FILE_FLAG_NO_BUFFERING with overlapped I/O - bytes read zero - c++

Question

I observe a weird behavior while using the flag FILE_FLAG_NO_BUFFERING with overlapped I/O.
I invoke a series of ReadFile() function calls and query their statuses later using GetOverlappedResult().
The weird behavior that I am speaking of is that even though file handles were good and ReadFile() calls returned without any bad error(except ERROR_IO_PENDING which is expected), the 'bytes read' value returned from GetOverlappedResult() call is zero for some of the files, and each time I run the code - it is a different set of files.
If I remove the FILE_FLAG_NO_BUFFERING, things start working properly and no bytes read value is zero.
Here is how I have implemented overlapped I/O code with FILE_FLAG_NO_BUFFERING.
long overlappedIO(std::vector<std::string> &filePathNameVectorRef)
{
long totalBytesRead = 0;
DWORD bytesRead = 0;
DWORD bytesToRead = 0;
std::map<HANDLE, OVERLAPPED> handleMap;
HANDLE handle = INVALID_HANDLE_VALUE;
DWORD accessMode = GENERIC_READ;
DWORD shareMode = 0;
DWORD createDisposition = OPEN_EXISTING;
DWORD flags = FILE_FLAG_OVERLAPPED | FILE_FLAG_NO_BUFFERING;
DWORD fileSize;
LARGE_INTEGER li;
char * buffer;
BOOL success = false;
for(unsigned int i=0; i<filePathNameVectorRef.size(); i++)
{
const char* filePathName = filePathNameVectorRef[i].c_str();
handle = CreateFile(filePathName, accessMode, shareMode, NULL, createDisposition, flags, NULL);
if(handle == INVALID_HANDLE_VALUE){
fprintf(stdout, "\n Error occured: %d", GetLastError());
fprintf(stdout," getting handle: %s",filePathName);
continue;
}
GetFileSizeEx(handle, &li);
fileSize = (DWORD)li.QuadPart;
bytesToRead = (fileSize/g_bytesPerPhysicalSector)*g_bytesPerPhysicalSector;
buffer = static_cast<char *>(VirtualAlloc(0, bytesToRead, MEM_COMMIT, PAGE_READWRITE));
OVERLAPPED overlapped;
ZeroMemory(&overlapped, sizeof(overlapped));
OVERLAPPED * lpOverlapped = &overlapped;
success = ReadFile(handle, buffer, bytesToRead, &bytesRead, lpOverlapped);
if(!success && GetLastError() != ERROR_IO_PENDING){
fprintf(stdout, "\n Error occured: %d", GetLastError());
fprintf(stdout, "\n reading file %s",filePathName);
CloseHandle(handle);
continue;
}
else
handleMap[handle] = overlapped;
}
// Status check and bytes Read value
for(std::map<HANDLE, OVERLAPPED>::iterator iter = handleMap.begin(); iter != handleMap.end(); iter++)
{
HANDLE handle = iter->first;
OVERLAPPED * overlappedPtr = &(iter->second);
success = GetOverlappedResult(handle, overlappedPtr, &bytesRead, TRUE);
if(success)
{
/* bytesRead value in some cases is unexpectedly zero */
/* no file is of size zero or lesser than 512 bytes(physical volume sector size) */
totalBytesRead += bytesRead;
CloseHandle(handle);
}
}
return totalBytesRead;
}
With FILE_FLAG_NO_BUFFERING absent, totalBytesRead value is 57 MB. With the flag present, totalBytesRead value is much lower than 57 MB and keeps changing each time I run the code ranging from 2 MB to 15 MB.

Answer 1

Your calculation of bytesToRead will produce 0 as a result when the file size is less than g_bytesPerPhysicalSector. So for small files you are requesting 0 bytes.