Creating an asynchronous "ReadFileMany()" on top of ReadFile() - c

Question

I'm trying to make a function, ReadFileMany, which imitates ReadFile's interface, which is given a list of (offset, length) pairs to read from, and which reads all portions of the file asynchronously.
I'm doing this inside ReadFileMany by using RegisterWaitForSingleObject to direct a thread pool thread to wait for I/O to complete, so that it can call ReadFile again to read the next portion of the file, etc.
The trouble I'm running into is that I can't seem to be able to mimic a certain behavior of ReadFile.
Specifically, file handles can themselves be used like events, without the need for an event handle:
OVERLAPPED ov = {0};
DWORD nw;
if (ReadFile(hFile, buf, buf_size, &nw, &ov))
{
if (WaitForSingleObject(hFile, INFINITE) == WAIT_OBJECT_0)
{
...
}
}
but of course, if the user waits on a file handle, he might get a notification that an intermediate read has complete, rather than the final read.
So, inside ReadFileMany, I have no choice but to pass an hEvent parameter to ReadFile for all but the last portion.
The question is, is there any way to still allow the user to wait for the file handle to become signaled when all the portions have been read?
At first the answer seems obvious: just avoid passing an event handle when reading the last portion!
That works fine if the read will be successful, but not if there are errors. If ReadFile starts suddenly returning an error on the last read, I will need to set the file handle to a signaled state manually in order to allow the reader to wake up from his potential call to WaitForSingleObject(hFile).
But there seems to be no way to set a file handle to a signaled state without performing actual I/O... so this is where I get stuck: is there any way for me to write this function so that it behaves like ReadFile on the outside, or is it impossible to do it correctly?

Answer 1

Assuming you are trying to read multiple sections of a single file, I would simply allocate an array of OVERLAPPED structures, one for each requested section of the file, kick off all of the ReadFile() calls at one time, and then use WaitForMultipleObjects() to wait for all of the I/Os to finish, eg:
struct sReadInfo
{
OVERLAPPED ov;
LPVOID pBuffer;
DWORD dwNumBytes;
DWORD dwNumBytesRead;
bool bPending;
sReadInfo()
{
memset(&ov, 0, sizeof(OVERLAPPED));
pBuffer = NULL;
dwNumBytes = 0;
dwNumBytesRead = 0;
bPending = false;
ov.hEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
if (!ov.hEvent) throw std::exception();
}
~sReadInfo()
{
CloseHandle(hEvent);
}
};
.
bool error = false;
try
{
std::vector<sReadInfo> ri(numSections);
std::vector<HANDLE> h;
h.reserve(numSections);
for(int i = 0; i < numSections; ++i)
{
ULARGE_INTEGER ul;
ul.QuadPart = ...; // desired file offset to read from
sReadInfo *r = &ri[i];
r->ov.Offset = ul.LowPart;
r->ov.OffsetHigh = ul.HighPart;
r->pBuffer = ...; // desired buffer to read into
r->dwNumBytes = ...; // desired number of bytes to read
if (!ReadFile(hFile, r->pBuffer, r->dwNumBytes, &r->dwNumBytesRead, &r->ov))
{
if (GetLastError() != ERROR_IO_PENDING)
throw std::exception();
r->bPending = true;
h.push_back(r->ov.hEvent);
}
}
if (!h.empty())
{
if (WaitForMultipleObjects(h.size(), &h[0], TRUE, INFINITE) != WAIT_OBJECT_0)
throw std::exception();
}
for (int i = 0; i < numSections; ++i)
{
sReadInfo *r = &ri[i];
if (r->bPending)
GetOverlappedResult(hFile, &r->ov, &r->dwNumBytesRead, FALSE);
// ...
}
}
catch (const std::exception &)
{
CancelIo(hFile);
return false;
}
return true;