Calling APIs

Calling APIs

What is an API?

API (Application Program Interface) is a function in DLL (Dynamic link library). Windows operating system is based on a couple of DLLs, such as Kernel32, User32, and GDI32. These APIs form the core part of Windows operating system. A DLL is a collection of functions (APIs). DLL is loaded into memory when any of the function is referred by an application. And once  a DLL is loaded in to memory DLL remains in memory and functions in DLL are dynamically bound to applications.  That means when an application calls a function in a DLL, the code of the function is not inserted into application (if it is done, it is called as static linking) and instead at run time the code of function is made available to application from DLL.


Dynamic linking is especially very efficient in Windows operating system, as a collection of application can run at the same time and may also use the same set of functions. So instead of placing one copy of each function in each application, it is better to provide one copy of functions and let all applications dynamically bind to functions.


Why to call APIs from Visual Basic?

Well, APIs are functions in DLLs and these APIs are what applications use indirectly or directly to get the job done in Windows OS. For instance, to show a window (a form in Visual Basic), to resize a window or to add an item to list box Windows use APIs.  All that is fine. But why should a Visual Basic programmer worry about all this? Because you say, I add an item using AddItem method of List box and show a form using Show method and so on. Accepted. But that is not the entire programming.


There are tasks (as you will see later) that you cannot accomplish just with commands of Visual Basic. You need to go beyond Visual Basic and access these APIs to get the job done. As you can see in figure 15.1, in most of the cases a command in Visual Basic ends up calling one of the APIs. But in some cases you do not find any command in Visual Basic that does the job for you. In this case you need to call APIs for VB application.

Figure 14.1: Calling API from Visual Basic application.

However, to call an API you need to know everything about API. First of all you should know whether there is any API that does the job that Visual Basic cannot do for you.  Then you must know the number of parameters, type of each parameter and the type of return value.  The following are the details you should have before you call an API.


  • The name of API
  • Details of Parameters such as number of parameters and type of each parameter
  • The library (DLL) in which API is available
  • What type of value the API returns?


How to call an API from Visual Basic?

In order to call an API from VB, you have to take two steps, they are :


  • Declaring API using Declare Function or Declare Sub
  • Calling API just like how you call a Visual Basic function or procedure.

Declaring an API

The first thing that you have to do to call an API is declare it in your project. This is generally done in code module. However, it can be done in General/Declarations of any module.


Here is the complete syntax of Declare statement.


Declare {Function|Sub}  FunctionName  Lib  “LibraryName” [Alias

“Aliasname”]   (Argument list)  [As Returntype]


The following sections will explain how to use each part of the syntax.


Function or Sub

Specifies whether API returns a value or not. If API returns a value then it is to be declared as a Function otherwise as a Subroutine. (Sub)



This is the name that you want to use to call API in your program. This name may be different from the actual name of the API. In this case, you also have to specify Alias and give actual name as the alias.


Lib “LibraryName”

Specifies the name of the library (DLL) in which the declared API is existing.  This indicates to Visual Basic where to look for API. The library should be available for Visual Basic. If it is not a standard library such as Kernel32, User32 or GDI32 then it is better you specify the complete path.


Note: Suffix 32 for a library name denotes 32-bit library. If you are using 16-bit library, i.e. for windows 3.x then you just have to give Kernel and not Kernel32.


Alias “AliasName”

Specifies the actual name of API when FuctionName is different from actual name. This enables you to change the name of API in your program so that name is more easier to call or more meaningful.


Argument List

Specifies the list of arguments (also called as parameters). For each argument you have to specify how argument is passed – by value or by reference, the name of argument and type of argument.


An argument is passed either by value or by reference. When a value is passed by value, its value is passed to function and changes made to the argument that corresponds to the parameter will not effect the value passed. That means, for example,  if you pass variable X to a function by value then changes made by function  argument will not effect the value of variable X.  On the other hand, if an argument is passed by reference, the reference(address) of the variable is passed to function and as the result any changes made to argument in function will effect the value of variable that is passed to function.


Keyword ByVal is used to specify pass by value and ByRef is used to specify pass by reference.


Passing Strings

For strings, ByVal means convert a Visual Basic string to C string. It doesn’t mean that the string is passed by value. String is always passed by reference as its address is passed and not the value.


Also note, that when you pass a string, you have to create required amount of space in Visual Basic string before it is passed to an API. Because APIs use C string convention, which assumes that the string points the location that is allocated to it.


Here is an example:


‘ declare a string of 255 locations

Dim st as string * 255

‘ call API and pass string to it

V = APIFunction(st)



The above example can also be rewritten in this way:


‘ Declare a normal Visual Basic string

Dim st as string


St = space (255)    ‘ create space for 255 spaces

V = APIFunction (st)


Data Types used in API calls

When we deal with APIs we deal with C data types. So we have to convert Visual Basic data types to C data types.  The following table lists out the C data types and corresponding Visual Basic data types.


