Use New Flags to Speed Up C Run-time Low-Level I/O Functions

The information in this article applies to:

• The C Run-time (CRT) included with: Microsoft Visual C++, 32-bit Edition, versions 1.0, 2.0, 2.1, and 4.0

SUMMARY

The low-level file open functions in the C Run-time library for the 32-bit Edition of Visual C++ accept some additional flags to allow you to take advantage of Win32-specific file system features. These flags are not available in the 16-bit version of Visual C++.

The low-level file open functions are: _open, _wopen, _sopen, and _wsopen. The new flags (and others) can be combined together with the bitwise OR operator and passed as the 'oflag' argument to the file open functions.

The file open functions call the Win32 function 'CreateFile' to actually open the file with the specified options. The following table relates these new C Run-time flags to the equivalent flags that are passed to CreateFile.

C Run-time Flag     CreateFile Flag

_O_SHORT_LIVED      FILE_ATTRIBUTE_TEMPORARY
_O_TEMPORARY        FILE_FLAG_DELETE_ON_CLOSE
_O_RANDOM           FILE_FLAG_RANDOM_ACCESS
_O_SEQUENTIAL       FILE_FLAG_SEQUENTIAL_SCAN

MORE INFORMATION

You can use these flags to improve file I/O performance in some cases. For example, if you need to read from a large file sequentially, and do not need to jump to different parts of the file, you most likely will improve the performance of your application by using the _O_SEQUENTIAL flag when opening the file. This translates to the FILE_FLAG_SEQUENTIAL_SCAN flag for CreateFile.

For more information, please refer to the following sources:

Microsoft Knowledge Base: Read these articles

   ARTICLE-ID: Q98756
   TITLE     : Increased Performance Using FILE_FLAG_SEQUENTIAL_SCAN

   ARTICLE-ID: Q103237
   TITLE     : Using Temporary File Can Improve Application Performance

   _sopen/_wsopen functions.  Search on "_open" and "_sopen."

Win32 Programmer's Reference, Volume 3, Function Reference: Read the description of WIN32 API CreateFile.