Microsoft KB Archive/165942

= How To Write Data to a File Using WriteFile API =

Article ID: 165942

Article Last Modified on 7/15/2004

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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

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This article was previously published under Q165942



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
 Start a new Standard EXE project. Add MODULE1.BAS.  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



