#include<stdio.h>
#include<stdlib.h>
#include<signal.h>

int main(){

	/* main is a function defined in the code segment of the program, and we 
	 * can't change code segment memory on modern operating systems.  It used
	 * to be allowed, so that it would be possible to write programs that modify
	 * themselves!  Nowadays, if you want to do that, you have to keep the 
	 * modified code on the stack or heap instead of in the code segment.  That 
	 * presents a barrier to hacking, especially when combined with a 
	 * non-executable stack.  At any rate, the when the next two lines attempt 
	 * to modify the memory location where main is located, the operating system
	 * will send us a signal, SIGSEGV (segment violation) instead of carrying out
	 * the modification.
	 *
	 * The separate variable and cast is to keep the compiler from catching our 
	 * attempt early.  This wouldn't compile:
	 * main = 10;
	 * It might compile if we left the type alone:
	 * int (*ref)() = main;
	 * ref = (void*)10;
	 * There are quite a few options to cause a segfault this way, but they are 
	 * generally two lines. */
	int* i = (int*)main;
	*i = 10; 

	return 0;
}