array padding

[Dan Pop]

Re: array padding

In <c8p03b08pd@new s3.newsguy.com> Chris Torek <nospam@torek.n et> writes:
[color=blue]
>I ran this on a Data General Eclipse[%] and it printed:[/color]

<snip>
[color=blue]
>[% Actually I do not have access to one, but this is what it might
>print, if I remember right.[/color]

Did anyone implement standard C on it? How about K&R C?

Dan
--
Dan Pop
DESY Zeuthen, RZ group
Email: Dan.Pop@ifh.de

[Chris Torek]

Re: array padding

>In <c8p03b08pd@new s3.newsguy.com> Chris Torek <nospam@torek.n et> writes:[color=blue][color=green]
>>I ran this on a Data General Eclipse[%] ...
>>[% Actually I do not have access to one, but this is what it might
>>print, if I remember right.[/color][/color]

In article <news:c8t6fr$c4 s$23@sunnews.ce rn.ch>
Dan Pop <Dan.Pop@cern.c h> asked:[color=blue]
>Did anyone implement standard C on it? How about K&R C?[/color]

I am not at all sure about Standard C. The machine I used ran
a (somewhat bizarre and twisted variation on) 4.2BSD Unix. The
C compiler implemented everything required to compile typical
Unix programs. I think that covers "K&R C" and then some -- the
compiler had enumerations and "unsigned char", for instance.

Quite a lot of our C code had to be fixed up not to make assumptions
about pointer formats.

Besides the pointer trickiness, the main thing I remember about
the Eclipse is that the output from perror() was broken. It printed
a two digit error number, followed by no whitespace, followed by
the string, so that one would get things like:

prog: cannot open foo.dat: 02No such file or directory

(I also remember something about bogus newlines, perhaps before the
"02".)

(This machine and the Pyramid were responsible for our group at
the University of Maryland making numerous fixes to varous programs.
The Pyramid found non-pointer bugs. For instance, MH used the
interesting calling sequence "parameter passing by register
assumption"[%], which was impossible on the Pyramid due to its
register windows. The Pyramid also exposed all the programs that
failed to "return 0" or "exit(0)" from main. Return values from
functions had to be stored in register "pr0" in the register-window
assigned to that function, and calling some other function did not
affect one's own pr0, only one's tr0. Hence, programs that on the
VAX returned whatever was left in r0 -- which was the return value
from the most-recently-called function -- instead returned their
first argument. For main(), this was the parameter argc.)

[% An illustration, not necessarily the actual MH code and using
ANSI C syntax rather than Classic C:

void f1(struct S *param) {
register int a, b;
register FILE *some_file;
...
some_file = fopen("some_pat h", "w");
f2(param, a);
...
}

void f2(register struct S *param, register int a) {
register FILE *fp; /* NOTE: NOT INITIALIZED! */
...
fprintf(fp, "%s: %d\n", param->zig, a);
...
}

This worked on the VAX, and even the PDP-11, because the machine's
registers were handed out in a fixed, known order. On the VAX,
"some_file" wound up in r9; on the PDP-11 it was in r3. In each
case, declaring "fp" third in f2() caused the C compiler to reserve
the same register -- which still held the opened FILE *! The VAX
C compiler gave you six registers, while the PDP-11 gave you only
three, so the actual "parameter passing by register assumption" in
MH was definitely in the first six "register" variables, and almost
certainly in the first three.]
--
In-Real-Life: Chris Torek, Wind River Systems
Salt Lake City, UT, USA (40°39.22'N, 111°50.29'W) +1 801 277 2603
email: forget about it http://web.torek.net/torek/index.html
Reading email is like searching for food in the garbage, thanks to spammers.

[EventHelix.com]

Re: array padding

C will allow you to use a negative index. The index into an array is
signed integer. I guess you can use negative indices if you make sure
that the memory before the start of the array is allocated for this.

I remember seeing use of negative in Tanenbaum's MINIX book. The
positive indices were used as user task pids and negative indices
were used for MINIX tasks.

Sandeep
--

http://www.EventHelix.com/EventStudio

EventStudio 2.0 - Generate Sequence Diagrams and Use Case Diagrams in PDF

[Al Bowers]

Re: array padding



EventHelix.com wrote:[color=blue]
> ... The index into an array is
> signed integer. I guess you can use negative indices if you make sure
> that the memory before the start of the array is allocated for this.
>[/color]

Doing this little trick is not C. A resource other than the
the ISO/IEC Internation Standard may suggest this and your compiler may
accept it in a way you expect. But that does not make it C. The C
Language describes this as undefined behavior.

The Standard specifically describes many circumstances of
undefined behavor. The following circumstance seems to make operation
clearly UB.

— Addition or subtraction of a pointer into, or just beyond,
an array object and an integer type produces a result that
does not point into, or just beyond, the same array object (6.5.6).


--
Al Bowers
Tampa, Fl USA
mailto: xabowers@myrapi dsys.com (remove the x to send email)

http://www.geocities.com/abowers822/

[Keith Thompson]

Re: array padding

eventhelix@hotm ail.com (EventHelix.com ) writes:[color=blue]
> C will allow you to use a negative index. The index into an array is
> signed integer. I guess you can use negative indices if you make sure
> that the memory before the start of the array is allocated for this.
>
> I remember seeing use of negative in Tanenbaum's MINIX book. The
> positive indices were used as user task pids and negative indices
> were used for MINIX tasks.[/color]

I'll bet that the "array" being indexed with a negative integer was
actually a pointer into the middle of an array object.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.