Author:

• Name: Don Yang
  Location: US - United States of America (United States)

Award: INABIAF award

To build:

    make all

To use:

    echo Message | ./prog > output.c; cc output.c; ./a.out

Try:

    ./try.sh

Judges’ remarks:

These compiler warnings are not a bug 🐞, they are a feature 🤓.

As an added bonus, there are some real Perls and Ruby gems in the code.

And while you are at it, open making.html in your favorite browser to watch how the author created this excellent winning entry. To watch obfuscation in action, we recommend that you:

• turn ON [x] Follow cursor
• make the window tall
• play at 60x

A fun challenge

Explain the purpose of the constant 114 in prog.c. Are there any alternative values that could replace 114 without affecting the program’s functionality?

The above fun challenge is still open. See the “Fun challenge Info” section for details.

Author’s remarks:

Synopsis

    ./prog "some message\n" "another message\n" > output.c
    gcc output.c -o decode

    gcc output.c -o /dev/null 2>&1 | ./decode
    clang output.c -o /dev/null 2>&1 | ./decode

    ruby output.c | ./decode
    perl output.c | ./decode

Details

This program is a message encoder. Run without arguments for a help message:

    ./prog

This program accepts one or two arguments, where each argument contains a text message to encode. For convenience, most C escape sequences are accepted and interpreted just as they would in C: \n, \r, \t, \a, \b, \v, and also \x with one or two hexadecimal digits. Note that while it’s possible to insert any byte value with \x, an embedded NUL (\x00) will terminate the input string, just like C strings.

Pass in one argument to generate C code to stdout:

    ./prog "Hello world\n" > output.c

The output might seem suspicious in that both GCC and Clang will output many warnings even at the default warning level:

    gcc output.c -o output

    output.c: In function 'main':
    output.c:4:4: warning: division by zero [-Wdiv-by-zero]
        4 |   O/0-O/0-
          |    ^
    output.c:4:8: warning: division by zero [-Wdiv-by-zero]
        4 |   O/0-O/0-
          |        ^
    ...

The reason why we got so many warnings is because we are using those warning messages to encode data. To recover the input message, pipe the warnings to the output program:

    gcc output.c -c -o /dev/null 2>&1 | ./output

(Remember to redirect stderr since that’s usually where compiler warnings go. Also, we want to send the binary output to /dev/null since we just want the warnings. We certainly don’t want to overwrite the decoder while it’s running.)

Encoding data using compiler warnings might seem like a fragile thing to do, but we like it because the warning messages for GCC and Clang are subtly different, such that it’s possible to encode two different messages in one file:

    ./prog "Message for GCC\n" "Message for Clang\n" > output2.c

    gcc output2.c -o decode

    gcc output2.c -c -o /dev/null 2>&1 | ./decode
    clang output2.c -c -o /dev/null 2>&1 | ./decode

• First message will be encoded in warning messages for GCC (11.1 or later, tested up to 15.2).
• Second message will be encoded in warning messages for Clang (tested up to 21.1.0). GCC 10.5 and earlier shares the same behavior as Clang.

For users who are missing GCC or Clang, and also to future-proof against changes in GCC/Clang warning messages, output code also includes extra decoders to simulate GCC/Clang behavior. By running output code with Ruby or Perl, we get diagnostics that are sufficiently similar to GCC/Clang warnings such that the decoder can recover the input messages:

    ruby output2.c | ./decode
    perl output2.c | ./decode

Miscellaneous features

Encoder and generated output have been verified to work under these environments:

• gcc 13.4.0 on Cygwin.
• gcc 11.4.0 on Linux.
• clang 20.1.8 on Cygwin.
• clang 11.0.1 on Linux.
• clang 10.0.0 on JSLinux.

The encoder and generated output have also been tested under these environments below. They compile and run just fine, but the compiler warning messages aren’t sufficient to fully exercise the program

• gcc 10.2.1 on Linux.
• gcc 9.3.0 on JSLinux.
• tcc 0.9.27 on JSLinux.

CRC32 of the source code is embedded in the code itself (44495630), which spells out DIV0.

Code layout is based on Kurumi, also known as Walnut, the elite hacker from “Lycoris Recoil”. The code is a bit larger than it needs to be due to the 50% dither pattern used for the hair.

Primary files

• prog.c - entry source code
• Makefile - entry Makefile
• prog.orig.c - original source code
• try.sh - script to try entry
• making.html - the making of prog.c

Secondary files

• 2025_yang3.tar.bz2 - download entry tarball
• README.md - markdown source for this web page
• .entry.json - entry summary and manifest in JSON
• .gitignore - list of files that should not be committed under git
• .path - directory path from top level directory
• .date-range - copyright date range for this directory
• index.html - this web page

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