Best abuse of CPP

The code for this entry can be found in prog.c

Judges' comments:

To use:

make
./prog

Try:

echo "International Obfuscated C Code Contest 2020" | ./prog 23209 > code.c

# compile without a PIN
cc code.c -o decode
# yes, you must compile with -DPIN=<number>

# compile with the wrong pin
cc -DPIN=555 code.c -o wrongcoded    # wait for it ...
./wrongcoded | hexdump -C
# Use the wrong PIN: You're gonna have a bad time.

# compile with a PIN
cc -DPIN=23209 code.c -o decoded    # wait for it ...
./decoded
# weeeeeeeeeeeeee! You are having a good time!

Selected Judges Remarks:

You might think of this entry as code obfuscater with a code. If you use the wrong code, you’re gonna have a bad time. What is compiling this entry about? Having a good .. time!

This winning entry also comes with a JavaScript timelapse spoiler that shows how this entry was written.

Author’s comments:

Synopsis

echo "Hello world" | ./mile 12345 > encoded.c

gcc -DPIN=12345 encoded.c -o ./decoded && ./decoded

Details

Mile is a C code generator. Run without arguments for the help message below:

usage: [PIN] < input.txt > output.c

where [PIN] is an integer in the range of 1..65535

When run with an encoding PIN (first command line argument), Mile will read bytes from stdin and generate C code on stdout. Example:

echo "Hello, world!" | ./mile 56789 > encoded.c

This generated code will reproduce the same input bytes on stdout when executed, if it was compiled with the same encoding PIN.

gcc -DPIN=56789 encode.c -o decoded

The generated code reconstructs all data at compile time by seeding a random number generator with the given PIN to reproduce the input bytes, and simply does one fwrite() call at run time to print those bytes. Not specifying a PIN is a compile time error. Specify the wrong PIN and output code will still compile, but will likely produce garbage instead of the input bytes. If the original input was empty then any PIN works, but you might get a compile time warning complaining about empty arrays.

Because all data is reconstructed at compile time, expect the compile time to be excruciatingly long if you try to encode too much data. Code generated by encoding a haiku will probably compile in seconds, a limerick may require tens of seconds, a sonnet will take minutes. Give Mile an epic poem and you can probably write a faster preprocessor before GCC is done (Clang probably already failed long before then).

I see that there is a lack of preprocessor abuse in recent years, probably because programming in preprocessor is too tedious. Mile is meant to help with that by producing C code filled with at least 98% preprocessor lines for any input. Now you too can enjoy long compile times with minimal effort.

Features

Output is randomized using system clock, so if you wait a second between runs, you should get a different generated code even for the same encoding PIN.
Customized bytecode encoding of preprocessor boilerplates, mostly for compression but should also make reverse engineering more fun.
CRC32 of the source code is embedded in the source code itself.
Code layout shaped to resemble “Mile” from “Didn’t I Say to Make My Abilities Average in the Next Life?!”. Mile is known to be able to talk to nanomachines directly, which is exactly what programming in preprocessor feels like.

Compatibility

Mile has been verified to work in these environments (also, compiles without warnings on all of these):

gcc 9.2.0 on Cygwin
gcc 8.3.0 on Linux
gcc 7.4.0 on Cygwin
gcc 6.1.0 on JSLinux
gcc 5.4.0 on Linux
clang 8.0.1 on Linux
clang 8.0.1 on Cygwin
tcc 0.9.27 on Linux
tcc 0.9.25 on JSLinux

Output of Mile is a bit more taxing, and only works with a subset of the above:

gcc: works just fine when using 8.3.0 and below, but may trigger warning for large files. gcc 9.2.0 and above may crash with large files.
clang: works fine for small files, fails on large files due to preprocessor bug.
tcc: works just fine on Linux. Fails silently on JSLinux.

For people eager to encode large files, an extra utility has been packaged with this entry to preprocess the generated output to make them more acceptable to typical compilers. View the cppp.cc source and then compile it with a c++ compiler of your choice.