The International Obfuscated C Code Contest (abbreviated IOCCC) is a computer programming contest for code written in C that is the most creatively obfuscated and held annually (when possible). It is described as "celebrating [C's] syntactical opaqueness".[1] The winning code for the 28th contest, held in 2024/25, was announced by live stream 2 Aug 2025.[2] Previous contests were held in the years 1984–1996, 1998, 2000, 2001, 2004–2006, 2011–2015, 2018–2020, and 2024.
| International Obfuscated C Code Contest | |
|---|---|
 The logo of the IOCCC | |
| Status | Active |
| Genre | Coding contest |
| Frequency | Semi-annually |
| Years active | 1984–1996, 1998, 2000, 2001, 2004–2006, 2011–2015, 2018–2020, 2024 |
| Inaugurated | 1984 |
| Founders | Landon Curt Noll, Larry Bassel |
| Most recent | 2024 |
| Next event | Dec 2025 |
| Website | www.ioccc.org |
Entries are evaluated anonymously by the current sitting judges, Leonid A. Broukhis & Landon Curt Noll. The judging process is documented in the competition guidelines[3] and consists of elimination rounds. By tradition, no information is given about the total number of entries for each competition. Winning entries are awarded with a category, such as "Worst Abuse of the C preprocessor" or "Most Erratic Behavior", and then announced on the official IOCCC website. The contest states that being announced on the IOCCC website is the reward for winning (plus bragging rights).
History
editThe IOCCC was started by Landon Curt Noll and Larry Bassel in 1984 while employed at National Semiconductor's Genix porting group. The idea for the contest came after they compared notes with each other about some poorly written code that they had to fix, notably the Bourne shell, which used macros to emulate ALGOL 68 syntax, and a buggy version of finger for BSD.[4] The contest itself was the topic of a quiz question in the 1993 Computer Bowl.[5] After a hiatus of five years starting in 2006, the contest returned in 2011.[6]
Compared with other programming contests, the IOCCC is described as "not all that serious" by Michael Swaine, editor of Dr. Dobb's Journal.[7]. Thought many IOCCC contestants may beg to differ.
Rules
editThe Rules[8] vary from year to year and are posted with a set of Guidelines[9] that attempt to convey additional enlightenment about the Rules. They are published on the IOCCC website. In addition all material including winning entries are published under Creative Commons license BY-SA 4.0 International.[10].
Hacking the contest rules is a tradition.
The Rules are often deliberately written with loopholes that contestants are encouraged to find and abuse.[9] Entries that take advantage of loopholes can cause the rules for the following year's contest to be adjusted.[9]
The most significant of the Rules is Rule 2a & 2b (originally Rule 1), gross & net source size limits. During the life time of the contest, Rule 2 has evolved[11] to accommodate subtle increases in source size limits. The 1984 contest started with a maximum source size of 512 bytes, which increased a few more times to 1536 bytes in 1991. In 1992, Rule 2 was split to distinguish between maximum overall size and maximum size ignoring white space and semicolons given certain conditions. In the early days, in order to make best use of space allowed, white space was stripped, often resulting in a compact blob of text, making it hard to read by humans, but served little purpose once passed through a C "pretty print" utility, which the judges did as part of their process. In 1992 the judges believed that form of obfuscation had played out and they wanted to encourage people to explore other ways of formatting the code, such as an ASCII image themed after the entry, or simply more traditionally indented C source. Around 2012/2013 the iocccsize(1)[12] tool was adopted by the contest and tweaked to aid both contestants and judges apply the Rule 2b counting algorithm.
Obfuscations employed
editEntries often employ strange or unusual tricks, such as using the C preprocessor to do things it was not designed to do[a] or avoiding commonly used constructs in the C programming language in favor of much more obscure ways of achieving the same thing.
