Q139640: Do Not Mix Operating System and CRT File Handles

Article: Q139640
Product(s): Microsoft C Compiler
Version(s): WinNT:2.0,2.1,2.2,4.0,5.0; Windows:2.0,2.1
Operating System(s): 
Keyword(s): kbcode kbCRT kbVC kbVC200 kbVC210 kbVC220 kbVC400 kbVC500 kbVC600
Last Modified: 31-JUL-2001

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The information in this article applies to:

- Microsoft Visual C++, versions 2.0, 2.1, 2.2, 4.0 
- Microsoft Visual C++, 32-bit Enterprise Edition, versions 5.0, 6.0 
- Microsoft Visual C++, 32-bit Professional Edition, versions 5.0, 6.0 
- Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
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SUMMARY
=======

In 32-bit Windows-based applications, Windows API functions (OpenFile,
CreateFile, and so on) return a file handle that cannot be substituted for a
handle returned by a C Runtime Function (_open, _wopen, and so on). You can
convert an operating system handle to a CRT handle by using the CRT function
_open_osfhandle() as demonstrated in the "Sample Code" section of this article.

NOTE: This distinction is not Win32 specific. However, some programmers using
16-bit products on 16-bit operating systems have mixed operating system and CRT
file handles.

MORE INFORMATION
================

The following sample code shows how to open a file stream from a file handle
returned by the OpenFile Windows API function.

Sample Code
-----------

  /* Compile options needed:none
  */ 
  #include <stdlib.h>
  #include <stdio.h>
  #include <fcntl.h>
  #include <io.h>
  #include <windows.h>

  void main( void )
  {
     HFILE OsFileHandle;
     FILE *stream;
     int CrtFileHandle;
     OFSTRUCT Buffer;
     int count = 0;
     char inbuf[128];

      if ( (OsFileHandle = OpenFile("test.dat", &Buffer, OF_READ))== -1){
          printf( "OpenFile Failed");
          exit(1);
          }

      /* convert OS file handle to CRT file pointer */ 

      if ( (CrtFileHandle=_open_osfhandle(OsFileHandle,_O_RDONLY))==-1){
          printf(  "_open_osfhandle Failed");
          exit(1);
          }

     /* Change handle access to stream access. */ 
     if( (stream = _fdopen( CrtFileHandle, "r" )) == NULL ) {
          printf(  "_fdopen Failed");
          exit( 1 );
        }

     while( fgets( inbuf, 128, stream ) != NULL )
        count++;

     /* After _fdopen, close with fclose */ 
     fclose( stream );
     printf( "Lines in file: %d\n", count );
  }

Additional query words: Windows 95

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Keywords          : kbcode kbCRT kbVC kbVC200 kbVC210 kbVC220 kbVC400 kbVC500 kbVC600 
Technology        : kbVCsearch kbVC400 kbAudDeveloper kbVC220 kbVC500 kbVC600 kbVC200 kbVC210 kbVC32bitSearch kbVC500Search
Version           : WinNT:2.0,2.1,2.2,4.0,5.0; Windows:2.0,2.1
Issue type        : kbinfo

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