mirror/ioccc
1
0 Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity

Merge pull request #191 from xexyl/1998-schweikh2

Browse Source 
Formatting fixes in 1998/schweikh2/README.md
This commit is contained in:
Landon Curt Noll authored and GitHub committed 2023-03-28 14:36:55 -07:00
2 parents c3fdc1b811 fb49513760
commit 45e679c147
1 file changed
+148 -113
Download Patch File Download Diff File Expand all files Collapse all files
1998/schweikh2/README.md
+148 -113
View File
@@ -14,31 +14,50 @@
make all
NOTE: Almost every compiler is supposed to be able to accept the code in
schweikh2.c; try compiling schweikh2.alt.c to see if your compiler
`schweikh2.c`; try compiling `schweikh2.alt.c` to see if your compiler
is fully standard-compliant.
### To run
## To run:
To use:
To use:
./schweikh2
or
<some number>
or
./schweikh2 integer_number
## Judges' comments
## Try:
Run the program with a small argument, like 5 or 10, several times.
Are the results different? Why?
./schweikh2 5
./schweikh2 5
Some non-gcc compilers that are not fully ANSI standard do not
compile this entry correctly.
./schweikh2
5
At the time this hint file was written, some gcc and egcs
implementations ran into problem when building the original entry:
What is the difference here?
What happens if you don't put in any number like the below?
./schweikh2
<ENTER>
## Judges' comments:
Run the program with a small argument, like 5 or 10, several times. Are the
results different? Why?
Some non-gcc compilers that are not fully ANSI standard do not compile this
entry correctly.
At the time this hint file was written, some gcc and egcs implementations ran
into problem when building the original entry:
make schweikh2.alt
This program often will produce an error of the form:
This program often will produce an error of the form:
gcc -ansi schweikh2.alt.c -o schweikh2.alt
as: Error: cca00NzV.s, line 58: Missing " at end of string
@@ -46,139 +65,155 @@ is fully standard-compliant.
as: Error: cca00NzV.s, line 58: malformed statement
*** Error code 1 (bu21)
because the line:
because the line:
??=line 10 ONE(O(1,1,2,6,0,6))
turns into the line:
turns into the line:
# 9 "01\012"
which causes gcc to give to the assembler the following two lines:
which causes gcc to give to the assembler the following two lines:
.stabs "whey.c
",132,0,0,Ltext1
and the lone " after the #.file line results in an assembler syntax error.
and the lone " after the #.file line results in an assembler syntax error.
In some cases one must compile using 'gcc -g':
In some cases one must compile using `gcc -g`:
make schweikh2.alt CFLAGS=-g
to trigger this error.
to trigger this error.
And with the proper text beyond the \012, it may be possible to have
all kinds of fun adding ``inline assembly'' via #line directives. :-)
For extra credit:
And with the proper text beyond the `\012`, it may be possible to have
all kinds of fun adding `inline assembly` via #line directives. :-)
For extra credit:
If your compiler has this bug, transform the line in the C
program to inject assembly instructions in such a way that
the program will compile.
If your compiler has this bug, transform the line in the C
program to inject assembly instructions in such a way that
the program will compile.
If you are able to do the above, try adding assembly instructions
so that it will compile, execute and do something interesting.
If you succeed in doing this (especially on FreeBSD, Linux or Sparc),
email the author for a free pat on the back.
If you are able to do the above, try adding assembly instructions
so that it will compile, execute and do something interesting.
## Author's comments
If you succeed in doing this (especially on FreeBSD, Linux or Sparc),
email the author for a free pat on the back.
What this program does
----------------------
NOTE: the author submitted the program as yarng so when trying the examples
below change `yarng` to `./schweikh2`.
My entry is a yarng, which stands for, you guessed it, yet another
random number generator, and is pronounced 'yawn' due to its
incredible speed. Seriously, it exploits the randomness of quantum
mechanical events that build up the hardware as well as some other
non-deterministic factors. Here's how it works:
## Author's comments:
1) an alarm is scheduled to be delivered in a second
2) an infinite loop is entered that just increments a counter
3) on delivery of the alarm signal, the counter's LSB is printed.
