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How To Write Data to a File Using WriteFile API

This article was previously published under Q165942
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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 flexibilityto write and read from files. This article demonstrates a technique towrite large amounts of data, in the form of a large array, to a binaryfile 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
kbVBp kbdsd kbDSupport KBWIN32SDK KBAPI
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Article ID: 165942 - Last Review: 12/04/2015 16:40:15 - Revision: 3.2

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