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Q61236: C 6.00 README: Increasing the Maximum Number of Open Files

Article: Q61236
Product(s): See article
Version(s): 6.00   | 6.00
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | | mspl13_c
Last Modified: 25-APR-1990

The following information is taken from the C Version 6.00 README.DOC
file.

Increasing the Maximum Number of Open Files
-------------------------------------------

C 6.00 allows you to increase the maximum number of files that may be
open for I/O (the default number is 20). To use this feature, you must
be running either OS/2 or DOS version 3.30 or later. Use the
procedures described in the remainder of this section to increase the
maximum number of open files.

Increasing File Handles
-----------------------

To increase the number of file handles, edit the start-up source file
CRT0DAT.ASM, which is provided in this release. Change the line

   _NFILE_ = 20

so that _NFILE_ is set to the desired maximum. For example, to
increase the maximum number of available file handles to 40, change
the line as shown here:

   _NFILE_ = 40

Note: Increasing the number of file handles allows you to use
low-level I/O functions, such as open and read, with more files.
However, it does not affect the number of stream-level I/O files (that
is, the number of FILE * streams).

Increasing Streams
------------------

To increase the number of streams, edit the source file _FILE.C.
Change the line

   #define _NFILE_ 20

to set _NFILE_ to the desired maximum. For example, to allow a maximum
of 40 streams, change the line as shown here:

   #define _NFILE_ 40

Increasing the number of streams allows you to use stream-level I/O
functions, such as fopen and fread, with more files.

Note: The number of low-level file handles must be greater than or
equal to the number of stream-level files. Thus, if you increase the
value of _NFILE_ in the module _FILE.C, you must also increase the
value of _NFILE_ in the module CRT0DAT.ASM.

Increasing the System Limit
---------------------------

To use more than 20 files at a time, you must increase the file limit
imposed on your process by the operating system.

To increase the system-wide limit, increase the number of files
available on your system as a whole by editing your system
configuration file (CONFIG.SYS). For example, to allow 100 open files
at a time on your system, put this statement in the configuration
file:

   FILES=120

To increase the process-by-process limit, you must also increase the
number of files the operating system makes available to your
particular process. To do this, edit CRT0DAT.ASM and enable the
commented-out code that is preceded by the appropriate description.

In the DOS version of CRT0DAT.ASM, for example, the commented-out code
appears as shown here:

   ;       mov     ah,67h
   ;       mov     bx,_NFILE_
   ;       callos

In the OS/2 version of CRT0DAT.ASM, the code appears as a call to
DOSSETMAXFH. Under OS/2, you must also enable the "extrn
DOSSETMAXFH:far" declaration that appears near the beginning of the
file.

In either case, remove the semicolon (;) comment characters.

Note: Under OS/2, you must take into account the fact that each
process has the potential to "own" open files. When planning how many
open files to allow on a system-wide basis, take this into account.

Using the Modified Start-Up Files
---------------------------------

After you modify CRT0DAT.ASM and/or _FILE.C, assemble or compile the
file(s). The start-up MAKEFILE contains sample command lines to
perform these jobs. Note that the object files will differ for OS/2
and DOS.

To use the new object files, either explicitly link your program with
the new CRT0DAT.OBJ and _FILE.OBJ file(s), or replace the CRT0DAT.OBJ
and _FILE.OBJ object(s) in the appropriate model of the C run-time
library.

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