WARNING: These rules are TENTATIVE

One might think of them as a beta release for the IOCCC that is about to open.

IMPORTANT: All tentative rules and guidelines are subject to change by the IOCCC judges at any time. See both the IOCCC news and the IOCCC Mastodon feed as sometimes the IOCCC judges mention changes there.

See our FAQ on “rules, guidelines, tools feedback” as well as our FAQ on “about asking questions” about these rules. You might also find the FAQ in general useful, especially the FAQ on “how to enter the IOCCC”.

The IOCCC is pending

While the IOCCC is not open now, there is a tentative opening date for the next IOCCC.

Comments and suggestions on these preliminary rules are welcome. See the FAQ for how to suggest, correct or provide feedback about these rules.

29th International Obfuscated C Code Contest Official Rules

Copyright (c) 2025 Leonid A. Broukhis and Landon Curt Noll.

All Rights Reserved. Permission for personal, education or non-profit use is granted provided this copyright and notice are included in its entirety and remains unaltered. All other uses MUST receive prior permission in writing by contacting the judges.

Rules Version

These IOCCC Rules are version 29.15 2025-12-02.

The | symbol indicates a change from the previous IOCCC.

Because the IOCCC29 rules was a substantial rewrite, only important changes from IOCCC28 have been marked.

Reminder: be SURE to read the IOCCC guidelines.

This IOCCC runs from 2025-12-03 20:25:12.045045 UTC to 2026-03-13 17:19:23.293137 UTC.

This contest will enter the pending state on 2025-12-03 20:25:12.045045 UTC, allowing people to register for the IOCCC.

This contest will enter the open state on 2025-12-15 18:21:24.273033 UTC, allowing people to upload submissions.

This contest will enter the judging state on 2026-03-13 17:19:23.293137 UTC, when people will no longer be allowed to upload submissions.

The above dates and times may change AT ANY TIME until the contest status is open.

The above timestamps are in “YYYY-MM-DD HH:MM:SS.micros UTC” format.

See Entering the IOCCC: the bare minimum you need to know.

See FAQ on “How can I comment or make a suggestion on IOCCC rules, guidelines and tools?”.

Your submission MUST be a C program.

See the Guidelines for Rule 1 - C program.

Rule 2 requires that your submission satisfy BOTH Rule 2a (Gross Size) and Rule 2b (Net Size).

During development this can be checked using iocccsize(1):

    iocccsize prog.c

See the Guidelines for Rule 2 - Size restrictions.

The overall maximum size of your prog.c program source MUST NOT exceed 4993 bytes.

See Entering the IOCCC: the bare minimum you need to know.

Check your source code using the latest official IOCCC size tool, iocccsize(1). The value printed MUST NOT exceed 2503.

See Rule 17 - Use mkiocccentry.

You MUST register, using a valid email address, in order to submit to the IOCCC. You may register while the contest is pending or open.

A few days after registering, you’ll receive your UUID username and temporary password, which you MUST change within 14 days.

See How to register for the IOCCC.

See Entering the IOCCC: the bare minimum you need to know.

See Guidelines for Rule 3 - Register for the IOCCC.

Your submission MUST include at least these files: Makefile, prog.c, remarks.md,.info.json, and .auth.json, the latter two of which are generated by mkiocccentry(1). See Rule 17 - Use mkiocccentry.

At a minimum the following requirements apply. Additional requirements may be imposed by mkiocccentry(1) updates that have yet to be listed below:

• .info.json and .auth.json MUST be generated by mkiocccenty(1) when packing a submission.

• Before forming the tarball, mkiocccentry(1) will run chksubmit(1) on your submission directory and if ANY problems are found (with .auth.json, .info.json or ANYTHING else), your tarball will NOT be formed.

• The Makefile, remarks.md,.info.json, and .auth.json MUST have a file size greater than 0 and MUST have content. Note that mkiocccentry(1) will check for file size but will NOT check for content. On the other hand chksubmit(1) will check that the .auth.json and .info.json are both valid JSON and are valid according to the IOCCC requirements.

