The Jargon File (version 4.4.7, 29 Dec 2003):

Obfuscated C Contest
 n.

    (in full, the ?International Obfuscated C Code Contest?, or IOCCC) An
    annual contest run since 1984 over Usenet by Landon Curt Noll and friends.
    The overall winner is whoever produces the most unreadable, creative, and
    bizarre (but working) C program; various other prizes are awarded at the
    judges' whim. C's terse syntax and macro-preprocessor facilities give
    contestants a lot of maneuvering room. The winning programs often manage to
    be simultaneously (a) funny, (b) breathtaking works of art, and (c)
    horrible examples of how not to code in C.

    This relatively short and sweet entry might help convey the flavor of
    obfuscated C:

    /*
     * HELLO WORLD program
     * by Jack Applin and Robert Heckendorn, 1985
     * (Note: depends on being able to modify elements of argv[],
     * which is not guaranteed by ANSI and often not possible.)
     */
    main(v,c)char**c;for(v[c++]="Hello, world!\n)";
    (!!c)[*c]&&(v--||--c&&execlp(*c,*c,c[!!c]+!!c,!c));
    **c=!c)write(!!*c,*c,!!**c);

    Here's another good one:

    /*
     * Program to compute an approximation of pi
     * by Brian Westley, 1988
     * (requires pcc macro concatenation; try gcc -traditional-cpp)
     */

    #define _ -F<00||--F-OO--;
    int F=00,OO=00;
    main()F_OO();printf("%1.3f\n",4.*-F/OO/OO);F_OO()
    
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    Note that this program works by computing its own area. For more digits,
    write a bigger program. See also hello world.

    The IOCCC has an official home page at http://www.ioccc.org/.


The Free On-line Dictionary of Computing (30 December 2018):

Obfuscated C Contest

    The International Obfuscated C Code Contest
   (IOCCC) is an annual contest run since 1984 over Usenet by
   Landon Curt Noll and friends.  The overall winner is whoever
   produces the most unreadable, creative, and bizarre (but
   working) C program.  Various other prizes are awarded at the
   judges' whim.  C's terse syntax and macro-preprocessor
   facilities give contestants a lot of maneuvering room.  The
   winning programs often manage to be simultaneously funny,
   breathtaking works of art and horrible examples of how *not*
   to code in C.

   This relatively short and sweet hello, world program
   demonstrates obfuscated C:

    /* HELLO WORLD program
     * by Jack Applin and Robert Heckendorn, 1985 */

    main(v,c)char**c;for(v[c++]="Hello, world!\n)";
    (!!c)[*c]&&(v--||--c&&execlp(*c,*c,c[!!c]+!!c,!c));
    **c=!c)write(!!*c,*c,!!**c);

   Here's another good one:

    /* Program to compute an approximation of pi
     * by Brian Westley, 1988 */

    #define _ -F<00||--F-OO--;
    int F=00,OO=00;
    main()F_OO();printf("%1.3f\n",4.*-F/OO/OO);F_OO()
    
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   Note that this program works by computing its own area.  For
   more digits, write a bigger program.

   [Jargon File]