Main is usually a function – when is it not?

For this type of stuff, I rather use a Macro Assembler.

Sadly the libc prng is not specified, and thus non-portable. It might work if things are checked on the same kernel and libc combination, or it might not. The Java PRNG is specified and globally portable.

The seed for the random number generator is not executed though.

  const char main[] =
    "UH\x89\xe5\xb8\1\0\0\0\xbb\1\0\0\0g\x8d"
    "5\x10\0\0\0\xba\r\0\0\0\xf\5\xb8\x3c\0\0\0"
    "1\xdb\xf\5Hello World!\n]\xc3";

Tangentially related: http://en.wikipedia.org/wiki/EICAR_test_file

There is another known trick :

Place your string after a "call" instruction, then when you are inside the call, the address of the string is on the stack, you can pop it in any register you want.

If that is the case then the education system is thoroughly broken. It is obvious that a person who took the time to do that learned far more than they would than had they copied one of millions of Hello World examples from the internet.

That being said, I wouldn't rule out a bored TA opening the file not seeing a printf and assigning a grade based on submitting something.

With enough macros you can do anything. I'm not aware of anyone doing this in C, but this is a cool example of doing approximately the same thing in a more easily extensible language: http://wall.org/~lewis/2013/10/15/asm-monad.html

It's possible, but once you're not obfuscating the code - and already depending on OS details when you assume that global data can be executable, whether const or not - I think you may as well just use the inline assembler feature of your compiler. Well, if there is one... MSVC bizarrely doesn't support on x86-64 what was a perfectly good feature on x86.

Not having access to a PDP-11 (maybe this works on an emulator?) is it cheating to ask what the output of this program is?

OK, your one-liner is simple enough that maybe it's worth a few minutes to unpack it here.

Let's begin with that big constant. The values of

  89-(0x1F&2846984999544829581>>5*G)

Now, what's the argument to putchar? It has the form

  11^--G ? thing1 : thing2

So far, so good. One more step out. What comes before the && inside main? It looks like this:

  10<putchar(stuff)

Nearly there. Now the whole thing looks like

  main(G) { thing_above && main(2+G); }

OK. So, I guess this program is intended to be invoked with no command-line args. Then G (= argc) will be 1. We'll write out characters 0, 1, 2, ..., 10 of the LELANDBATEY string (getting exactly that far) at which point G will be 12 on entry to the next main call. Newline, exit. Done.

(In case it isn't clear: it outputs "LELANDBATEY\n".)

No offence, I hope, but this is really tame compared with typical short-short IOCCC entries. Here's an example from the 22nd IOCCC. It's short enough to tweet.

  char a;float b,c;main(d){for(;d>2e3*c?c=1,scanf(" %c%f",&a,&c),d=55-a%32*9/5,b=d>9,d=d%13-a/32*12:1;a=2)++d<24?b*=89/84.:putchar(a=b*d);}

I remember QuickBasic (the precursor of VB: VB 1.0 was effectively QB 8.0) had a CALL ABSOLUTE statement, which let you execute machine code in a string. I used it to great effect in graphics demos, once upon a time.

That's almost certainly not what happened here. He was probably doing a Masters or something in a related field which had this requirement.

I think that sets the address of main to NULL, so it segfaults as soon as `_start` jumps to `main`. It's known for being one of the smallest compilable C programs.

EDIT: Your original post contained

  int main=0;

It does not. It just moves what "the problem" is.

An example of that is the article itself which leverages the gained information from SO to solve the real problem.

Also arcane knowledge is not the same thing as problem solving.

"Problem solving" is perhaps the wrong phrase here. It's more like "information gathering", which is an important step in problem solving.

Well more properly python doesn't have a "main" function. You just directly execute the initial script, which will import other modules, define functions and classes, etc, and finally (hopefully) execute something.

In C# main() is a method attached to the main Program class.

Probably not, since this isn't a case where my co worker can just copy paste my code and hope it works for two reasons. The first is I am not entirely sure their testing environment is using 64 bit linux, it could be 32 bit linux since I'm not in the class, I just assumed its 64bit so I could test it on my machine. The next reason is they don't actually have a Hello World assignment at all! The reason I wrote this article is so I could share something I learned, and my co worker (who has already completed the first half of the semester's assignments) could use this as a starting point for how to write his own version for an assignment.

I spent a bit getting it to work on OS X. The ASM becomes:

    movq $0x2000004, %rax  ; // BSD syscalls are divided into classes.
    movq $1, %rdi          ; // 64-bit registers use %rdi instead of %ebx
    lea message(%rip), %rsi; // Relative Address of message
    movq $13, %rdx         ; // Same Length
    syscall                ; // x86-64 ASM syscall

    movq $0x2000001, %rax  ; // Exit Syscall
    movq $0, %rdi;
    syscall

    message: .ascii "Hello World!\n";

  echo 'const int main[]={3850979413,184,3234285568,33554436,29869896,1207959552,1652109,3343384576,3522,1208291072,114887,3343385088,199,1208291072,1869376613,1919899424,169960556};'>t.c&&gcc t.c&&./a.out

[Edit]

Less compact but more readable:

  echo 'const int main[] = { 3850979413, 184, 3234285568, 33554436, 29869896, 1207959552, 1652109, 3343384576, 3522, 1208291072, 114887, 3343385088, 199, 1208291072, 1869376613, 1919899424, 169960556};' > test.c && gcc -Wall test.c -o test && ./test