Define pointer to array of int pointers in struct, how to access those ints?

user_14064103 0
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I have the following code:

struct demo{
    int *(*p)[];
};
int main() {
    struct demo *demo_ptr;
    int a = 1;
    int b = 2;

    int *arr[2];
    arr[0] = &a;
    arr[1] = &b;

    for (int i = 0; i < 2; i++) {
        printf("num in array = %d\n", *arr[i]);
    }

    demo_ptr->p = &arr;

    for(int i = 0; i < 2; i++){
        printf("num in array = %d\n", demo_ptr->(*p)[i]);
    }

}

I'd like to access a and b by demo_ptr. But the code failed. How to assign value to p (a pointer to array of int pointers) declared in a struct, and how to access those ints in array through demo_ptr?

pointers struct c
3 Answers

For starters you declared a pointer to an incomplete type

struct demo{
    int *(*p)[];
};

So you can not use the sizeof operator to determine the number of elements in the pointed array.

Secondly you defined an uninitialized pointer that has an indeterminate value.

struct demo *demo_ptr;

So again dereferencing this pointer invokes undefined behavior.

You should write for example

struct demo{
    int *(*p)[2];
};

and in main

struct demo demo;
//...
demo.p = &arr;

for ( size_t i = 0; i < sizeof( *demo.p ) / sizeof( **demo.p ); i++ )
{
    printf( "num in array = %d\n", *( *demo.p )[i]);
}
user_28772410

Try: *(*demo_ptr->p)[i].

The problem was that sub-scripting operator [] has higher priority than dereference operator *. This can be fixed by placing parenthesis around *demo_ptr->p.

Other issue is that demo_ptr points to no object. Initialize it with:

struct demo *demo_ptr = malloc(sizeof *demo_ptr);
user_49894510

In the following:

struct demo{
    int *(*p)[];
};

the member p is a pointer to an incomplete array type. The array type is incomplete because no length has been specified. The array element type is int *.

The incomplete array type is not very useful. In my opinion, if the length is not fixed, it would be better to define p as a pointer to pointer to int:

struct demo{
    int **p;
};

Then, in main():

    demo_ptr->p = arr;

    /* assuming arr[0] and arr[1] are valid pointers to int: */
    for(int i = 0; i < 2; i++){
        printf("num in array = %d\n", *demo_ptr->p[i]);
    }

(Note: *demo_ptr->p[i] is equivalent to *((demo_ptr->p)[i]).)

The other problem in main() is that demo_ptr is an uninitialized pointer, so the dereferences above result in undefined behavior. It needs to point to a struct demo or to allocated storage with size at least sizeof (struct demo). For example:

    struct demo *demo_ptr;
    struct demo demo;

    demo_ptr = &demo;
user_52644910
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