C Data Type Size

Visual Basic Data Type

BOOL 32 Boolean
BYTE 8 Byte
Char 8 String * 1
Double 64 Double
DWORD 32 Long
Float 32 Single
HANDLE 32 Long
Int 32 Long
Long 32 Long
LPTSTR 32 No equivalent
LPCTSTR 32 No equivalent
Short 16 Integer
UINT 32 Long
ULONG 32 Long
USHORT 16 Integer
UCHAR 8 String * 1
WORD 16 Integer

Table 14.1: C data types and their equivalent data types in Visual Basic.



What is a Handle?

As long as you use Visual Basic command you live under cover. You do not know what is really happening outside Visual Basic. But the moment you decide to call an API, you need to understand what is happening under the hood. Handle is one of them. VB allows you to access object using object oriented syntax. But what’s really happening in Windows is concealed.


Every window is assigned and identified by a handle, which is a 32 bit number ( long integer). Windows OS identifies windows using handle. Remember each control, such as Text box, Command button, is also a window. These windows are called as control windows. So each control and form has a handle using which Windows OS identifies the controls and forms.


If you ever have to take the handle of a control or a form in Visual Basic, use hWnd property of the form or control. This property returns the handle of the form or control that is assigned by Windows OS.



Getting API declaration from WIN32API.TXT

When you want to get declarations of standard APIs, you can use WIN32API.TXT text file. It contains Visual Basic declarations for all standard APIs, constants, and Types.


Use program API Text View to load the text file and get required declarations. API Text Viewer is an Add-in. So it is to be first added to VBIDE.


You can run it using the following steps:


  1. Select Add-Ins menu and choose Add-In Manager
  2. Visual Basic displays Add-In Manager as shown in figure 14.2.
  3. Double click on VB 6 API Viewer to load it into VBIDE.
  4. Then click on Ok to close Add-In Manager window. This adds API Viewer to Add-Ins menu.
  5. Now again select Add-Ins menu and select API Viewer option.
  6. API Viewer starts. It doesn’t display anything initially. See figure 14.3.
  7. Select File-> Load Text File… option and select TXT file.
  8. API Viewer displays all the function declarations form WIN32API.TXT as shown in figure 14.2.

Figure 14.2: Loading Visual Basic 6 API Viewer.

Figure 14.3: API Viewer displaying WIN32API.TXT.

Sample Application

We will develop an application to display information regarding the following in different tabs.


  • Windows Operating system
  • Processor and Memory
  • Selected Disk


Each tab displays information regarding one of the above mentioned topics. We will use TabStrip control to displays multiple tabs.


What is TabStrip Control?

TabStrip control is used to display multiple pages of information in one page by displaying one page at a time. Each page is called as a tab. Each tab is like a mini-form. Each tab contains a title. When user clicks on the title of the tab, the page that corresponds to the tab will be displayed.


To load TabStrip control into project:


  1. Select Project -> Components
  2. In Components dialog box, check Microsoft Windows Common Controls 6.0.
  3. Click on


When a TabStrip control is placed on the form, it has only one tab. You need to create additional tabs, if you want, using the following procedure.


To create tabs in TabStrip control:

  1. Invoke property pages of TabStrip control by selecting Properties option in Context menu.
  2. Select Tabs tab in Property Pages.
  3. Click on Insert Tab button to add a new tab. Repeat the process for each tab.

Note: TabStrip is not a container control. You have to use other containers such as Frame control to place controls on Tabs of TabStrip control.


We will understand more about TabStrip control, as we will proceed.


Designing User Interface

As I have already mentioned in this application user interface is consisting of TabStrip control with three different tabs. First we will create three tabs using the procedure explained above, and then change Caption property of first tab to  “Windows Information”, second tab to “System Information” and third tab to “Disk Information”.


See figure 14.4, 14.5 and 14.6 to know how these three tabs appear at runtime, what is the information displayed in each of these tabs and how controls are arranged.


Figure 14.4:Windows Information Tab.

Figure 14.5: System Information Tab.

Figure 14.6: Disk Information tab displaying information regarding selected drive.

Follow the steps given below to create required controls:


Create a Frame and change the following properties


Name        Frames

Caption     “” (Null string)

Index       1


Index property is used to create a control array. Control array concept explained in chapter 8. A control array is a collection of controls that have the same name but different values for Index property. For all controls in a control array there is only one event procedure for each event.


Place second frame and change the following properties.


Name        Frames

Index       2

Caption     “” (Null string)


Place one more frame (third frame) and change the following properties.


Name        Frames

Index       3

Caption     “” (Null string)


Adding Control to Frames

After frames are added, place required controls on each frame. See figure 17.4, 17.5 and 17.6 for layout of the controls. Most of the controls are Labels. We set BorderStyle property to 1-Fixed Single for labels that display the values.