• Your submitted source code MUST be called prog.c. When the Makefile compiles your code, the executable (if applicable) MUST be called prog. The Makefile may compile your source code into other filenames as well, if you want.

• As prog, prog.alt, prog.alt.o, prog.o may be created by your Makefile, your submission MUST NOT include those files in your top directory.

• Your submission may NOT have any file that starts with COPYING, COPYRIGHT, or LICENSE as those files are reserved by IOCCC.

• Your submission may NOT have in the top directory: prog.orig.c, README.md or index.html as those files are reserved by the IOCCC.

• Your submission may NOT have in the top directory any file that matches a file glob pattern form [0-9][0-9][0-9][0-9]_*.tar.bz2 as those files are reserved by IOCCC.

• Your submission may ONLY contain regular files and directories. Other file types such as symlinks, sockets etc. are NOT allowed.

• All directories MUST have permission mode of 0755. All files MUST have the file mode of 0444, EXCEPT for files that end in .sh which must have a mode of 0555.

• All files that end in .md MUST be in markdown format.

• All filenames are treated as case insensitive. For example, REMARKS.MD, and Remarks.md, and remarks.md are considered by mkiocccenty(1) to be the SAME file. When preparing a submission on a case sensitive filesystem, ensure that the submission will work on a case insensitive filesystem.

NOTE: by ‘top directory’ we mean the directory with prog.c, Makefile, remarks.md and the mkiocccentry(1) generated files .auth.json and .info.json and NOT the topdir arg to mkiocccentry(1).

NOTE: mkiocccentry(1) will reject any directory which has prohibited files.

The remarks.md and Makefile are explained in more detail in the Guidelines.

See IOCCC markdown guidelines.

See Rule 17 - Use mkiocccentry.

See the FAQ on “What permissions may my files be and what if I need different permissions?”.

See the FAQ on “What are the detailed recommendations for a submission Makefile?”.

See the Guidelines on the mkiocccentry toolkit.

Your submission MUST NOT modify the filenames or content of any of your original submission, such as prog.c, Makefile, or ANY other supplied files. To modify files, your program can make a copy or you can use the Makefile to setup working copies. Any working copies MUST be cleaned up by the Makefile clobber target, so as to restore the directory to its original state.

With the exception of the /tmp and the /var/tmp directories, assuming the directory name or filename does NOT start with a . (dot), your submission MUST NOT create or modify directories and files ABOVE the current directory.

The Makefile clobber target MUST restore your submission to its original form, including the removal of any created files and directories.

Your code should be able to run under a SUS environment.

I am NOT a Rule, I am a free(void *human); ‼️

    while (!(ioccc(rule(you(are(number(6)))))) {
        ha(); ha(); ha();
    }

See Rule 19 - Prime.

You, the author(s), MUST own the contents of your submission, OR, IF you do NOT own the work, then you MUST have permission from the original owner(s) to its use. If you submit ANY content that is owned by others, you MUST detail that ownership, i.e., who owns what, and document in your remarks.md the permission you obtained from them.

You are permitted to use tools to write your code.

See Rule 16 - Anonymous Judging, and Guidelines for Rule 7 - Original Work.

The submit server will accept submissions ONLY from those who have registered and ONLY while the contest is open and ONLY after you have changed your initial password.

The submit server places a total size limit of 3999971 bytes PER submission.

Each account has ten (10.000) submission slots (0..9).

Each submission slot should be annotated by the server as to whether it was received, processed, malformed, etc. Please take note of any errors, rectify, and resubmit.

In order to avoid potential packaging errors, it is highly recommended to use mkiocccentry(1), AND to check from time to time the status on the submit server, to see if any tests run by the server failed to validate your submission.

See Rule 3 - Registering for the IOCCC, Rule 15 - GNU Makefile, and Rule 17 - Use mkiocccentry.

See Guidelines for Rule 8 - Submitting requirements.

