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

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Article ID: 131324 - View products that this article applies to.
This article was previously published under Q131324
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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
98756Increased Performance Using FILE_FLAG_SEQUENTIAL_SCAN
103237Using Temporary File Can Improve Application Performance
Visual C++ Books Online: Read the descriptions of the _open/_wopen and
   _sopen/_wsopen functions.  Search on "_open" and "_sopen."
				
Visual C++ Run-Time Library Reference, Part 2, Run-Time Reference: Read the descriptions of _open/_wopen and _sopen/_wsopen.

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

Properties

Article ID: 131324 - Last Review: December 9, 2003 - Revision: 2.0
APPLIES TO
  • The C Run-Time (CRT), when used with:
    • Microsoft Visual C++ 1.0 Professional Edition
    • Microsoft Visual C++ 2.0 Professional Edition
    • Microsoft Visual C++ 2.1
    • Microsoft Visual C++ 4.0 Standard Edition
Keywords: 
kbcrt KB131324
Retired KB Content Disclaimer
This article was written about products for which Microsoft no longer offers support. Therefore, this article is offered "as is" and will no longer be updated.

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