C Beginner question: Pointer arithmetic > cleaning up once you are done

Question

I'm slowly getting the hang of pointers. But there are still some questions I have.

Is it possible to cause memory leaks when using pointer arithmetic because you are shifting the actual position of where the pointer is pointing to?

I mean, if say I count upwards to copy a string char by char, would I need to count down so C "knows" where the pointer used to point to?

Thanks Frank

Answer 1

A memory leak is possible when using malloc() or similar functions and not calling free() when it's needed. free() should always be called with the pointer returned by malloc(), so yes if you do something like this:

int* ptr = malloc(sizeof(int));
free(ptr + 1);

causes undefined behaviour. Could be a memory leak, maybe a segmentation fault, anything is possible.

Answer 2

Memory is allocated on the heap. A pointer is just that, a pointer to the location in memory. You need to know the address of the start of the allocated memory in order to free it later.

This is because the information about allocated memory (e.g. how much was allocated) needs to be remembered by the memory management system so it knows how much to free later, and to prevent it from allocating the same block to another malloc call. The start address of the memory is what identifies it.

If you want to fiddle with the pointer, take a copy of it and don't modify the original pointer.

int *x = malloc(...);
int *y = x;

... pointer arithmetic with y

free(x);