Contributions have included source code formatted to resemble images, text, etc., after the manner of ASCII art, preprocessor redefinitions to make code harder to read, and self-modifying code. In several years an entry was submitted that required a new definition of some of the rules for the next year, regarded as a high honor. An example is the world's shortest self-reproducing program. The entry was a program designed to output its own source code, and which had zero bytes of source code. When the program ran, it printed out zero bytes, equivalent to its source code.[15]
In the effort to take obfuscation to its extremes, contestants have produced programs which skirt around the edges of C standards, or result in constructs which trigger rarely used code path combinations in compilers. As a result, several of the past entries may not compile directly in a modern compiler, and some may cause crashes.
Examples
editWithin the code size limit of only a few kilobytes, contestants have managed to do complicated things – a 2004 winner turned out an operating system.[16]
Toledo Nanochess
editToledo Nanochess is a chess engine created by Mexican software developer Oscar Toledo Gutiérrez, a five-time winner of the IOCCC. In accordance with IOCCC rules, it is 1255 characters long. The author claims that it is the world's smallest chess program written in C.
The source code for Toledo Nanochess and other engines is available.[17] Because Toledo Nanochess is based on Toledo's winning entry from the 18th IOCCC (Best Game[18]), it is heavily obfuscated.[19]
On February 2, 2014, the author published the book Toledo Nanochess: The commented source code, which contains the fully commented source code.[20]
As of February 7, 2010, it appears to be one of only two chess engines written in less than 2 kilobytes of C that are able to play full legal chess moves, along with Micro-Max by Dutch physicist H. G. Muller. In 2014 the 1 kilobyte barrier was broken by Super Micro Chess[21] – a derivative of Micro-Max – totaling 760 characters (spaces and newlines included).[22] There is also a smaller version of Toledo's engine, the Toledo Picochess, consisting of 944 non-blank characters.
Source code excerpt
B,i,y,u,b,I[411],*G=I,x=10,z=15,M=1e4;X(w,c,h,e,S,s){int t,o,L,E,d,O=e,N=-M*M,K
=78-h<<x,p,*g,n,*m,A,q,r,C,J,a=y?-x:x;y^=8;G++;d=w||s&&s>=h&&v 0,0)>M;do{_ o=I[
p=O]){q=o&z^y _ q<7){A=q--&2?8:4;C=o-9&z?q["& .$ "]:42;do{r=I[p+=C[l]-64]_!w|p
==w){g=q|p+a-S?0:I+S _!r&(q|A<3||g)||(r+1&z^y)>9&&q|A>2){_ m=!(r-2&7))P G[1]=O,
K;J=n=o&z;E=I[p-a]&z;t=q|E-7?n:(n+=2,6^y);Z n<=t){L=r?l[r&7]*9-189-h-q:0 _ s)L
+=(1-q?l[p/x+5]-l[O/x+5]+l[p%x+6]*-~!q-l[O%x+6]+o/16*8:!!m*9)+(q?0:!(I[p-1]^n)+
!(I[p+1]^n)+l[n&7]*9-386+!!g*99+(A<2))+!(E^y^9)_ s>h||1<s&s==h&&L>z|d){p[I]=n,O
[I]=m?*g=*m,*m=0:g?*g=0:0;L-=X(s>h|d?0:p,L-N,h+1,G[1],J=q|A>1?0:p,s)_!(h||s-1|B
-O|i-n|p-b|L<-M))P y^=8,u=J;J=q-1|A<7||m||!s|d|r|o<z||v 0,0)>M;O[I]=o;p[I]=r;m?
