C Arrays, behind the scene !!

An Array in C is a collection of similar types of data elements. It is a built-in data type and is widely used by developers. Here, we will cover the array declarations, the memory layout, array decay, and multi-dimensional arrays.

Array Declaration

The array is declared with the following syntax -

dataType arrayName [array_size];

An array can be defined by specifying the initial value of the array elements, the compiler can identify the array size if not specified explicitly.

“Array definitions and declarations.”

💡 C99 and beyond support dynamic array declaration. The size of the array depends on the user-provided input.

Memory Layout

A contiguous memory space is allocated for array elements and can be accessed via an array index.

“Memory layout”

💡 The array size is a compile-time constant!

The ith element of an array can be accessed in a constant time via arr[i]. It is located at the address arr + i * sizeof(int). Here, the arr is the address of the first element in the array, in the above case 0x400.

arr, &arr, and &arr[0] all yield the same value (the address of the first element of the array and it is the same as the address of the array itself). Hence, arr[i] is effectively translated to *(arr + i) i.e. dereferencing element at the address (a+i).

According to the pointer arithmetic arr + i will be calculated to arr + i * sizeof(int).

“arrays.c”

🤯 Note: arr[i] = *(arr + i) = *(i + arr) = i[arr]

One can calculate the array size using sizeof() function. Where does array size come from? Is it stored anywhere?

Answer - No!

The arr is an object of type int[5], which is well known to the compiler at compile time. When you ask the compiler sizeof(arr) it returns 5 * sizeof(int) = 5 * 4 = 20. So the array size can be calculated with sizeof(arr) / sizeof(int).

Array and Pointers

Array and pointers are generally used interchangeably. An array passed to a function is normally captured as a pointer to the data type. When an array is passed, the address of the first element is passed and captured in a pointer.

Array Decay

A loss of type and dimension of an array is known as Array Decay. The C standard dictates that arrays passed as parameters decay to a pointer to the first element. Hence, The argument int a[] in the helper function on line 3 is the same as int *b. So the sizeof operator in line 5 will return the size of an integer pointer, which on my machine is 0x8. The programmer needs to pass the array size explicitly to access the array correctly.

Multidimensional Array

An N-dimensional array is an array of arrays. It will be mapped to one-dimensional memory addresses.

dataType arrayName [size1][size2][size3]...[sizeN];

For example - A 2D array defined as follows can be viewed as a table of two rows and three columns.

int arr[2][3] = {{1, 2, 3}, {4, 5, 6}};

“arrays.c”

The arr[i][j] will be placed at (arr + i * size of row + j * size of an array element).

Now you try -

int arr[2][3] = {{1, 2, 3}, {4, 5 ,6}};
int **pv = arr;
printf("0x%lx, 0x%lx, 0x%lx\n", arr, arr + 1, *(arr + 1));
printf("0x%lx, 0x%lx, %d\n", pv, pv + 1, *(pv + 1));

Assume 0x400 to be the starting address of the given 2d array. What do you think the print statements will output?