Binaryworld - How To Read/Write Data File Using ReadFile and WriteFile API ... [ VB -> Files & Folder ]

The Win32 API allows binary files to be opened and written using the CreateFile, ReadFile, and WriteFile APIs. These functions offer increased flexibility to write and read from files. This article demonstrates a technique to write large amounts of data, in the form of a large array, to a binary file all at once instead of element by element.

Step-By-Step Example

- Create a standard exe project
- Add the following code in form1

Click here to copy the following block

Const GENERIC_WRITE = &H40000000
Const GENERIC_READ = &H80000000
Const FILE_ATTRIBUTE_NORMAL = &H80
Const CREATE_ALWAYS = 2
Const OPEN_ALWAYS = 4
Const INVALID_HANDLE_VALUE = -1
Const FILE_NAME = "TEST.DAT" 'This can be any file that does not
'currently exist.

Private Type MyType
  value As Integer
End Type

Private Declare Function ReadFile Lib "kernel32" (ByVal hFile As Long, _
    lpBuffer As Any, ByVal nNumberOfBytesToRead As Long, _
    lpNumberOfBytesRead As Long, ByVal lpOverlapped As Long) As Long

Private Declare Function CloseHandle Lib "kernel32" ( _
    ByVal hObject As Long) As Long

Private Declare Function WriteFile Lib "kernel32" ( _
    ByVal hFile As Long, lpBuffer As Any, _
    ByVal nNumberOfBytesToWrite As Long, _
    lpNumberOfBytesWritten As Long, ByVal lpOverlapped As Long) As Long

Private Declare Function CreateFile Lib "kernel32" _
    Alias "CreateFileA" (ByVal lpFileName As String, _
    ByVal dwDesiredAccess As Long, ByVal dwShareMode As Long, _
    ByVal lpSecurityAttributes As Long, _
    ByVal dwCreationDisposition As Long, _
    ByVal dwFlagsAndAttributes As Long, ByVal hTemplateFile As Long) _
    As Long

Private Declare Function FlushFileBuffers Lib "kernel32" ( _
    ByVal hFile As Long) As Long

Private Sub fillArray(anArray() As MyType)
  Dim x As Integer

  For x = 0 To UBound(anArray)
    anArray(x).value = x
  Next x
End Sub

Private Sub readArray(Fname As String, anArray() As MyType)
  Dim fHandle As Long
  Dim fSuccess As Long
  Dim sTest As String
  Dim lBytesRead As Long
  Dim BytesToRead As Long

  'Get size of data to write
  BytesToRead = (UBound(anArray) + 1) * LenB(anArray(0))
  'Get a handle to a file Fname.
  fHandle = CreateFile(Fname, GENERIC_WRITE Or GENERIC_READ, _
      0, 0, OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL, 0)
  'Here you should test to see if you get a file handle or not.
  'CreateFile returns INVALID_HANDLE_VALUE if it fails.
  If fHandle <> INVALID_HANDLE_VALUE Then
    fSuccess = ReadFile(fHandle, anArray(LBound(anArray)), _
        BytesToRead, lBytesRead, 0)
    'ReadFile returns a non-zero value if it is successful.
    'Now you just close the file.
    fSuccess = CloseHandle(fHandle)
  End If
End Sub

Private Sub writearray(Fname As String, anArray() As MyType)
  Dim fHandle As Long
  Dim fSuccess As Long
  Dim sTest As String
  Dim lBytesWritten As Long
  Dim BytesToWrite As Long
  'Get the length of data to write
  BytesToWrite = (UBound(anArray) + 1) * LenB(anArray(0))
  'Get a handle to a file Fname.
  fHandle = CreateFile(Fname, GENERIC_WRITE Or GENERIC_READ, _
      0, 0, OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL, 0)
  'Here you should test to see if you get a file handle or not.
  'CreateFile returns INVALID_HANDLE_VALUE if it fails.
  If fHandle <> INVALID_HANDLE_VALUE Then
    fSuccess = WriteFile(fHandle, anArray(LBound(anArray)), _
        BytesToWrite, lBytesWritten, 0)
    'Check to see if you were successful writing the data
    If fSuccess <> 0 Then
      'Flush the file buffers to force writing of the data.
      fSuccess = FlushFileBuffers(fHandle)
      'Close the file.
      fSuccess = CloseHandle(fHandle)
    End If
  End If
End Sub

Private Sub Form_Load()
  Dim fHandle As Integer
  Dim T(1000) As MyType  'Define a large array of data
  Dim S(1000) As MyType  'Define another large array

  fillArray T       'Fill the array with some values
  writearray FILE_NAME, T 'Write the entire array to disk
  MsgBox "Written to the disk"
  readArray FILE_NAME, S 'Read into a different array
  MsgBox "Read from the disk"
End Sub