4) while some other counter is not zero, go to 1)
5) exit
What this program does
----------------------
The yarng therefore produces output at a rate of 1 baud. The number of
bits may be specified by an argument and defaults either to UINT_MAX
(least likely) or UINT_MAX+1 (most likely). Extra credit: why the
different values? How long would you need for an empirical test?
My entry is a yarng, which stands for, you guessed it, yet another
random number generator, and is pronounced 'yawn' due to its
incredible speed. Seriously, it exploits the randomness of quantum
mechanical events that build up the hardware as well as some other
non-deterministic factors. Here's how it works:
For example:
1. an alarm is scheduled to be delivered in a second
2. an infinite loop is entered that just increments a counter
3. on delivery of the alarm signal, the counter's LSB is printed.
4. while some other counter is not zero, go to 1)
5. exit
$ yarng 32
10111001010101110110010101001111
$ yarng 5
10011
The yarng therefore produces output at a rate of 1 baud. The number of
bits may be specified by an argument and defaults either to `UINT_MAX`
(least likely) or `UINT_MAX+1` (most likely). Extra credit: why the
different values? How long would you need for an empirical test?
Why I think this is obfuscated
------------------------------
For example:
-Ever seen a 'do for ... while' loop?
-I avoided 'int' like the plague. Instead I used storage class specifiers
(register, auto) to get implicit int. The Standard allows implicit int
even in casts as you can see in '(void(*)(register))main'. Where implicit
int is impossible, like in the 'int main' declaration I have int split
across two lines. All indent programs I use insert space at the beginning
of the continuation line and thereby introduce a syntax error.
-main (ab)used as a signal handler. Shocking. Yeah, this leads to undefined
behaviour. I have yet to see a system where it does not work. If it
doesn't work for you, I'd appreciate if you mail me about it.
Uh-oh, schweikh did it, uttered "Works for me". Don't quote me on that!
-Even if you know how the program works, you just can't predict the output.
-Chained use of token pasting operators ## with shuffled arguments
-So you think you can't assign to __LINE__ and __FILE__? Not so. The
#line preprocessor directive does the trick. But saying '#line 10 "01\n"'
is a little lame and I came up with '??=line 10 ONE(O(1,1,2,6,0,6))'
-__LINE__ is used as the signal number argument to signal(). If indenting
or other editing changes the number of lines the program is likely to be
killed after one second... Thanks to POSIX for specifying SIGALRM as 14.
You're SOL if your system does not conform. Get a real OS.
-All strings (or characters) needed are crammed into __FILE__ using
octal notation and then extracted using pointer + offset notation.
-Use of a # (null) preprocessor directive (hidden as a %: digraph)
Delete this line and prepare for a core dump.
-Identifiers reused with different capitalization: one, One, ONE, zero,
Zero. ONE is particularly nasty because it appears as a macro taking
an arg and as a plain identifier in main's parameters. The rule is that
whenever ONE is followed by '(' it must be the macro.
-A quote from Kernighan and Plauger, "The Elements of Programming Style",
2nd Ed., page 21: "A useful way to decide if some piece of code is
clear or not is the ``telephone test.'' If someone could understand your
code when read aloud over the telephone, it's clear enough. If not then
it needs rewriting."
Now try the telephone test for 'one = zero = atoi (ONE[zero-1])'
-Don't you love the aesthetics of symmetry: write (1, 1+__FILE__+1, 1)
(telephone test this as well; is it 1, One, ONE, one? Maybe won?).
```sh
$ yarng 32
10111001010101110110010101001111
$ yarng 5
10011
```
Where does the randomness come from?
------------------------------------
Why I think this is obfuscated
------------------------------
Each bit obviously depends on the number of times the loop body is
executed between two calls of the signal handler. But what determines
this number?
- Ever seen a 'do for ... while' loop?
+ The accuracy of the clock driving the CPU
+ The accuracy of the clock driving the timer responsible for alarm signals
+ On multitasking OS's: The total time the process is scheduled
to run between successive invocations of the signal handler.
