Why does pointer arithmetic produce incorrect results?

I am trying to move a pointer forward 4096 bytes at a time like so:

#include <stdint.h>
#include <stdio.h>
void calculate(void* a)
{
    uintptr_t stride = 4096; 
    uint64_t *buff = (uint64_t*)a;
    for (uintptr_t offset = 0; offset < 20000; offset += stride)
    {
        fprintf(stdout, "%s: stride = 0x%x offset = 0x%x buff = %p offset+buff = %p\n", __func__, stride, offset, buff, offset+buff);
    }
}

int main(void)
{
    void* address = 0x7f5b588d1000;

    calculate(address);

    return 0;
}

However this is the current output:

calculate: stride = 0x1000 offset = 0x0 buff = 0x7f5b588d1000 offset+buff = 0x7f5b588d1000
calculate: stride = 0x1000 offset = 0x1000 buff = 0x7f5b588d1000 offset+buff = 0x7f5b588d9000

As you can see it says:

0x7f5b588d1000 + 0x1000 = 0x7f5b588d9000

Why is this computation wrong? The result should obviously be 0x7f5b588d2000

>Solution :

Pointer arithmetic is done in units of the pointed-to type. This means adding stride to a pointer of type uint64_t moves the pointer up by stride elements, not stride bytes, so the pointer value increases by stride * sizeof(uint64_t).

You should change the type of buff to uint8_t * or simply unsigned char *. That will move the address up by the given number of bytes.

uint8_t *buff = a;

Note also that it’s not necessary to cast to or from a void *.