Entries requiring human interaction to be initially compiled are NOT permitted. However, see the guidelines on rule 9 as you may suggest alternative compilation steps to do something fun or cute.

Each entry MUST automate the build process using bash, gmake, gcc and/or clang and other commonly available tools under an SUS environment.

NOTE: any reference to gmake(1) can be any GNU Make compatible tool.

NOTE: it is better to use the ${MAKE} variable as then one may override the make command.

NOTE: it is better to NOT modify ${CC} in your Makefile as this makes your program more portable.

See Rule 15 - GNU Makefile.

See Guidelines for Rule 9 - No interactive compiling allowed.

Programs that require special privileges (setuid(2), setgid(2), super-user, special owner, special group, etc.) are HIGHLY DISCOURAGED. We do NOT guarantee these functions will behave as you expect on our test platforms.

If your program needs special permissions you MUST document this fact and explain why it is needed in your submission’s remarks.md file, preferably close to the top, because mkiocccentry(1) will NOT set those modes.

Furthermore, if you need a supplementary program that you include to have these permissions, you will also have to explain this as well, because these bits are NOT allowed in submissions and those bits will NOT be copied by mkiocccentry(1) either.

See Rule 4 - Required files, and Rule 5 - Do NOT modify files.

Legal abuse of the rules is somewhat encouraged, but comes with risks. A submission that, in the opinion of the Judges, violates the rules WILL BE DISQUALIFIED. Submissions that plan to abuse the rules MUST justify why their rule abuse is legal (or interesting) in the remarks.md file.

Submission packaging abuse will NOT be tolerated and for this reason, amongst many other reasons, it is recommended that you use mkiocccentry(1). See also Rule 17 - use mkiocccentry.

During development you can check your prog.c size limits using iocccsize(1), e.g., iocccsize -v1 prog.c. Overriding problems detected by any of the IOCCC tools risks that the submission will be rejected.

If you do submit a rule abusing or violating submission, then consider also submitting an alternate version in a different submission server slot that does NOT violate the rules in case your rule abuse excuse is rejected.

IMPORTANT: If you do submit such an alternate non-rule abusing version, PLEASE indicate that in your remarks.md file of your non-rule abusing version so that the Judges don’t think you uploaded a duplicate into a wrong slot by mistake.

See Guidelines for Rule 11 - Legal rule abuse.

Use of UTF-8 is supported by C11 standard and its updates and so too by the IOCCC.

See Guidelines for Rule 12 - UTF-8.

Any C source that fails to compile because lines contain carriage-returns (CTRL+M, \r) in particular as part of DOS/Windows style newlines might be rejected. Please do NOT put trailing CTRL+M in prog.c, Makefile, or remarks.md files, but instead end lines with Unix newline (line-feed, CTRL+J, \n).

You are permitted, in order to try and squeeze your prog.c under a Rule 2a Gross Size and/or Rule 2b Net Size limits, to NOT end source with a newline. If you need to do this, please document that in your remarks.md file and if your compiler complains about this, document this too and update your Makefile to account for this.

See Guidelines for Rule 13 - No carriage returns in prog.c.

Submissions that are substantially similar to previous winning IOCCC entries are discouraged. See the FAQ on “What types of entries have been frequently submitted to the IOCCC?.

You are allowed to resubmit to a later contest, a submission that didn’t win the IOCCC. If you do so, then you would be WELL ADVISED to try and enhance and improve your submission. If you do resubmit something that didn’t previously win, then you are encouraged to mention this in your remarks.md file as this sometimes helps.

The Makefile MUST adhere to GNU Makefile syntax. It’s highly recommended that you copy the Makefile.example as a template, renaming it to Makefile. The primary target of the Makefile MUST be prog and the source file is prog.c. The C compiler and preprocessor MUST be invoked using ${CC} and ${CPP} macros respectively.

The clobber target MUST clean and restore the submission directory tree to its original state, such as removing temporary files or any modified data files. Don’t assume that git tool chain is installed, i.e. do NOT expect git clean -f to work. To be clear: do NOT use git to clean out files as this WILL cause problems in the winner repo!

