Do Not Mix Operating System and CRT File Handles

• Microsoft Visual C++, 32-bit Edition, versions 2.0, 2.1, 2.2, 4.0, 5.0

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 );