Self modifying code demo

addtwo.c

#include <stdio.h>
#include <unistd.h>
#include <sys/mman.h>

int AddTwo(int input) {
	return input + 2;
}

void patchThisShizzle() {
	int pageSize = sysconf(_SC_PAGE_SIZE); //size of a page. Smallest unit that
	//I can un-protect so I can write to it
	mprotect((void*) ((((long) &AddTwo) / pageSize) * pageSize), pageSize * 2, 
		PROT_READ | PROT_WRITE | PROT_EXEC);
	//unprotect the two pages of memory that contains the code for AddTwo by
	//rounding down the pointer to the nearest page
	//set the property to read, write, and execute
	unsigned char* addInstrPtr = ((unsigned char*) &AddTwo);
	for(;;addInstrPtr++) { //for every byte in the function code
		if (*addInstrPtr == 2) { //if the byte is 2
			*addInstrPtr = 3; //change it to 3
			break;
		}
	}
}

int main() {
	patchThisShizzle();
	int out = AddTwo(2);
	printf("%d\n", out);
	return 0;
}