Determining the Size of an Array from a Pointer in C

When working with arrays in C , it can be necessary to determine their size. However, when passing an array as a function argument, it decays into a pointer, leaving no explicit size information.

Incorrect Approaches

• sizeof list/sizeof(int): This method attempts to determine the size of the array using the sizeof operator, but it divides by the size of int instead of the array's actual size.
• list.size(): This method cannot be used because it is not a member function of the int array type.

Determining the Size Correctly

The correct approach to determine the size of an array from a pointer is to pass the size as an additional argument.

int largest(int *list, int highest_index)
{
  // ...
}

To use this function, pass the array and its size as arguments:

int array[] = {1, 2, 3, 4, 5};
int size = sizeof(array) / sizeof(int); // Calculate array size
int max = largest(array, size);

Array/Pointer Equivalence

C inherits array/pointer equivalence from C, which allows arrays to decay to pointers when passed to functions. This behavior means that the pointer contains no size information.

Memory Layout

Understanding the memory layout of arrays is crucial. When an array is declared, contiguous memory is allocated to store its elements. The pointer to the first element implies the beginning of the array, but it does not carry information about its size.

Conclusion

To accurately determine the size of an array from a pointer in C , it is necessary to pass the size as an additional argument to the function that processes the array. This approach ensures that the function has access to the required information.