C dynamic memory allocation and sizeof()

Question

I\'m having some trouble understanding the difference between these two code segments: I allocate space for an array of integers dynamically within my code with the followin

Answer 1

well, int *arr declares a pointer, a variable which keeps the address of some other variable, and its size is the size of an integer because it's a pointer, it just have to keep the address, not the pointee itself.
int arr[8] declares an array, a collection of integers. sizeof(arr) refers to the size of the entire collection, so 8*sizeof(int).
Often you hear that "array and pointers are the same things". That's not true! They're different things.

Answer 2

sizeof(arr) is the same as sizeof(int*), i.e. the size of a single pointer. You can however calculate arr_sz as ... cnt!

Answer 3

If you have int arr1[8] the type of arr1 (as far as the compiler is concerned) is an array ints of size 8.

In the example int * arr2 the type of arr2 is pointer to an integer.

sizeof(arr1) is the size of an int array
sizeof(arr2) is the size of an int pointer (4 bytes on a 32 bit system, 8 bytes on a 64 bit system)

So, the only difference is the type which the compiler thinks that variable is.

Answer 4

int *arr;   ----> Pointer
int arr[8]; ----> Array

First up what you got there - int *arr is a pointer, pointing to some bytes of memory location, not an array.

The type of an Array and a Pointer is not the same.

In another function where I pass in arr, I would like to determine the size (number elements) in arr. When I call

int arr_sz = sizeof(arr)/sizeof(int);

it only returns 1, which is just the number of bytes in an int for both arguments I am assuming (4/4)=1. I just assumed it would be the same as using an array

Even if it is assumed to be an Array -- that's because Arrays get decayed into pointers when passed into functions. You need to explicitly pass the array size in functions as a separate argument.

Go through this:

Sizeof an array in the C programming language?

There is a difference between a static array and dynamic memory allocation.

The sizeof operator will not work on dynamic allocations. AFAIK it works best with stack-based and predefined types.

Answer 5

You can't use sizeof with memory pointers:

int *arr = calloc(cnt, sizeof(int));

But it's ok to use it with arrays:

int arr[8];

Answer 6

Mike,

arr is a pointer and as such, on your system at least, has the same number of bytes as int. Array's are not always the same as pointers to the array type.