*m=*g,*g=0:g?*g=9^y:0;}_ L>N){*G=O _ s>1){_ h&&c-L<0)P L _!h)i=n,B=O,b=p;}N=L;}
n+=J||(g=I+p,m=p<O?g-3:g+2,*m<z|m[O-p]||I[p+=p-O]);}}}}Z!r&q>2||(p=O,q|A>2|o>z&
!r&&++C*--A));}}}Z++O>98?O=20:e-O);P N+M*M&&N>-K+1924|d?N:0;}main(){Z++B<121)*G
++=B/x%x<2|B%x<2?7:B/x&4?0:*l++&31;Z B=19){Z B++<99)putchar(B%x?l[B[I]|16]:x)_
x-(B=F)){i=I[B+=(x-F)*x]&z;b=F;b+=(x-F)*x;Z x-(*G=F))i=*G^8^y;}else v u,5);v u,
1);}}
Pi
editBelow is a 1988 entry which calculates pi by looking at its own area:[23]
#define _ -F<00||--F-OO--;
int F=00,OO=00;main(){F_OO();printf("%1.3f\n",4.*-F/OO/OO);}F_OO()
{
_-_-_-_
_-_-_-_-_-_-_-_-_
_-_-_-_-_-_-_-_-_-_-_-_
_-_-_-_-_-_-_-_-_-_-_-_-_-_
_-_-_-_-_-_-_-_-_-_-_-_-_-_-_
_-_-_-_-_-_-_-_-_-_-_-_-_-_-_
_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_
_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_
_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_
_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_
_-_-_-_-_-_-_-_-_-_-_-_-_-_-_
_-_-_-_-_-_-_-_-_-_-_-_-_-_-_
_-_-_-_-_-_-_-_-_-_-_-_-_-_
_-_-_-_-_-_-_-_-_-_-_-_
_-_-_-_-_-_-_-_
_-_-_-_
}
(This entry was written in K&R C; it does not work correctly in ANSI C without some changes.[24])
Flight simulator
editAnother example is the following flight simulator, the winner of the 1998 IOCCC,[25] as listed and described in Calculated Bets: Computers, Gambling, and Mathematical Modeling to Win (2001)[26] and shown below:
#include <math.h>
#include <sys/time.h>
#include <X11/Xlib.h>
#include <X11/keysym.h>
double L ,o ,P
,_=dt,T,Z,D=1,d,
s[999],E,h= 8,I,
J,K,w[999],M,m,O
,n[999],j=33e-3,i=
1E3,r,t, u,v ,W,S=
74.5,l=221,X=7.26,
a,B,A=32.2,c, F,H;
int N,q, C, y,p,U;
Window z; char f[52]
; GC k; main(){ Display*e=
XOpenDisplay( 0); z=RootWindow(e,0); for (XSetForeground(e,k=XCreateGC (e,z,0,0),BlackPixel(e,0))
; scanf("%lf%lf%lf",y +n,w+y, y+s)+1; y ++); XSelectInput(e,z= XCreateSimpleWindow(e,z,0,0,400,400,
0,0,WhitePixel(e,0) ),KeyPressMask); for(XMapWindow(e,z); ; T=sin(O)){ struct timeval G={ 0,dt*1e6}
; K= cos(j); N=1e4; M+= H*_; Z=D*K; F+=_*P; r=E*K; W=cos( O); m=K*W; H=K*T; O+=D*_*F/ K+d/K*E*_; B=
sin(j); a=B*T*D-E*W; XClearWindow(e,z); t=T*E+ D*B*W; j+=d*_*D-_*F*E; P=W*E*B-T*D; for (o+=(I=D*W+E
*T*B,E*d/K *B+v+B/K*F*D)*_; p<y; ){ T=p[s]+i; E=c-p[w]; D=n[p]-L; K=D*m-B*T-H*E; if(p [n]+w[ p]+p[s
]== 0|K <fabs(W=T*r-I*E +D*P) |fabs(D=t *D+Z *T-a *E)> K)N=1e4; else{ q=W/K *4E2+2e2; C= 2E2+4e2/ K
*D; N-1E4&& XDrawLine(e ,z,k,N ,U,q,C); N=q; U=C; } ++p; } L+=_* (X*t +P*M+m*l); T=X*X+ l*l+M *M;
XDrawString(e,z,k ,20,380,f,17); D=v/l*15; i+=(B *l-M*r -X*Z)*_; for(; XPending(e); u *=CS!=N){
XEvent z; XNextEvent(e ,&z);
++*((N=XLookupKeysym
(&z.xkey,0))-IT?
