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From: jpn@genrad.com (John P. Nelson)
Newsgroups: comp.os.msdos.misc
Subject: Re: Max files open
Message-ID: <38897@genrad.UUCP>
Date: 8 Oct 90 17:59:22 GMT
References: <659@demott.COM> <1990Oct5.223922.2798@herald.usask.ca>
Sender: news@genrad.UUCP
Reply-To: jpn@genrad.UUCP (John P. Nelson)
Organization: GenRad, Inc., Concord, Mass.
Lines: 58

>>> No matter what I set "FILES=" in CONFIG.SYS
>>>the maximum files that I can open is 15. Is there any way in which this
>>>can be increased.
>> 
>It also depends on your DOS version.  Some versions of DOS (as recently
>as 3.20) allow FILES=[>20] but in reality could only accomodate 20
>files.

This answer is VERY misleading.

Every version of DOS (at least since 2.0, I don't know about 1.0)
allows you to set FILES greater than 20.  This has no effect on the
per-application file limit of 20:  FILES is a system-wide parameter.
The FILES parameter was primarily intended to allow more file
descriptors on systems that use lots of TSRs or multitaskers.

On the other hand, it has always (since DOS 2.0) been possible to
increase the size of the per-task file descriptor limit, on a task by
task basis.  The per-task limit of 20 files is caused by the fact that
the default file descriptor table is located in the PSP, which itself
is a fixed length item.  However, the PSP contains both the
fixed-length table, and a pointer to that table (and a length
variable).  It is possible to create a larger task file descriptor
table by 1.  allocating a block of memory for it, 2.  copying the
contents of the fixed-length file descriptor table from the PSP,  3.
changing the pointer in the PSP to point to the new table, and 4.
changing the length in the PSP to the new length.

All that happened in DOS 3.3 is that Microsoft sanctioned this
technique by adding a new DOS service that does the above 4 steps for
you.  (As I recall, though, at least some copies of DOS 3.3 do not
implement this correctly, and will fail for certain table size
requests).  However, the default application file descriptor table size
is still 20 (since the PSP hasn't changed) and the old technique of
doing the 4 steps manually still works.

>Your best bet would be to upgrade your version of DOS.

Actually, I doubt that this will help very much.

First, unless an application is written to check the version of DOS,
and to USE the table-size function, you don't gain any benefit from
upgrading to a new version of DOS.  Since applications which use the
"manual" technique work with ANY version of DOS, this is a much more
attractive technique for the programmer to use.

In any case, this doesn't help someone who is using a binary executable,
not a program he has written himself.

Even for the programmer who wants to use this technique, there are
pitfalls.  For instance, high level language libraries may assume that
the fixed-length table is being used, and may not be able to handle the
larger table.

     john nelson

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