为结构成员赋值

Question

Wwhy cant I not assign value like this

A1->a |= 5;

#include <stdio.h>

typedef struct {
    int a;
    int b;
    int c;
} A;

int main(){
    A *A1 = (A *) 0x100; //this is for example, an address of a register

    printf("A1.a is at %p\n", &(A1->a));
    printf("A1.b is at %p\n", &(A1->b));
    printf("A1.c is at %p\n", &(A1->c));

    A1->a |= 5; //this line doesnt work
    printf("A1.a = %d ", A1->a);
    return 0;
}

A1->a |= 5; this line doesnt work, cant any body help me understand why? I am trying to learn embedded programing and its programming style.

Answer 1

A *A1 = (A *) 0x100;

Your problem is this line. You cannot reference to an address that you do not own it. You do not know in this address, you can read or write the data.

you can reference to an address of a variable:

A A0;
A *A1 = &A0;

Answer 2

If the code is supposed to run on machine with Memory Management Unit active, then you will end up with a segmentation fault because the address you're trying to access is not mapped to your process.Keep in mind, you cannot access memory beyond the memory space you've been allocated (unless you request it, malloc, mmap, etc).

If it is supposed to run on a machine without MMU active (on MCU) then you can freely access any memory address (unless you do not have any Memory Unit Protection active for that region). Basically you're trying to access three registers (0x100, 0x104, 0x108, suppose you run on 32 bit machine). Please take into account that "int" is a signed number, what happens if your register has bit 31 set to "1"? The result will be interpreted as signed. Now try to use bitwise operation on signed numbers.!!! (certainly not the result you're expecting) .

#include <stdint.h>

typedef struct {
   uint32_t a;
   uint32_t b;
   uint32_t c;
} A;

volatile A *A1 = (volatile A*)0x100;

Just to make sure, do you really want to use bitwise?

A1->a | = 5 ; // if a = 1 => a = 7

If you're expecting to set the 5th bit, then you need to do:

A1->a |= (1 << 5); // if a = 1 => a = 33