N-LT? UP-N?& E:&
J:& u: &h); --*(
DN -N? N-DT ?N==
RT?&u: & W:&h:&J
); } m=15*F/l;
c+=(I=M/ l,l*H
+I*M+a*X)*_; H
=A*r+v*X-F*l+(
E=.1+X*4.9/l,t
=T*m/32-I*T/24
)/S; K=F*M+(
h* 1e4/l-(T+
E*5*T*E)/3e2
)/S-X*d-B*A;
a=2.63 /l*d;
X+=( d*l-T/S
*(.19*E +a
*.64+J/1e3
)-M* v +A*
Z)*_; l +=
K *_; W=d;
sprintf(f,
"%5d %3d"
"%7d",p =l
/1.7,(C=9E3+
O*57.3)%0550,(int)i); d+=T*(.45-14/l*
X-a*130-J* .14)*_/125e2+F*_*v; P=(T*(47
*I-m* 52+E*94 *D-t*.38+u*.21*E) /1e2+W*
179*v)/2312; select(p=0,0,0,0,&G); v-=(
W*F-T*(.63*m-I*.086+m*E*19-D*25-.11*u
)/107e2)*_; D=cos(o); E=sin(o); } }
This program needs the following command line on a Linux system to be compiled:[25]
$ cc banks.c -o banks -DIT=XK_Page_Up -DDT=XK_Page_Down \
-DUP=XK_Up -DDN=XK_Down -DLT=XK_Left -DRT=XK_Right \
-DCS=XK_Return -Ddt=0.02 -lm -lX11 -L/usr/X11R6/lib
In order to run the binary file (banks) it has to be supplied with a .sc scenery file via stdin input:[25]
$ cat pittsburgh.sc | ./banks
Akari
editBelow is a 2011 entry which downsamples PGM, PPM images and ASCII art (of Akari from YuruYuri) by Don, Yang:[27]
/*
+
+
+
+
[ >i>n[t
*/ #include<stdio.h>
/*2w0,1m2,]_<n+a m+o>r>i>=>(['0n1'0)1;
*/int/**/main(int/**/n,char**m){FILE*p,*q;int A,k,a,r,i/*
#uinndcelfu_dset<rsitcdti_oa.nhs>i/_*/;char*d="P%" "d\n%d\40%d"/**/
"\n%d\n\00wb+",b[1024],y[]="yuriyurarararayuruyuri*daijiken**akkari~n**"
"/y*u*k/riin<ty(uyr)g,aur,arr[a1r2a82*y2*/u*r{uyu}riOcyurhiyua**rrar+*arayra*="
"yuruyurwiyuriyurara'rariayuruyuriyuriyu>rarararayuruy9uriyu3riyurar_aBrMaPrOaWy^?"
