Read and write to a memory location

Question

After doing lot of research in Google, I found below program

#include 

int main()
{
    int val;
    char *a = (char*) 0x1000;
          


        

Answer 1

That is the correct way to write data to memory location 0x1000. But in this day and age of virtual memory, you almost never know the actual memory location you want to write to in advance, so this type of thing is never used.

If you can tell us the actual problem you're trying to solve with this, maybe we can help.

Answer 2

You can write to a specific memory location.

But only when you have the rights to write to that location.

What you are getting, is the operating system forbidding you to write to 0x1000.

Answer 3

You are doing it except on your system you cannot write to this memory causing a segmentation fault.

A segmentation fault (often shortened to segfault), bus error or access violation is generally an attempt to access memory that the CPU cannot physically address. It occurs when the hardware notifies an operating system about a memory access violation. The OS kernel then sends a signal to the process which caused the exception. By default, the process receiving the signal dumps core and terminates. The default signal handler can also be overridden to customize how the signal is handled.

If you are interested in knowing more look up MMU on wikipedia.

Here is how to legally request memory from the heap. The malloc() function takes a number of bytes to allocate as a parameter. Please note that every malloc() should be matched by a free() call to that same memory after you are done using it. The free() call should normally be in the same function as where you called malloc().

#include <stdio.h>
int main()
{
    int val;
    char *a;

    a = (char*)malloc(sizeof(char) * 1);

    *a = 20;
    val = (int)*a;
    printf("%d", val);

    free(a);

    return 0;
}

You can also allocate memory on the stack in a very simple way like so:

#include <stdio.h>
int main()
{
    int val;
    char *a;
    char b;

    a = &b;
    *a = 20;
    val = (int)*a;

    printf("%d", val);

    return 0;
}