HOWTO: Write Data to a File Using WriteFile API

• Microsoft Visual Basic Control Creation, Learning, Professional, and Enterprise Editions for Windows, version 5.0
• Microsoft Visual Basic Standard, Professional, and Enterprise Editions, 32-bit only, for Windows, version 4.0

SUMMARY

The Win32 API allows binary files to be opened and written using the OpenFile 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