How To Write Data to a File Using WriteFile API

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Article ID: 165942 - View products that this article applies to.
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SUMMARY

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.

MORE INFORMATION

  1. Start a new Standard EXE project. Add MODULE1.BAS.
  2. Type the following code into MODULE1.BAS:
          Public Const GENERIC_WRITE = &H40000000
          Public 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.
    
          Type MyType
             value As Integer
          End Type
    
          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
    
          Declare Function FlushFileBuffers Lib "kernel32" ( _
            ByVal hFile As Long) As Long
    
          Sub fillArray(anArray() As MyType)
             Dim x As Integer
    
             For x = 0 To UBound(anArray)
                anArray(x).value = x
             Next x
          End Sub
    
          Sub Main()
             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
             readArray FILE_NAME, S           'Read into a different array
          End Sub
    
          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
    
          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
    
    						

REFERENCES

Microsoft Win32 SDK

Win32 Programmers Reference
ReadFile
CreateFile
WriteFile
FlushFileBuffers
CloseFile

Properties

Article ID: 165942 - Last Review: July 15, 2004 - Revision: 3.2
APPLIES TO
  • Microsoft Visual Basic 6.0 Learning Edition
  • Microsoft Visual Basic 6.0 Professional Edition
  • Microsoft Visual Basic 6.0 Enterprise Edition
  • Microsoft Visual Basic 5.0 Control Creation Edition
  • Microsoft Visual Basic 5.0 Learning Edition
  • Microsoft Visual Basic 5.0 Professional Edition
  • Microsoft Visual Basic 5.0 Enterprise Edition
  • Microsoft Visual Basic 4.0 Standard Edition
  • Microsoft Visual Basic 4.0 Professional Edition
  • Microsoft Visual Basic 4.0 32-Bit Enterprise Edition
Keywords: 
kbhowto kbapi KB165942

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