Can I use CreateFile, but force the handle into a std::ofstream?
Question
Is there any way to take advantage of the file creation flags in the Win32 API such as FILE_FLAG_DELETE_ON_CLOSE
or FILE_FLAG_WRITE_THROUGH
as described here http://msdn.microsoft.com/en-us/library/aa363858(VS.85).aspx , but then force that handle into a std::ofstream?
The interface to ofstream is obviously platform independent; I'd like to force some platform dependent settings in 'under the hood' as it were.
Solution
It is possible to attach a C++ std::ofstream
to a Windows file handle. The following code works in VS2008:
HANDLE file_handle = CreateFile(
file_name, GENERIC_WRITE,
0, NULL, CREATE_ALWAYS,
FILE_ATTRIBUTE_NORMAL, NULL);
if (file_handle != INVALID_HANDLE_VALUE) {
int file_descriptor = _open_osfhandle((intptr_t)file_handle, 0);
if (file_descriptor != -1) {
FILE* file = _fdopen(file_descriptor, "w");
if (file != NULL) {
std::ofstream stream(file);
stream << "Hello World\n";
// Closes stream, file, file_descriptor, and file_handle.
stream.close();
file = NULL;
file_descriptor = -1;
file_handle = INVALID_HANDLE_VALUE;
}
}
This works with FILE_FLAG_DELETE_ON_CLOSE
, but FILE_FLAG_WRITE_THROUGH
may not have the desired effect, as data will be buffered by the std::ofstream
object, and not be written directly to disk. Any data in the buffer will be flushed to the OS when stream.close()
is called, however.
OTHER TIPS
Some of these flags are also available when using _fsopen / fopen:
FILE* pLockFile = _fsopen(tmpfilename.c_str(), "w", _SH_DENYWR );
if (pLockFile!=NULL
{
// Write lock aquired
ofstream fs(pLockFile);
}
Here we open the file so when doing a flush, then it writes through (And it is deleted when closed):
FILE* pCommitFile = fopen(tmpfilename.c_str(), "wcD");
if (pCommitFile!=NULL)
{
// Commits when doing flush
ofstream fs(pCommitFile);
}