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DOS 2.0 HAS PROBLEMS WITH REDIRECTION OF I/O
There are problems in DOS 2.0 with the redirection of I/O and
piping for programs that use the original DOS 1.1 INT 21 function
calls for input. This problem is readily apparent to users of C
language packages such as Computer Innovation C-86, Lattice C, or
Microsoft C (you'd think they would get it right!). One problem
is that all output to the screen is redirected, even keyboard echo.
Correct operation would redirect all program output for the screen
(stdout) to the specified >file, but the echo of keyboard input would
still be sent to the screen. Instead, both the keyboard echo and
the program output are sent to the redirected >file. Thus, if you
run programs such as the CAT.C (K&R,page 154) example that Microsoft
distributes with their C; or COPYIO.C (K&R,page 15) with the output
redirected to a file, you will get the following results:
1. Under DOS 1.1, keyboard input is echoed to the screen
as you type and each line appears in the >file once as
expected.
2. Under DOS 2.0, keyboard input is not echoed to the
screen, but each line appears in the >file twice!
This situation is handled correctly in DOS 2.0 if the new INT
21 function call 3F is used. This can be demonstrated by redirecting
output for the DOS 2.0 function MORE - it works as desired.
The redirecting of input to these programs doesn't work properly
either. If the file has not been edited with debug to end with a
control-Z, the program will hang up at the end of the <input file.
You must reboot the system to continue! Also, if you pipe the output
of the first program into a second program, the final output will
contain each line four times, doubled spaced after the second line!
These problems do not occur for programs that use the new DOS 2.0
calls for I/O, such as SORT and MORE.
The question now is how do you fixup C programs to run under
DOS 2.0 and not redirect keyboard echo to the stdout file? The easiest
way for C compilers that include their own redirection code is to
change their redirection symbols from <, >, and >> to something else. Then
DOS 2.0 won't do the redirection, so the C code will be able to do
it correctly. With the Microsoft C compiler, this is easily accomplished
by modifying three lines of code in _MAIN.C. A good choice is to
modify _MAIN.C so that it redirects on the symbols {, }, and }}.
The only restriction is that these symbols then should not be used
in filenames. With these changes, the user can choose to let either
DOS <, >, >> or C {, }, }} do the redirecting. The modified
version of _MAIN.C is compiled to obtain a new _MAIN.OBJ, which can
either be put into the library MC.LIB to replace the original _MAIN
by using the LIB.EXE utility, i.e. LIB MC.LIB -_MAIN+_MAIN
or it can be kept separate. If kept separate, remember to include
it in the list of .OBJ files specified in the LINK call, i.e.
LINK c _main myprogram.
The three lines to change in Microsoft C's _MAIN are:
case '{':
case '}':
if (*line == '}')
Kludgy, yes, but it works better than before!! WHR 9-26-83
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