The Makefile MUST use a syntax that is compatible with bash(1) and GNU make(1). You are encouraged to use SHELL= bash in your Makefile; please add a space between the = and the value such as:

    SHELL= bash

Assume that commands commonly found in Single UNIX Specification environments and systems are available in the search $PATH.

Avoid command options that aren’t part of SUS, such as GNU long name options (such as --example) or extensions like grep -P.

Do NOT assume that . (current directory) is in the $PATH environment variable.

See Rule 4 - Required files.

See FAQ on “What are the detailed recommendations for a submission Makefile?”.

See Guidelines for Rule 15 - GNU Makefile.

Your source code, data files, remarks, and program output MUST NOT identify the author(s) of your code (unless you are Peter Honeyman or pretending to be Peter Honeyman). In order to be impartial the Judges do NOT know who is submitting programs to the IOCCC until final selection is made, and only submissions that won.

Even if you are a past winner, you MUST NOT reveal your identity deliberately or by accident anywhere in your submission. Failure to comply may result in the submission being disqualified.

TL;DR: use mkiocccentry(1) to package your submission.

The files, directories, and paths MUST conform to the limits imposed by mkiocccentry(1), including but NOT limited to, their names, count, path length, directory tree depth, and permissions.

The submission tree may have at most 13 directories and maximum directory depths of 4. The maximum number of files, excluding prog.c, remarks.md, Makefile, prog.alt.c, try.sh, try.alt.sh, .info.json and .auth.json is 31.

The maximum file-path component is 38 and the maximum path length is 60.

The submission MUST be a .txz compressed tarball (i.e., tarball compressed by xz(1)) as constructed by mkiocccentry(1), which MUST be within [1..3999971] bytes in size.

The tarball MUST be a v7 format tarball, and any supplementary tarballs you provide MUST ALSO be v7 format.

The sum of all file lengths, after unpacking from the tarball, MUST be less than 27651*1024 (28314624) bytes.

The submission MUST pass all txzchk(1) checks, and the tarball filename MUST pass all fnamchk(1) checks as well.

You may NOT submit a tarball created by the -d, -e OR by the -s seed options of mkiocccentry(1).

The unpacked submission directory MUST pass the checks performed by the minimum required version of chksubmit(1).

Submissions may NOT use a mkiocccentry toolkit command line options that are labelled as “for TESTING purposes”, OR labelled as “for the use by the IOCCC judges only”, in their respective man pages.

Submissions may ONLY use mkiocccentry toolkit command lines that confirm to the command’s -h “print help message”, again notwithstanding the options for testing and the IOCCC judges.

The prog.c file should pass the Rule 2 - Size restrictions checks performed by iocccsize(1).

The .auth.json and .info.json files MUST be exactly what the minimum required mkiocccentry(1) version generates.

Any submission that fails to pass the checks performed by the minimum required version of the mkiocccentry toolkit WILL be rejected!

See Rule 4 - Required files.

See the FAQ on “What is the mkiocccentry tool, how do I obtain it and how do I use it?”.

See the Guidelines for Rule 17 - Use mkiocccentry.

The entirety of your submission MUST be submitted under the following license:

CC BY-SA 4.0 DEED Attribution-ShareAlike 4.0 International.

See Rule 7 - Original Work.

This 19th rule, while prime, is reserved for future abuse 😁. Additional rules, both humorous and otherwise, may be added below this rule in future contests.

See Rule 6 - Free Rule.

Further Reading

Please read the Guidelines next for further commentary on these rules.

Read the FAQ on “Entering the IOCCC: the bare minimum you need to know” for an overview of how to enter the contest.

See also the FAQ in general.

And now for something completely different.

Consider joining the IOCCC Discord Server for discussions about The Rules, questions for the Judges, and C in general.

Leonid A. Broukhis chongo (Landon Curt Noll) /\cc/\