However, in third frame (Disk information), we have to place a DriveListbox control, which allows user to select the drive for which the user wants to get details.


Adding a module to contain API declarations

Generally we place all API declarations in a code module. Now we add a code module to the project using Project -> Add Module.


Add the Type declarations shown in table 14.2 using API Viewer.


To get the Type declaration from API Viewer, do the following:


  1. Load Win32Api.txt using File-> Load Text File…
  2. Select Types in API Types dropdown list box.
  3. Select one type at a time and click on Add button to add the Type declaration to Selected Items.
  4. Repeat the process until all types listed in table 14.2 are added.


At the end of the above process all three Type declarations should be copied to General/Declarations of the code module, as shown in listing 14.1.


Here is the list of Types taken from API Viewer and what they contain.




MEMORYSTATUS Contains fields that return the information regarding total physical memory, available memory etc.
SYSTEM_INFO Contains fields that return information regarding number of processors, type of processor etc.
OSVERSIONINFO Contains fields that return information regarding platform id, major and minor version etc.

Table 14.2: Type declarations required for sample application.


Add API declarations for the APIs listed in table 14.3 using API Viewer.


To add API declarations, do the following:


  1. Select Declares in API Type dropdown list box.
  2. Select each function listed in table 14.3 and click on Add button to send its declaration to Selected Items.

To transfer selected items to code module in Visual Basic project:


  1. Once all Types and Declarations are copied to Selected Items, click on Copy button to copy selected items to clipboard.
  2. Then activate code module in Visual Basic project and select General/Declarations.
  3. Choose Edit->Paste in VB IDE to paste Type declarations and API declarations that are copied into clipboard, into code module.


dwLength As Long

dwMemoryLoad As Long

dwTotalPhys As Long

dwAvailPhys As Long

dwTotalPageFile As Long

dwAvailPageFile As Long

dwTotalVirtual As Long

dwAvailVirtual As Long

End Type



dwOemID As Long

dwPageSize As Long

lpMinimumApplicationAddress As Long

lpMaximumApplicationAddress As Long

dwActiveProcessorMask As Long

dwNumberOrfProcessors As Long

dwProcessorType As Long

dwAllocationGranularity As Long

dwReserved As Long

End Type



dwOSVersionInfoSize As Long

dwMajorVersion As Long

dwMinorVersion As Long

dwBuildNumber As Long

dwPlatformId As Long

szCSDVersion As String * 128 ‘Maintenance string for PSS usage

End Type


Public Declare Function GetWindowsDirectory Lib “kernel32” Alias   “GetWindowsDirectoryA” (ByVal lpBuffer As String, ByVal nSize As Long) As Long


Public Declare Function GetTempPath Lib “kernel32” Alias “GetTempPathA” (ByVal nBufferLength As Long, ByVal lpBuffer As String) As Long

Public Declare Sub GetSystemInfo Lib “kernel32” (lpSystemInfo As SYSTEM_INFO)


Public Declare Function GetUserName Lib “advapi32.dll” Alias “GetUserNameA” (ByVal lpBuffer As String, nSize As Long) As Long


Public Declare Function GetComputerName Lib “kernel32” Alias “GetComputerNameA” (ByVal lpBuffer As String, nSize As Long) As Long


Public 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


Public Declare Function GetVersionEx Lib “kernel32” Alias “GetVersionExA” (lpVersionInformation As OSVERSIONINFO) As Long


Public Declare Function GetVolumeInformation Lib “kernel32” Alias “GetVolumeInformationA” (ByVal lpRootPathName As String, ByVal lpVolumeNameBuffer As String, ByVal nVolumeNameSize As Long, lpVolumeSerialNumber As Long, lpMaximumComponentLength As Long, lpFileSystemFlags As Long, ByVal lpFileSystemNameBuffer As String, ByVal nFileSystemNameSize As Long) As Long


Public Declare Sub GlobalMemoryStatus Lib “kernel32” (lpBuffer As MEMORYSTATUS)


Public Declare Function GetDriveType Lib “kernel32” Alias “GetDriveTypeA” (ByVal nDrive As String) As Long

Listing 14.1: Type declarations and API declarations.

Here is the list of functions used in the sample application and what they do?




GetWindowsDirectory Returns the name of the directory into which Windows OS is installed.
GetTempPath Returns path for temporary directory (temp).
GetSystemInfo Returns information regarding system into a variable of SYSTEM_INFO type.
GetUserName Returns the name of the current user.
GetComputerName Returns the name of the computer.
GetDiskFreeSpace Returns the space details of the given disk.
GetVersionEx Returns details regarding windows version into a variable of type OSVERSIONINFO.
GetVolumeInformation Returns the details of the specified volume.
GlobalMemoryStatus Returns memory statistics into a variable of type MEMORYSTATUS.
GetDriveType Returns the type of the specified drive.

