C++ Seeking with CreateFile instead of fopen

Question

I would like to exchange FILE* with HANDLE=CreateFile() to test the speed difference.

I am trying to change my code now.

Could somebody tell me how I chould change these 2 lines to reflect that I am dealing with a handle now and not a FILE* anymore?

fseek(myFile,iBytePos,SEEK_SET);
fread(&SomeValues[0],iByteCount,1,myFile);

I tried

LARGE_INTEGER l;

l.QuadPart=iBytePos;
SetFilePointer(myFile,l.LowPart,&l.HighPart,FILE_BEGIN);

DWORD dw;
BOOL result = ReadFile(myFile,&SomeValues[0],iByteCount,&dw,NULL);

but something is not correct. I must have gone wrong somewhere.

Thank you very much for the help!

Answer 1

The Windows API equivalent for fread is ReadFile and for fseek is SetFilePointer. If you merely replace those calls the performance difference will be slim, if any. In contrast to fseek, SetFilePointer supports files > 2GB. If you don't need that the call is simply:

SetFilePointer(myFile, iBytePos, NULL, FILE_BEGIN);

You can speed up disk I/O using the Windows API by exploiting its greater flexibility. If you have lots of seek-read-seek-read operations you might benefit from using ReadFileScatter instead.

Another potential route for optimization would be to use Asynchronous I/O. A comparison can be found at Synchronous and Asynchronous I/O.

Answer 2

For fseek you can use SetFilePointerEx or SetFilePointer.

For fread you can use ReadFile or ReadFileEX

You can find help here

http://msdn.microsoft.com/en-us/library/windows/desktop/aa365542(v=vs.85).aspx