"*]/f]`;hvroai<dp/f*i*s/<ii(f)a{tpguat<cahfaurh(+uf)a;f}vivn+tf/g*`*w/jmaa+i`ni("/**
*/"i+k[>+b+i>++b++>l[rb";int/**/u;for(i=0;i<101;i++)y[i*2]^="~hktrvg~dmG*eoa+%squ#l2"
":(wn\"1l))v?wM353{/Y;lgcGp`vedllwudvOK`cct~[|ju {stkjalor(stwvne\"gt\"yogYURUYURI"[
i]^y[i*2+1]^4;/*!*/p=(n>1&&(m[1][0]-'-'||m[1][1] !='\0'))?fopen(m[1],y+298):stdin;
/*y/riynrt~(^w^)],]c+h+a+r+*+*[n>)+{>f+o<r<(-m] =<2<5<64;}-]-(m+;yry[rm*])/[*
*/q=(n<3||!(m[2][0]-'-'||m[2][1]))?stdout /*]{ }[*/:fopen(m[2],d+14);if(!p||/*
"]<<*-]>y++>u>>+r >+u+++y>--u---r>++i+++" <)< ;[>-m-.>a-.-i.++n.>[(w)*/!q/**/)
return+printf("Can " "not\x20open\40%s\40" "" "for\40%sing\n",m[!p?1:2],!p?/*
o=82]5<<+(+3+1+&.(+ m +-+1.)<)<|<|.6>4>-+(> m- &-1.9-2-)-|-|.28>-w-?-m.:>([28+
*/"read":"writ");for ( a=k=u= 0;y[u]; u=2 +u){y[k++ ]=y[u];}if((a=fread(b,1,1024/*
,mY/R*Y"R*/,p/*U*/)/* R*/ )>/*U{ */ 2&& b/*Y*/[0]/*U*/=='P' &&4==/*"y*r/y)r\}
*/sscanf(b,d,&k,& A,& i, &r)&& ! (k-6&&k -5)&&r==255){u=A;if(n>3){/*
]&<1<6<?<m.-+1>3> +:+ .1>3+++ . -m-) -;.u+=++.1<0< <; f<o<r<(.;<([m(=)/8*/
u++;i++;}fprintf (q, d,k, u >>1,i>>1,r);u = k-5?8:4;k=3;}else
/*]>*/{(u)=/*{ p> >u >t>-]s >++(.yryr*/+( n+14>17)?8/4:8*5/
4;}for(r=i=0 ; ;){u*=6;u+= (n>3?1:0);if (y[u]&01)fputc(/*
<g-e<t.c>h.a r -(-).)8+<1. >;+i.(<)< <)+{+i.f>([180*/1*
(r),q);if(y[u ]&16)k=A;if (y[u]&2)k--;if(i/*
("^w^NAMORI; { I*/==a/*" )*/){/**/i=a=(u)*11
&255;if(1&&0>= (a= fread(b,1,1024,p))&&
")]i>(w)-;} { /i-f-(-m--M1-0.)<{"
[ 8]==59/* */ )break;i=0;}r=b[i++]
;u+=(/**>> *..</<<<)<[[;]**/+8&*
(y+u))?(10- r?4:2):(y[u] &4)?(k?2:4):2;u=y[u/*
49;7i\(w)/;} y}ru\=*ri[ ,mc]o;n}trientuu ren (
*/]-(int)'`';} fclose( p);k= +fclose( q);
/*] <*.na/m*o{ri{ d;^w^;} }^_^}}
" */ return k- -1+ /*\' '-`*/
( -/*}/ */0x01 ); {;{ }}
; /*^w^*/ ;}
If the program is run using its own source as the input, the result is:
$ ./akari akari.c
int
*w,m,_namori=('n');
#include<stdio.h>/*;hrd"% dnd4%"*/
/**/int(y),u,r[128*2/*{y}icuhya*rr*rya=
*/];void/**/i(){putchar(u);}int/**/main(/*
"(n"l)?M5{YlcpvdluvKct[j skao(tve"t"oYRYR"
*/int(w),char**n){for(m =256;--m;r[m]/*
"<*]y+u>r>u+y-u-r+i+" ) ;>m.a.i+n>()/q*/
=25<(31&( m -1))||64-( m &192)||2>w?m:(2+
m/*"*,/U// R/)/U * & /Y/0/U/=P &=/"*/)\
&16?m-13 : 13+ m) ;u=+10 ;for(;(m=/*
*>/()/{ p u t-s +(yy*+ n1>7?/:*/
getchar ())+1 ;i() ){if(10/*
"wNMR;{ I/=/" )/{*/==u*1
)i(); if(m-10){
u=/*> *./<)[;*/8*
4;i(); }u=r[ m];}return(
* *n/*{i ;w; }_}
( -*/ *00 ) ; }
$ ./akari akari.c > ./akari.small
$ ./akari ./akari.small
wm_aoi(n)
/*ity,,[2*/{}char*y=
(")M{lpduKtjsa(v""YY"
"*yuruyuri") ;main(/*
/",U/ R)U* Y0U= ="/\
*/){puts (y+ 17/*
"NR{I=" ){/=*
=* */);/*
**/{ ;;}}
$
$ ./akari ./akari.small > ./akari.smaller
$ ./akari ./akari.smaller
main
(){puts("Y"
"U RU YU "\
"RI" )/*
*/ ;}
$
See also
editNotes
editReferences
edit- ^ Palmer, Geoff (November 1, 2004). "Beyond the command line". PC World New Zealand. Fairfax Media, Fairfax New Zealand Limited. Archived from the original on February 10, 2013. Retrieved 2013-04-07.