- I avoided 'int' like the plague. Instead I used storage class specifiers
(register, auto) to get implicit int. The Standard allows implicit int even in
casts as you can see in `(void(*)(register))main`. Where implicit int is
impossible, like in the `int main` declaration I have int split across two
lines. All indent programs I use insert space at the beginning of the
continuation line and thereby introduce a syntax error.
How about the randomness of the yarng? To measure these properties I
counted the number of bit sequences and compared them to what I would
expect from a generator with uniform distribution (all sequences
equally likely to occur).
- `main` (ab)used as a signal handler. Shocking. Yeah, this leads to undefined
behaviour. I have yet to see a system where it does not work. If it doesn't work
for you, I'd appreciate if you mail me about it. Uh-oh, schweikh did it,
uttered "Works for me". Don't quote me on that!
It is interesting to note that yarng's maximum deviations are less
than or equal to rand()'s. Randomness however tends to degrade under
high load averages on my system (Linux). This could be avoided by
allowing the process one second of execution time instead of wall
clock time (using the SIGVTALRM sent by the virtual timer
instead of SIGALRM).
- Even if you know how the program works, you just can't predict the output.
I'm not surprised. Yarng in the end depends on fluctuations of the
electrical field and the heat distribution in the oscillators, the
implementation and state of the scheduling algorithm, the system
load, number and temporal distribution of asynchronous events and
last but not least the phase of the moon. God _does_ play dice.
Type yarng and watch Him roll.
- Chained use of token pasting operators ## with shuffled arguments.
- So you think you can't assign to `__LINE__` and `__FILE__`? Not so. The `#line`
preprocessor directive does the trick. But saying `#line 10 "01\n"` is a little
lame and I came up with `??=line 10 ONE(O(1,1,2,6,0,6))`.
- `__LINE__` is used as the signal number argument to signal(). If indenting or
other editing changes the number of lines the program is likely to be killed
after one second... Thanks to POSIX for specifying SIGALRM as 14. You're SOL if
your system does not conform. Get a real OS.
- All strings (or characters) needed are crammed into `__FILE__` using octal
notation and then extracted using pointer + offset notation.
- Use of a `#` (null) preprocessor directive (hidden as a `%:` digraph) Delete
this line and prepare for a core dump.
- Identifiers reused with different capitalization: `one`, `One`, `ONE`, `zero`,
`Zero`. `ONE` is particularly nasty because it appears as a macro taking an arg
and as a plain identifier in `main`'s parameters. The rule is that whenever
`ONE` is followed by `(` it must be the macro.
- A quote from Kernighan and Plauger, "The Elements of Programming Style", 2nd
Ed., page 21: "A useful way to decide if some piece of code is clear or not is
the 'telephone test.' If someone could understand your code when read aloud
over the telephone, it's clear enough. If not then it needs rewriting." Now try
the telephone test for 'one = zero = atoi (ONE[zero-1])'.
- Don't you love the aesthetics of symmetry: `write (1, 1+__FILE__+1, 1)`
(telephone test this as well; is it 1, One, ONE, one? Maybe won?).
Where does the randomness come from?
------------------------------------
Each bit obviously depends on the number of times the loop body is
executed between two calls of the signal handler. But what determines
this number?
- The accuracy of the clock driving the CPU.
- The accuracy of the clock driving the timer responsible for alarm signals.
- On multitasking OS's: The total time the process is scheduled to run between
successive invocations of the signal handler.
How about the randomness of the yarng? To measure these properties I
counted the number of bit sequences and compared them to what I would
expect from a generator with uniform distribution (all sequences
equally likely to occur).
It is interesting to note that yarng's maximum deviations are less
than or equal to rand()'s. Randomness however tends to degrade under
high load averages on my system (Linux). This could be avoided by
allowing the process one second of execution time instead of wall
clock time (using the `SIGVTALRM` sent by the virtual timer
instead of `SIGALRM`).
I'm not surprised. Yarng in the end depends on fluctuations of the
electrical field and the heat distribution in the oscillators, the
implementation and state of the scheduling algorithm, the system
load, number and temporal distribution of asynchronous events and
last but not least the phase of the moon. God _does_ play dice.
Type yarng and watch Him roll.
## Copyright and CC BY-SA 4.0 License: