ioccc/2004/vik2

Best Abuse of CPP:

Daniel Vik
926 Genter Street
La Jolla, Ca 92037
USA

http://www.vik.cc/

To build:

make

To run:

./vik2

Try:

echo "Do or do not. There is no try."

Judges' remarks:

The fun thing about CPP abuses is that, in most cases, running the preprocessor on them doesn't help you decipher the obfuscated code. That's assuming your preprocessor can take it at all.

Author's remarks:

Introduction

It's been a while since the last pre-processor killer won the ioccc. I thought it was time to relive hour long compile times so I made a program that does (almost) all computation in the pre-processor. The program even causes gcc (and other pre-processors) to crash when configured to do maximum number of computations. The default configuration of the makefile is set to make the program compile in a not too long time.

I've got cpp to crash in a couple of diffenent ways. Most common is to get a segmentation fault, but a couple of times it just got stuck in some infinite loop.

I actually had to write my own pre-processor to verify that the program was doing the right thing...

Since it takes a very long time to compile the program, there is a symbol that can be defined to shorten the compile time and avoid compiler errors.

Features

The program implements an 11-bit ALU in the pre-processor. The ALU has two registers, X and Y, for calculations and one status register SR. Each bit in the registers are represented by one symbol that are either defined or undefined.

The ALU has six instructions:

CLR_X - Clear register X

CLR_Y - Clear register Y

ADD   - Calculates X + Y and stores  the result in X.  If the result
    is larger than 11 bits, the CARRY bit in SR is set otherwise
    the CARRY bit is cleared.

SUB   - Calculates X - Y and stores the result in X. If Y was larger
    than X,  the CARRY bit in SR is set otherwise the  CARRY bit
    is cleared.

TST   - If X == Y the EQ bit in SR is set,  otherwise  the EQ bit is
    cleared. If X < Y the LT bit in SR is set,  othervise the LT
    bit is cleared.

INC2  - Calculates X + 2 and stores  the result in X.  The CARRY bit
    is handled as in the ADD instruction.

Internally the ALU implements a one bit adder with a carry in and carry out. This adder is the core of the ALU. The ADD instruction uses the one bit adder to create the 11-bit adder. The SUB instruction is implemented by adding ~Y to X and then add 1. Finally the TST instruction calculates X-Y and checks the result and the CARRY bit in the SR register to set the EQ and LT bits.

All calculations are made using only the #ifdef, #infdef, #else, #endif, #define and #undef. I never assign a value to a defined symbol. The logic is done by assuming that a defined symbol is 1 and an undefined symbol is 0. (I guess this means that I have unlimited memory as long as I only store zeroes (!)).

The application that uses the pre-processor ALU is a prime number generator. The application uses the same pre-processor directives. Needed variables are stored in memory represended by one defined or undefined symbol per bit. When doing arithmetics, the variables are loaded into the X and Y registers by defining / undefining the X and Y register bits. Then the calcualtion is made and the result is moved back to the memory.

The choice of application may be a bit unfortunate since we've already seen a winning entry calculating prime nubmers in the pre-processor. There is a big difference though. The 1988 applin entry uses pre- processor arithmethic (in a very nice way though). This program only defines and undefines symbols in a well defined ;) way and doesn't do any pre-processor arithmetics.

The reason why I choosed to do a prime number application is that the algorithm can be written to do a lot of calculations without a lot of nesting (gnu cpp allows only 200 nested includes).

Build and Run

First, do a first pre processing of the initial definitions. The FNAME symbol must be defined to the name of the output file. If you want to speed up the compilation, define the STOP symbol to anything from _3 to _10. (The default value is _10 which will make the compilation take some time.) Prime numbers up to 2^STOP will be calculated.

cpp '-DSTOP=_5' '-DFNAME="tmp1.c"' prog.c tmp1.c

Then use cpp or any other pre-processor to do the actual work. This will take some time....

cpp tmp1.c > tmp2.c

Finally compile the pre processed file

cc -O tmp2.c -o prog

Then run the program to reveil all prime numbers up to 32 (if you used STOP=_5 as above):

./prog

Verification

I wasn't able to compile the full program which calculates prime numbers up to 1024 using gnu cpp (or any other pre-processor) so I had to write my own. Knowing how the program works it was quite easy to reduce the compilation time from several hours to about one second. I also added some statistics to my home made pre-processor and I found that calculating prime numbers up to 1024 makes the program include itself over 6.8 million times (see the statistics table below).

Obfuscation

Well, since 98.5% of the program is preprocessor directives, the main obfuscation is the use of preprocessor directives.

Doing arithmetics without acutally using any arithmetic operations makes the program quite hard to follow.

In order to actually do the calculations, the program includes itself a lot.

The symbol names are made as short as possible to meet the space limits. This of course adds to the obfuscation (even though the program is quite obfuscated with longer, readable macro names.

Finally, I made the 1.5% c code a bit harder to read by breaking all lines after three characters.

The program has 1029 lines which makes it the longest entry (so far).

Even though the pre-processed and indented output is easier to read, it is quite hard to find the code in it. The size of the pre-processed file is several Mb when using gnu cpp.

Limitations

In order to fit the program within the space limits, I had to define the #define, #endif, and some other directives as single character symbols. This does not compile on all pre-processors but using gnu cpp as a first compilation step solves this.

Statistics

The table below contains some interesting statistics about the compilation of the program. The intermediate file size is the size of the output file of the cpp pre-processor. The time taken to pre process the program were measured using gnu cpp on my 2.0MHz P4 with 512Mb ram and a lot of disk.

    Generated   Recursive  Max nest  Intermediate
Level   prime nrs   includes   count     file size          Time

  _3     < 8           2 322       9         74 748            4"
  _4     < 16          4 672       9        182 652            9"
  _5     < 32         14 237       9        629 121           32"
  _6     < 64         43 748       9      1 998 134         1'41"
  _7     < 128       149 851      11      6 823 575         5'50"
  _8     < 256       526 874      17     24 028 933        20'37"
  _9     < 512     1 920 272      38     65 527 808(*)  1h 42'19"(*)
 _10     < 1024    6 826 389     114     63 782 912(*)  2h 47'58"(*)

 (*) Before cpp crashed.

Copyright and CC BY-SA 4.0 License:

This file is Copyright (c) 2023 by Landon Curt Noll. All Rights Reserved. You are free to share and adapt this file under the terms of this license:

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