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i see the question on a c++ programming context, i check for a solution and one of my friend give me this code its works perfect but i can't understand it's logic and also how it's works. i asked to him about it but he also don't know how the program is actually works, i think he is also take this solution from somewhere. Anybody can explain the logic behind this i mean in the line (&main + (&exit - &main)*(j/1000))(j+1); ? 

#include <stdio.h> #include <stdlib.h> void main(int j) { printf("%d\n", j); (&main + (&exit - &main)*(j/1000))(j+1); }

Thanks in advance

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    Can not recursively calling main in C++. Commented Nov 4, 2014 at 9:32
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    Thanks for your quick replay. i try it in c++ but it will not work but in c its works perfectly.but i don't know how its works .i mean its logic Commented Nov 4, 2014 at 9:34
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    Is this a standardmain signature? Commented Nov 4, 2014 at 9:40
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    Seems like this code is "cheating" by using a recursive function call to create the counting loop (it's not an explicit loop, but there's still a loop there). Still, nice solution. Commented Nov 4, 2014 at 10:37
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    The pointer subtraction is undefined behaviour. Commented Nov 4, 2014 at 10:47

2 Answers 2

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It works as follows:

Performs the int division j/1000, which will return 0 always while j is smaller than 1000. So the pointer operation is as follows:

&main + 0 = &main, for j < 1000.

Then it calls the resulting function pointed by the pointer operations passing as parameter j+1. While j is less than 1000, it will call main recursively with parameter one more than the step before.

When the value of j reaches 1000, then the integer division j/1000 equals to 1, and the pointer operation results in the following:

&main + &exit - &main = &exit.

It then calls the exit function, which finishes the program execution.

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3 Comments

does the exit function must be written explicitly? or it can be deduced from the main?
The exit function is from the C std library, defined in stdlib.h.
It basically "linearly interpolates" the difference from the MAIN function to the EXIT function, with integer-division so that the switch from A to B is instantaneous once 1001 has been reached - and the integer is increased each time from the recursive call of MAIN.
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I go with the explanation already given but it would be easier to understand if written as below:

void main(int j) { if(j == 1001) return; else { printf("%d\n", j); main(j+1); } }

The above code does the same as already written code.

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4 Comments

Realy this is more simple than the above .but logic is different
The idea is the same though. The one you have has just been obfuscated, this is the canonical way to do recursion :- though you wouldn't usually use main to do it as it technically isn't a valid main signature iirc
really this is more simple method rather than above one. thanks for your replay
@ArunPrasanth np. The idea behind this is recursion and this would make you understand the code easily instead of the code which you have posted

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