How To Write Data to a File Using WriteFile API

How To Write Data to a File Using WriteFile API
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SUMMARY
The Win32 API allows binary files to be opened and written using the CreateFile,...
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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.
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MORE INFORMATION
Start a new Standard EXE project. Add MODULE1.BAS. Type the following code into...
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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

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REFERENCES
Microsoft Win32 SDK Win32 Programmers Reference ReadFileCreateFileWriteFileFlush...
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Microsoft Win32 SDK Win32 Programmers Reference
ReadFileCreateFileWriteFileFlushFileBuffersCloseFile
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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