Question

I tried the below program to make the pointer to point to a particular address and to store a value in that address.When i make the pointer to contain the value for the assigned address i’m getting a run time error asking me to close the program.
Is it not possible to assign a value to the address 0x6778.why is it so? In what situations does this needed? Please help me understand.

int *p=(int*)0x6778;    
printf("The address is:%x",p);

When tried to do *p=1000 i am getting the error.

Answer 1

There are many reasons why this could give you an error:

1. The address 0x6778 might not be part of this process’s virtual memory — it might not really “exist”. You could read more about virtual memory, but basically addresses don’t refer directly to physical bytes — they have to be translated in a table, and that table might not have an entry for your address.

2. If it is mapped, it might be on a read-only page

3. If it’s mapped and writable, it might corrupt some other part of your program, causing a segfault soon after.

In general, you probably can’t write to an arbitrary address in a user-level application. Of course, if you’re running a kernel or embedded system, ignore this answer, as it totally does not apply 😉