How to append data on a file in win32 - file

Question

I've searched to do this but I can't find what am I doing incorrectly.I'm trying to make this function appends data every time it's called but it always do it once. If the file doesn't exist it creates a new one and write on file for ONLY once if the file exist it does nothing (or maybe overwrite)
void WriteToFile (char data[],wchar_t filename[] )
{
HANDLE hFile;
DWORD dwBytesToWrite = (DWORD)strlen(data);
DWORD dwBytesWritten ;
BOOL bErrorFlag = FALSE;
hFile = CreateFile((LPCWSTR)filename, // name of the write
GENERIC_WRITE, // open for writing
0, // do not share
NULL, // default security
CREATE_NEW, // create new file only
FILE_ATTRIBUTE_NORMAL, // normal file
NULL); // no attr. template
if (hFile == INVALID_HANDLE_VALUE)
{
DisplayError(TEXT("CreateFile"));
_tprintf(TEXT("Terminal failure: Unable to open file \"%s\" for write.\n"), filename);
return;
}
bErrorFlag = WriteFile(
hFile, // open file handle
data, // start of data to write
dwBytesToWrite, // number of bytes to write
&dwBytesWritten, // number of bytes that were written
NULL); // no overlapped structure
if (FALSE == bErrorFlag)
{
DisplayError(TEXT("WriteFile"));
printf("Terminal failure: Unable to write to file.\n");
}
else
{
if (dwBytesWritten != dwBytesToWrite)
{
// This is an error because a synchronous write that results in
// success (WriteFile returns TRUE) should write all data as
// requested. This would not necessarily be the case for
// asynchronous writes.
printf("Error: dwBytesWritten != dwBytesToWrite\n");
}
else
{
_tprintf(TEXT("Wrote %d bytes to %s successfully.\n"), dwBytesWritten, filename);
}
}
CloseHandle(hFile);
}
And this is where I call the function in WM_COMMAND
//When a menu item selected execute this code
case IDM_FILE_SAVE:
saveBool = true;
char Str[] = "this is my own data";
wchar_t filename[] = L"data.txt";
WriteToFile(Str, filename);
break;

Answer 1

if the file exist it does nothing
As it should be. Per the CreateFile() documentation:
CREATE_NEW
1
Creates a new file, only if it does not already exist.
If the specified file exists, the function fails and the last-error code is set to ERROR_FILE_EXISTS (80).
If the specified file does not exist and is a valid path to a writable location, a new file is created.
For what you are attempting to do, use OPEN_ALWAYS instead:
OPEN_ALWAYS
4
Opens a file, always.
If the specified file exists, the function succeeds and the last-error code is set to ERROR_ALREADY_EXISTS (183).
If the specified file does not exist and is a valid path to a writable location, the function creates a file and the last-error code is set to zero.
You can use the FILE_APPEND_DATA access specifier to have CreateFile() automatically seek to the end of the file after creating/opening it (otherwise, you have to seek manually using SetFilePointer/Ex()) before you then write new data to the file.
Try this:
void WriteToFile (char *data, wchar_t *filename)
{
HANDLE hFile;
DWORD dwBytesToWrite = strlen(data);
DWORD dwBytesWritten ;
BOOL bErrorFlag = FALSE;
hFile = CreateFileW(filename, // name of the write
FILE_APPEND_DATA, // open for appending
FILE_SHARE_READ, // share for reading only
NULL, // default security
OPEN_ALWAYS, // open existing file or create new file
FILE_ATTRIBUTE_NORMAL, // normal file
NULL); // no attr. template
if (hFile == INVALID_HANDLE_VALUE)
{
DisplayError(TEXT("CreateFile"));
wprintf(L"Terminal failure: Unable to create/open file \"%s\" for writing.\n", filename);
return;
}
while (dwBytesToWrite > 0)
{
bErrorFlag = WriteFile(
hFile, // open file handle
data, // start of data to write
dwBytesToWrite, // number of bytes to write
&dwBytesWritten, // number of bytes that were written
NULL); // no overlapped structure
if (!bErrorFlag)
{
DisplayError(TEXT("WriteFile"));
printf("Terminal failure: Unable to write to file.\n");
break;
}
wprintf(L"Wrote %u bytes to \"%s\" successfully.\n", dwBytesWritten, filename);
data += dwBytesWritten;
dwBytesToWrite -= dwBytesWritten;
}
CloseHandle(hFile);
}