Question 285421 in Visual Studio Code Question posted in
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Visual Studio Code – Compiler allocates same memory for pointers to struct in embedded C

I have two a structure for which I have created two instances of structure using pointer to struct and I am expecting them to return two different values based on the input . If I execute this code in visual studio code , I see the int_struct_pt1 and int_struct_pt2 has the same addresses in the memory . So when int_struct_pt2 receives it value int_struct_pt1 is overwritten by the same value of int_struct_pt2.

I am working on STM32F4 , so I do not want to use the dynamic memory allocation . Is there any other way I can get these data without being overwritten or how do I handle the memory allocation .

//int_struct.h
//Declaration in header file
typedef struct {
    int a;
    int b;
} int_struct;

int_struct * func(int a);

//int_app.c
//Pointer to struct in source file 
static int_struct* int_struct_pt1;
static int_struct* int_struct_pt2;
int_struct_pt1 = func(10);
int_struct_pt2 = func(16);

//int_struct.c
static int_struct int_struct_app;
int_struct * func(int a ){
    int_struct_st.a = a;
    int_struct_st.b = 15;
    return &int_struct_st;
}

Thanks

Tried creating different variables . Debugged and saw the pointers had the same memory address.
My expectation is to get the best method in C to perform this function without being overwritten

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2

Answers


  1. 0 
    static int_struct int_struct_st;

    Has only one instance and if you call your function it will always return the same address of the same object.

    You need to create the struct every time this function is called:

    Generally (as it is a small MCU medical project), you should implement statically allocated memory pools allocator/deallocator which will not fragment the heap. This is a very trivial example of the simplest implementation:

    #define MAXINITS 4

static int_struct ist[MAXINITS];
static int taken;

int_struct * func(int a )
{
    if(taken < MAXINITS)
    {
        ist[taken].a = a;
        ist[taken++].b = 15;
        return &ist[taken -1];
    }
    return NULL;
}

    or with malloc

    int_struct * func(int a ){
    int_struct *st = malloc(sizeof(*st);
    if(st)
    {
        st -> a = a;
        st -> b = 15;
    }
    return st;
}
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  2. 0

    You can change func to be an initialization function:

    void func(int_struct *st, int a ){
    st->a = a;
    st->b = 15;
}

    Elsewhere, define a variable for each instance:

    static int_struct int_struct_1;
static int_struct int_struct_2;

    If you really must use pointer variables:

    static int_struct* int_struct_pt1 = &int_struct_1;
static int_struct* int_struct_pt2 = &int_struct_2;

    Call the function to initialize the int_struct:

        func(&int_struct_1, 10);
    func(&int_struct_2, 16);

    or:

        func(int_struct_pt1, 10);
    func(int_struct_pt2, 16);
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