Q51434: HOWTO: Calculate Available File Handles at Run Time

Article: Q51434
Product(s): Microsoft C Compiler
Version(s): 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;
Operating System(s): 
Keyword(s): kbCRT kbVC
Last Modified: 25-JUL-2001

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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 
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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:

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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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