFree Lib "kernel32" (ByVal hMem As Long) _

As Long

Private Declare Function GlobalUnlock Lib "kernel32" (ByVal hMem As Long) _

As Long

Then, when the user clicks Command2, we can use the memory pointer we created in the program in the previous

topic, hMemoryPointer, with MoveMemory, to copy the string to the buffer:

Private Sub Command2_Click()

Call MoveMemory(inbuffer, hMemoryPointer, DataLength)

...

Now we can move the string to Text2 from inbuffer:

Private Sub Command2_Click()

Call MoveMemory(inbuffer, hMemoryPointer, DataLength)

Text2.Text = inbuffer

...

Finally, we unlock and free the memory we’ve used:

Private Sub Command2_Click()

Call MoveMemory(inbuffer, hMemoryPointer, DataLength)

Text2.Text = inbuffer

GlobalUnlock (hMemoryPointer)

GlobalFree (hMemory)

End Sub

That’s it—when you run the program, as shown in Figure 23.10, and click the Store Text In Memory button, the

text in the top text box, Text1, is stored in memory. When you click the Read Text From Memory button, the

string is read back and displayed in the bottom text box, Text2, as you see in Figure 23.10. Now we’re using

memory with the Windows API.

Figure 23.10 Storing and reading memory data.

The code for this example is located in the winmemory folder on this book’s accompanying CD-ROM.

Making A Window Topmost

You can use SetWindowPos to make a window “topmost,” which means it’ll always stay on top of other

windows:

Function SetWindowPos Lib "user32" (ByVal hwnd As Long, ByVal _

hWndInsertAfter As Long, ByVal x As Long, ByVal y As Long, ByVal _

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Visual Basic 6 Black Book:Connecting To The Windows API And Visual C++

cx As Long, ByVal cy As Long, ByVal wFlags As Long) As Long

Here’s what the arguments to this function mean:

• hwnd—Handle of the window to work with.

• hWndInsertAfter—Handle to the window that will precede this window in the Z-order. This parameter

can be a window handle set to one of the following values: HWND_BOTTOM (= 1; places the window at

the bottom of the Z-order), HWND_TOP (= 0; places the window at the top of the Z-order),

HWND_TOPMOST (= –1; places the window above all non-topmost windows), or

HWND_NOTOPMOST (= –2; repositions the window to the top of all non-topmost windows).

• x—Specifies the new position of the left side of the window.

• y—Specifies the new position of the top of the window.

• cx—Specifies the new width of the window.

• cy—Specifies the new height of the window.

• wFlags—Specifies sizing and positioning options, as shown in Table 23.5.

Table 23.5SetWindowPos flags.

Constant

Meaning

SWP_DRAWFRAME (= &H20)

Draws a frame (defined when the window was

created) around the window.

SWP_FRAMECHANGED (= &H20)

Sends a WM_NCCALCSIZE message to the

window, even if the window’s size is not being

changed. If this flag is not specified,

WM_NCCALCSIZE is sent only when the

window’s size is being changed.

SWP_HIDEWINDOW (=&H80)

Hides the window.

SWP_NOACTIVATE (= &H10)

Does not activate the window. If this flag is not set,

the window is activated and moved to the top of

either the topmost or the non-topmost group

(depending on the setting of the hWndInsertAfter

parameter).

SWP_NOCOPYBITS (= &H100)

Discards the entire contents of the client area. If this

flag is not specified, the valid contents of the client

area are saved and copied back into the client area

after the window is sized or repositioned.

SWP_NOMOVE (= 2)

Retains current position (ignores the x and y

parameters).

SWP_NOOWNERZORDER (= &H200)

Does not change the owner window’s position in the

Z-order.

SWP_NOREDRAW (= 8)

Does not redraw changes. If this flag is set, no

repainting of any kind occurs.

SWP_NOREPOSITION (= &H200)

Same as SWP_NOOWNERZORDER.

SWP_NOSIZE (= 1)

Retains current size (ignores the cx and cy

parameters).

SWP_NOZORDER (= 4)

Retains current ordering (ignores hWndInsertAfter).

SWP_SHOWWINDOW (=&H40)

Displays the window.

