Q47105: Best to Avoid C Run-Time Library Routines in TSRs

Article: Q47105
Product(s): See article
Version(s): 5.10
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# G890601-19569 | mspl13_c
Last Modified: 30-AUG-1989

Question:

I have two questions about using the C run-time library in a TSR
(terminate-and-stay-resident program):

1. Will I encounter any problems with calling the following routines
   from a TSR? What about other library routines?

      _strncmp    __aFFalshr   __chkstk   _sprintf  _free
      __acrtused  _memcpy      _realloc   _strlen   _malloc
      _strcmp     __aFFaulshr

2. What DOS system calls (INT 21) do the C library references shown
   above generate? I want to avoid calling DOS because it's not
   re-entrant.

Response:

Microsoft does not recommend using standard library routines in TSRs
at all, even though they may appear to be safe. The main problem is
that although they may be safe in this version of the compiler, they
may change in future versions. Microsoft does not support using the
run-time code in a TSR.

In particular, you can have severe problems with chkstk, sprintf,
malloc, realloc, and free. It is likely that the other routines won't
present too much of a problem, but without digging through the source
and knowing EXACTLY what you're doing, there is no way to be sure.

There is, however, an excellent alternative. Microsoft offers the
run-time library source for sale. Call our sales number at (800)
426-9400 for pricing and to order. With this source, you have almost
total control over what your TSR looks like and how it acts. You can
inspect each routine you use to ensure that it won't cause any
interactions you don't want, and you can modify those routines that
do. You're also better insulated against later modifications to the
run-time library; since you control the code, you can modify it when
you need to. This is by far your best alternative. You'll KNOW what's
going on and you'll be able to ensure that you can maintain your
applications. The only routines not included in this source are the
floating-point and graphics routines.

You may be able to find a source for routines written to support TSR
programming even though Microsoft does not offer such software. Look
for ads in the programmer-oriented PC magazines and/or call some of
the programming-oriented software dealers.

The run-time source allows you to determine which functions call DOS
via INT 21h and which don't. Having the source also allows you to come
up with alternative strategies for those functions that make calls.

The "MS-DOS Encyclopedia" (published by Microsoft Press) is an
excellent source of information about writing TSRs.

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