How does this "hello world!" program work?

Question

int main(void)
{ 
    return('yes', *"no", **main, *********printf) ("hello world!\n") *0; 
}

outputs hello world!, but how does it actually work?

`return('yes', *"no", **main, *********printf) ("Its magical!\n") *0;` By the way, i compiled it and it actually works ._. — Warty, Apr 19 '10 at 18:56
you have a strange style to write software!You will never receive a job at me. :-) — Oleg, Apr 19 '10 at 18:59
As a pedantic remark, in ANSI C (in C89/90 as well) calling a variadic function (`printf` in this case) without declaring a prototype first is undefined behavior. So, from the pedantic point of view your program works only accidentally. You need to include `stdio.h` for it to become a defined C program. — AnT stands with Russia, May 08 '10 at 20:18

rampion · Answer 1 · 2010-04-19T19:02:35.047

46

Two things really:

Function pointers don't dereference the same as other pointers. *main == main
A comma separated list returns the value of the last element in the list

So if we simplify the pointers:

int main(void)
{ 
    return('yes', *"no", main, printf) ("hello world!\n") *0; 
}

And using the last element in the list as the value of the list

int main(void)
{ 
    return printf("hello world!\n") *0; 
}

printf returns the number of characters printed

int main(void)
{ 
    return 13 *0; 
}

And 13*0 is left as an exercise to the reader.

edited Apr 19 '10 at 19:02

answered Apr 19 '10 at 18:56

rampion

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Some friends of mine were having a mustache growing contest, and I had to shave. So I wore a fake mustache that day. – rampion Apr 19 '10 at 19:01
8

to be pedantic: function pointers dereference just fine, but the resulting function designator will be immediately converted back to a function pointer – Christoph Apr 19 '10 at 19:06
@rampion: I can't understand the last exercise example you've given. What do you mean by that? Please help me. Output is 0. – Destructor May 06 '15 at 12:47
meet: what does that mean `main` returns? – rampion May 06 '15 at 13:12
@rampion: oh, main() always return 0. I understood. Thanks – Destructor May 06 '15 at 13:14

score 12 · Answer 2 · edited Apr 19 '10 at 19:05

('yes', *"no", **main, *********printf)

will evaluate to *********printf, because comma operator evaluates its operands and returns value of last expression. *********printf is equal to printf, as dereferencing function pointer results in the same function pointer; it does nothing.

Next, result of first parenthesis, printf, is applied to ("hello world!\n") which results in text printed to screen. printf function returns number of characters written. That number is then multiplied with 0 and product is returned by main function.

How does this "hello world!" program work?

2 Answers2

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