Table 14.3: APIs used in the sample application.


Writing code to get required information

Write three user-defined functions, GetSystemInformation, GetDiskInformation, and GetWindowsInformation.  These user-defined functions call APIs and get the required information. These functions also populate corresponding labels with the required information. Listing 14.2 shows the code for these three user-defined functions.


Public Sub GetWindowsInformation()



Dim wdir As String * 255

Dim wlen As Long

‘ get windows platform


wlen = GetWindowsDirectory(wdir, 255)

lblwindir.Caption = Left(wdir, wlen)

wlen = GetTempPath(255, wdir)

‘ get

lbltemppath.Caption = Left(wdir, wlen)

winfo.dwOSVersionInfoSize = Len(winfo)

wlen = GetVersionEx(winfo)


With winfo

If .dwPlatformId = 0 Then

lblwinplatform.Caption = “Windows 3.X”

ElseIf .dwPlatformId = 1 Then

lblwinplatform.Caption = “Windows 95”

ElseIf .dwPlatformId = 2 Then

lblwinplatform.Caption = “Windows NT”


lblwinplatform.Caption = “Unknown”


End If


lblversion.Caption = .dwMajorVersion & “.” & .dwMinorVersion


End With


End Sub


Public Sub GetSystemInformation()


Dim sinfo As SYSTEM_INFO


Dim slen  As Long

Dim cname As String * 100

Dim rcode As Long


‘ call API to get information


GetSystemInfo sinfo

With sinfo

lblprocessor.Caption = .dwProcessorType

End With

‘ get memory information

minfo.dwLength = Len(minfo)

GlobalMemoryStatus minfo

With minfo

lbltotalmemory.Caption = .dwTotalPhys

lblfreememory.Caption = .dwAvailPhys

lblutilization.Caption = .dwMemoryLoad

End With


‘ get computer name

slen = 100

rcode = GetComputerName(cname, slen)

lblcomputername.Caption = Left(cname, slen)

‘ get current user name


slen = 100

rcode = GetUserName(cname, slen)

lblcuruser.Caption = Left(cname, slen)



End Sub


Private Sub Drive1_Change()

GetDiskInformation Left(, 1) & “:\”

End Sub


Private Sub Form_Load()


x = TabStrip1.Left + 40

Y = TabStrip1.Top + 300

For i = 1 To 3

frames(i).Visible = False

frames(i).Move x, Y, TabStrip1.Width, TabStrip1.Height – 300


frames(1).Visible = True


‘ get the information



‘ get current drives information

GetDiskInformation Left(CurDir, 3)

End Sub


Private Sub TabStrip1_Click()

‘ make all frames invisible

For i = 1 To 3

frames(i).Visible = False


‘ make the selected frame visible

frames(TabStrip1.SelectedItem.Index).Visible = True

End Sub


Public Sub GetDiskInformation(drive As String)

Dim vname As String * 100

Dim vserial As Long

Dim mcl As Long

Dim vfsys As Long

Dim vfsysname As String * 100

Dim x As Long

Dim dtype As Long

Dim dt As String

‘ get volume information

x = GetVolumeInformation(drive, vname, 100, vserial, mcl, _

vfsys, vfsysname, 100)


lblvolume.Caption = vname

lblserial.Caption = vserial

lblFilesystem.Caption = vfsysname


‘ get disk information


x = GetDiskFreeSpace(drive, nspc, bps, fc, tnc)

lblsectors.Caption = nspc

lblbytespersector.Caption = bps

lblfreeclusters.Caption = fc

lblclusters.Caption = tnc

‘ get drive type

dtype = GetDriveType(drive)


Select Case dtype

Case 0

dt = “Unknown”

Case 1

dt = “Not Available”

Case 2

dt = “Removable”

Case 3

dt = “Fixed”

Case 4

dt = “Remote”

Case 5

dt = “CDROM”

Case 6

dt = “RAM Disk”

End Select


lbldrivetype.Caption = dt

End Sub

Listing 14.2: Code for Sample application.


When you run the project, you will see the first tab (Windows Information) as we have shown that in Load event of the form. Whenever user clicks on any other tab, Click event of TabStrip control occurs. We take Index of the selected item tab and use that index to display the frame that has the same index. See the code for Click event of TabStrip1 (see listing 14.1) for complete code.


Whenever user selects a different drive letter in DriveListBox of Disk Information tab, we invoke GetDiskInformation by sending the name of the selected drive.


The ability to call an API from Visual Basic application is a very important facility. In several cases you find that what you want, cannot be done in Visual Basic and you need to call an API. What we have seen in our sample application is a tiny list of APIs. There are thousands of APIs. Try to understand as many APIs as possible. Because, using APIs you can push limits.