DOCUMENT:Q51434  25-JUL-2001  [visualc]
TITLE   :HOWTO: Calculate Available File Handles at Run Time
PRODUCT :Microsoft C Compiler
PROD/VER:MS- DOS:6.0,6.00a,6.00ax,7.0;OS/2:6.0,6.00a;WIN3X:1.0,1.5;WINNT:1.0,2.0;
OPER/SYS:
KEYWORDS:kbCRT kbVC

======================================================================

-------------------------------------------------------------------------------
The information in this article applies to:

 - The C Run-Time (CRT), included with:
    - Microsoft C for MS-DOS, versions 5.1, 6.0, 6.0a, 6.0ax 
    - Microsoft C for OS/2, versions 5.1, 6.0, 6.0a 
    - Microsoft C/C++ for MS-DOS, version 7.0 
    - Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5 
    - Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0 
-------------------------------------------------------------------------------

SUMMARY
=======

The following code sample allows you to determine how many CRT file handles are
available at run time for a particular process. The function makes use of the
predefined pointers _iob and _lastiob, which are set to point to the first and
the last input/output information blocks, respectively. The I/O blocks are
examined to determine whether they are in use, and a total number of available
file handles is produced. There is no equivalent C library function currently
available.

NOTE: This is not necessarily an indication of the maximum number of physical
files that can be opened. The system-wide limit is set in MS-DOS by FILES= in
the CONFIG.SYS file. In OS/2, this limit is arbitrarily governed by system
resources and is modified for each process by DosSetMaxFH().

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

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

      /*
      Compile options needed: /DNO_EXT_KEYS under Windows NT, none otherwise

      NOTE: This code will not compile unless it is compiled as a .C file
            and not a .CPP.
      */ 

      #define FILE struct _iobuf
      #ifndef NO_EXT_KEYS           /* Extensions enabled */ 
          #define _CDECL  cdecl
          #define _NEAR   near
      #else                         /* Extensions not enabled */ 
          #define _CDECL
          #define _NEAR
      #endif                        /* NO_EXT_KEYS */ 

      #define _IOREAD 0x01          /* Open for read bit */ 
      #define _IOWRT  0x02          /* Open for write bit */ 
      #define _IORW   0x80          /* Open for read/write bit */ 

      FILE                          /* File handle table entry */ 
      {
          char *_ptr;
          int   _cnt;
          char *_base;
          char  _flag;
          char  _file;
       }_NEAR _CDECL _iob[], /*Set to first I/O block at run time*/ 
                     *_lastiob;   /* Set to last I/O block */ 

      /*
         All of the above definitions were taken from the STDIO.H header
         file except for _lastiob[], which is not defined. This information
         was extracted to make the example more clear.
      */ 

      /*
       - The following macro will check the availability of a file handle
         by examining the _flag member of the I/O block
      */ 

      #define inuse(s) ((s)->_flag & (_IOREAD|_IOWRT|_IORW))

      /*
       - The following routine loops through the total number of I/O blocks
         and checks the flags to see if it is used or not. The number of
         unused handles is returned, which can be 1 to the maximum number of
         file handles as set by the operating system or the FILES= command
         in the CONFIG.SYS file.
      */ 

      int Number_Of_Handles(void)
      {
         FILE *stream = _iob;
         int count;

         count = 0;
         do
            if (inuse(stream)) count++;
         while(stream++ < _lastiob);

         return(((_lastiob - _iob)+1)-count);
      }

      void main(void)
      {
         int i;

         i = Number_Of_Handles();
      /* i is now set to the number of available file handles */ 
      }

The table of I/O blocks that is being checked here was allocated at run time
according to the maximum number of file handles allowed per process.

Additional query words:

======================================================================
Keywords          : kbCRT kbVC 
Technology        : kbVCsearch kbAudDeveloper kbCRT
Version           : MS- DOS:6.0,6.00a,6.00ax,7.0;OS/2:6.0,6.00a;WIN3X:1.0,1.5;WINNT:1.0,2.0;
Issue type        : kbhowto

=============================================================================

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