Q47741: Closing stdprn and stdaux May Not Increase Available Handles
Article: Q47741
Product(s): See article
Version(s): 5.10
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# G890728-23559 | mspl13_c
Last Modified: 16-AUG-1989
Question:
We are trying to close the handles for stdaux and stdprn so that we
can open additional files before hitting the limit of the FILES=
parameter in CONFIG.SYS (which we set to 11). In that mode, regardless
of whether or not we close handles 3 and 4 (stdaux and stdprn), we are
able to open only 8 files (FILES= - 3). The "MS-DOS Encyclopedia"
states that stdin, out, and err are mapped to one device, but use
three handles.
We noticed that if FILES= in CONFIG.SYS is set to 20 or above, the
program DOES get additional files opened when handles 3 and 4 are
closed. Apparently we're getting some sort of interaction between the
process file-handle table/limit (of 20) and the system table/setting
from CONFIG.SYS.
How can we gain access to those two file handles, keeping our FILES=
setting in CONFIG.SYS at 11? We want to be able to open 10 files in
our program, with FILES= set to 11.
Response:
Due to a limitation of DOS, you can't do this. The workaround is to
simply set FILES=13, for the reasons described below.
In order to open a file using the C run time, ALL of the following
conditions must be true:
1. There must be a space in the C run-time library's internal file
tables. This is normally limited to 20 files, but this limit can be
increased by modifying the start-up code if you're using DOS 3.30
or later.
2. There must be DOS file handles available to your particular
process. Again, the normal limit is 20, but this can be increased
with a call to function 67h of INT 21h (only under DOS 3.30 or
later). This can also be modified in the start-up code.
3. Finally, there must be files available from DOS's system-wide pool,
as set by FILES= in CONFIG.SYS. (This defaults to 8, but can be set
as high as 20 under DOS Version 3.30 and earlier or 255 under
Versions 3.30 and later.)
The results of comparing a C program that uses the C run-time library
and a MASM program that calls DOS directly are shown below. Each was
run with stdprn and stdaux, closed and open.
C Program MASM Program
--------- ------------
FILES= W/o Closing W/ Closing W/o Closing W/ Closing
------ ----------- ---------- ----------- ----------
17 (Note 1) 14 14 14 14
18 (Note 1) 15 15 15 15
19 15 16 16 16
20 15 17 17 17
21 (Note 2) 15 17 17 17
Please note the following:
1. The formula for the number of files you can open for FILES=n, where
n is less than 19, is n - 3.
2. No matter how high you set FILES=, you'll be limited to these
numbers in C unless you modify the start-up code as described in
README.DOC. In MASM, you'll be limited to these numbers unless you
raise the per-process limit on file handles by calling INT 21h,
function 67h.
3. Running the program from a batch file doesn't change these numbers;
however, reduce each number by 1 for each input or output
redirection you do.
4. The MASM program doesn't get any additional handles freed up when
it closes handles 3 and 4 because, contrary to what the "MS-DOS
Encyclopedia" implies, DOS won't allow you to reuse those two
handles.
Without closing handles, C programs give you a constant 15 or n - 3,
whichever is smaller. The upper limit is 15 because, unless you modify
the start-up code, the limit is 20 minus the five default files.
When you close handles 3 and 4, they are closed for C but not for DOS.
Therefore, you can open exactly as many handles as DOS normally
allows.
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