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

i observe weird behavior while using flag file_flag_no_buffering overlapped i/o. invoke series of readfile() function calls , query statuses later using getoverlappedresult().

the weird behavior speaking of though file handles , readfile() calls returned without bad error(except error_io_pending expected), 'bytes read' value returned getoverlappedresult() call 0 of files, , each time run code - different set of files. if remove file_flag_no_buffering, things start working , no bytes read value zero.

here how have implemented overlapped i/o code 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 , 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 cases unexpectedly 0 */                 /* no file of size 0 or lesser 512 bytes(physical volume sector size) */             totalbytesread += bytesread;             closehandle(handle);         }     }      return totalbytesread; } 

with file_flag_no_buffering absent, totalbytesread value 57 mb. flag present, totalbytesread value lower 57 mb , keeps changing each time run code ranging 2 mb 15 mb.

your calculation of bytestoread produce 0 result when file size less g_bytesperphysicalsector. small files requesting 0 bytes.