- ^ "Previous IOCCC Winners with spoilers". IOCCC.org. International Obfuscated C Code Contest. Retrieved 2023-05-01.
- ^ "Judging Process" (HTML). IOCCC.org. International Obfuscated C Code Contest. Retrieved 2025-08-27.
- ^ "How it started". IOCCC.org. International Obfuscated C Code Contest. Retrieved 2025-08-27.
- ^ "Top Execs Fail To Compute Correctly". San Jose Mercury News. California. May 15, 1993. p. 1A. Archived from the original on 2014-08-08. Retrieved 2025-08-08 – via Newsbank.
- ^ a b Jackson, Joab (November 15, 2011). "Obfuscated Code Contest Returns". PC World. Retrieved 2023-05-01.
- ^ Swaine, Michael (May 1, 2008). "There Must be Contest". Dr. Dobb's Journal. Retrieved 2025-08-27.
- ^ "Rules". IOCCC.org. International Obfuscated C Code Contest.
- ^ a b c "Guidelines". IOCCC.org. International Obfuscated C Code Contest.
- ^ "IOCCC License Employed". IOCCC.org. International Obfuscated C Code Contest.
- ^ "Rule 2 Eras". IOCCC.org. International Obfuscated C Code Contest.
- ^ "iocccsize". GitHub.com. Anthony C Howe.
- ^ Spinellis, Diomidis (October 5, 2006). "Code Finessing". Dr. Dobb's Journal. Retrieved 2023-05-01.
- ^ "IOCCC 2004 – Best Abuse of CPP". IOCCC.org. International Obfuscated C Code Contest. Retrieved 2023-05-01.
- ^ "iocc smr" (plain text). IOCCC.org. International Obfuscated C Code Contest. 1994. Retrieved 2025-08-05.
- ^ "gavin.hint3" (plain text). IOCCC.org. International Obfuscated C Code Contest. 2004. Retrieved 2023-05-01.
- ^ "Toledo Nanochess and Toledo Picochess". nanochess.org.
- ^ "Winners: The authors of winning IOCCC entries". IOCCC.org. International Obfuscated C Code Contest.
- ^ "Nanochess partially de-obfuscated version". GitHub. Retrieved 1 May 2023.
- ^ Toledo Gutiérrez, Oscar (2014). Toledo Nanochess: The commented source code. Lulu. ISBN 978-1-304-86437-6.
- ^ "Super Micro Chess Engine - Home Page - Extremely Compact C Source and Executable Program Size". smmax.sourceforge.net.
- ^ "Super Micro FIDE 760". sourceforge.net.
- ^ "westley.c". IOCCC.org. International Obfuscated C Code Contest. 1988.
- ^ Using gcc, compile with the following command line: or
$ gcc -traditional-cpp -o r r.c
(The source file is$ gcc -E r.c | sed 's/- -/--/g' > r2.c ; gcc -o r2 r2.c
r.c)[original research?] - ^ a b c Carl Banks' Blog: IOCCC Flight Simulator. blog.aerojockey.com. Retrieved 2023-05-01.
- ^ Skiena, Steven (2001). Calculated Bets: Computers, Gambling, and Mathematical Modeling to Win. The Mathematical Association of America. pp. 152, 153. ISBN 978-0521009621.
- ^ "Winners: The authors of winning IOCCC entries". IOCCC.org. International Obfuscated C Code Contest. Retrieved 2023-05-01.