Let’s see an example. Here, we’ll size a window to 100 x 100 pixels and make it a topmost window. We start by

declaring SetWindowPos in our program, as well as the constants we’ll use, SWP_SHOWWINDOW,

SWP_DRAWFRAME, and HWND_TOPMOST:

Private Declare Function SetWindowPos Lib "user32" (ByVal hwnd As Long, _

ByVal hWndInsertAfter As Long, ByVal x As Long, ByVal y As Long,

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Visual Basic 6 Black Book:Connecting To The Windows API And Visual C++

ByVal cx As Long, ByVal cy As Long, ByVal wFlags As Long) As Long

Const HWND_TOPMOST = -1

Const SWP_SHOWWINDOW = &H40

Const SWP_DRAWFRAME = &H20

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Visual Basic 6 Black Book:Connecting To The Windows API And Visual C++

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Visual Basic 6 Black Book

(Publisher: The Coriolis Group)

Author(s): Steven Holzner

ISBN: 1576102831

Publication Date: 08/01/98

Bookmark It

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Table of Contents

Now when the window loads, we can make the window topmost this way:

Private Sub Form_Load()

retVal = SetWindowPos(Me.hwnd, HWND_TOPMOST, 100, 100, _

100, 100, SWP_DRAWFRAME Or SWP_SHOWWINDOW)

End Sub

That’s all it takes. Now the window will stay on top of other windows, as you see in Figure 23.11. Our topmost

program is a success.

Figure 23.11 Making a window topmost.

Determining Free And Total Disk Space

The Testing Department is calling again. Your program, SuperDuperDataCrunch, is overflowing users’ disks.

Can’t you check how much disk space is available before writing out your 800MB database? Hmm, you think,

how would that work?

You can use the GetDiskFreeSpace function to check how much space there is on a disk:

Private Declare Function GetDiskFreeSpace Lib "kernel32" Alias _

GetDiskFreeSpaceA (ByVal lpRootPathName As String, _

lpSectorsPerCluster As Long, lpBytesPerSector As Long, _

lpNumberOfFreeClusters As Long, lpTotalNumberOfClusters As Long) _

As Long

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Here’s what this function’s arguments mean:

• lpRootPathName—The disk you want to check (for example, “c:\”)

• lpSectorsPerCluster—The number of sectors per cluster on the disk

• lpBytesPerSector—The number of bytes per sector

• lpNumberOfFreeClusters—The number of free clusters

• lpTotalNumberOfClusters—The total number of clusters

To find the total free space, you multiply SectorsPerCluster * BytesPerSector * NumberOfFreeClusters. To find

the total space on the drive, you multiply SectorsPerCluster * BytesPerSector * TotalNumberOfClusters.

Let’s see an example. Here, we’ll determine the free space on the c: drive. We start by declaring

GetDiskFreeSpace:

Private Declare Function GetDiskFreeSpace Lib "kernel32" Alias _

"GetDiskFreeSpaceA" (ByVal lpRootPathName As String,_

lngSectorsPerCluster As Long, lngBytesPerSector As Long, _

lngNumberOfFreeClusters As Long, lngTotalNumberOfClusters As Long)_

As Long

Next, when the form loads, we fill the variables lngSectorsPerCluster, lngBytesPerSector,

lngNumberOfFreeClusters, and lngTotalNumberOfClusters with GetDiskFeeSpace for drive C:

Private Sub Form_Load()

Dim lngSectorsPerCluster As Long

Dim lngBytesPerSector As Long

Dim lngNumberOfFreeClusters As Long

Dim lngTotalNumberOfClusters As Long

retVal = GetDiskFreeSpace("c:\", lngSectorsPerCluster, _

lngBytesPerSector, lngNumberOfFreeClusters, lngTotalNumberOfClusters)

...

Finally, we can display the total number of free bytes in a label, Label1, this way:

Private Sub Form_Load()

Dim lngSectorsPerCluster As Long

Dim lngBytesPerSector As Long

Dim lngNumberOfFreeClusters As Long

Dim lngTotalNumberOfClusters As Long

retVal = GetDiskFreeSpace("c:\", lngSectorsPerCluster, _

lngBytesPerSector, lngNumberOfFreeClusters, _

lngTotalNumberOfClusters)

Label1.Caption = "Free space on drive c: " & _

Str(lngSectorsPerCluster * lngBytesPerSector * _

lngNumberOfFreeClusters) & " bytes"

End Sub

When you run the program, you’ll see the number of free bytes on the computer’s C: drive, something like the

display in Figure 23.12.

Figure 23.12 Determining free disk space.

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