Dynamically setting pointer to int giving incorrect size

Question

I have a struct defined:

struct Query {  
   int *pages;
   int currentpage;
};


struct Query *new = malloc(sizeof(struct Query));

new->pages = malloc(sizeof(int) * 4);

I then checked the size of new->pages to ensure that the size was 4 by doing:

    int size = sizeof(new->pages)/sizeof(new->pages[0]);

However, size keeps returning 2 as opposed to 4.

Would anyone know what I might be doing wrong?

Thanks for your help.

Answer 1

Your expectation is wrong.

sizeof(new->pages) in

int size = sizeof(new->pages)/sizeof(new->pages[0]);

doesn't return the total number of int elements. Just the size of the int pointer (int*). Since sizeof(int*) is 8 and sizeof(int) is 4 on your platform, it returns 2.

By the way, you should use size_t for size as sizeof operator returns a size_t and %zu is the format specifier (in printf()) to print size_t.

Answer 2

You are seeing this because the sizeof operator yields the size of the type of its operand in bytes, and the type of new->pages is a pointer to integer, as defined by the struct, so it evaluates to the size of the pointer, rather than the array.

Note the difference by the example:

int arr[4] = {0,1,2,3};
int *p = arr;
printf("%zu\n", sizeof(arr)); // prints 16
printf("%zu\n", sizeof(p)); // prints 4

The convention of getting number of elements of an array using sizeof(arr)/sizeof(arr[0]) works ONLY on arrays, it does not work on pointers, you have to keep track the length on your own.