Reading/Writing self heap

Question

Could the own heap space be readed? could the software be self modified in memory?

I write some code to show the subject,

am I reading own code at memory? how (if possible) to write it and change instruction on runtime?

#include<stdio.h>
#include<stdint.h>

volatile int addressBase;
uint8_t read(int address);


int main(void) {

    printf("Helium word");

    addressBase=(int)&main;        
    printf("[%X]", read( 0 ));         
    getchar();

    return 0;
}


uint8_t read(int address)
{

       const uint8_t *addr;                        
       addr=(const unsigned char *)(addressBase+(int)address);
       return (*addr);
}

Answer 1

Memory is normally divided into read-only instruction memory and writable data memory which may or may not be executable. If you application wants to write and then run its own code (like a JIT compiler), you will probably need to use some operating specific method of obtaining the necessary memory block. So yes you can read your own instructions but I highly doubt you will be able to modify them.

Answer 2

You can read and write heap space at your own risk.

Self-modifiying code might be a useful trick in restricted, small environments like small embedded systems. Modern desktop or server CPUs however do not like self-modifying code at all because it breaks instruction caching, prefetching and pipelining. One anecdote: TI-Scheme ran blazingly fast on 386 CPUs. It used self-modifying code. 486 CPUs introduced instruction caching and TI-Scheme crashed.