Get address of a static struct member from another header file

Question

I am having trouble putting together some code in which I wish to have the following:

• Have a header file myHeader.h in which I create a type struct myStruct with members int number1, int number2 and int number3. This file would also contain a "getter" prototype for getting the address of each of the members in a struct instance.

• A translation unit (file.c) in which I declare a static instance of struct myStruct, i.e. static struct myStruct myStructInstance. This translation unit defines the "getter" functions (I will illustrate with a code example at the end of the post).

• Another header file anotherHeader.h in which I wish to - now this is a tricky bit that's causing problems for me - get the addresses of each of the members of a static structure and use them for something.

Here's an example to show what I am talking about.

myHeader.h

struct myStruct{
    int number1;
    int number2;
    int number3;
};

int* get_number1(void);
int* get_number2(void);
int* get_number3(void);

file.c

#include <myHeader.h>

static struct myStruct myStructInstance = {
        .number1 = 0,
        .number2 = 0,
        .number3 = 0
};

int* get_number1(void){
    struct myStruct* ptr_myStructInstance = &(myStructInstance);
    int* number1Address = &(ptr_myStructInstance->number1);
    return number1Address;
}

int* get_number2(void){
    struct myStruct* ptr_myStructInstance = &(myStructInstance);
    int* number2Address = &(ptr_pfcVariables->number2);
    return number2Address;
}

int* get_number3(void){
    struct myStruct* ptr_myStructInstance = &(myStructInstance);
    int* number3Address = &(ptr_myStructInstance->number3);
    return number3Address;
}

anotherHeader.h

#include <myHeader.h>

int* pNumber1 = get_number1();

int* pNumber2 = get_number2();

int* pNumber3 = get_number3();

The problem is that the code shown above throws "initializer element is not constant" error in anotherHeader.h. I've read a few answers as to why we cannot use non-constant expressions when initializing variables with static storage duration, however I do not understand how this relates to my code.

Answer 1

I've read a few answers as to why we cannot use non-constant expressions when initializing variables with static storage duration, however I do not understand how this relates to my code.

A function call is a non-constant expression.

int* pNumber1 = get_number1();

The above code is trying to initialize a global variable with a non-constant expression.

Answer 2

You simply can't call code in the global scope.

However, you could easily accomplish the same thing as what you appear to be trying to do by simply extending myHeader.h with:

extern int *pNumber1, *pNumber2, *pNumber3;

and then allocating and initializing these in file.c with:

int *pNumber1 =  &myStructInstance.number1,
    *pNumber2 =  &myStructInstance.number2,
    *pNumber3 =  &myStructInstance.number3;