Getting a pointer to a memory address from a different pointer C

Question

I'm writing my own memory allocater. I'm getting a pointer to mapped memory from mmap. From there, I want to be able to get a pointer to a different part of that mapped memory (like x bytes away from the current pointer). How can I do this? Memory management is very confusing to me.

Answer 1

mmap gives you a void* and C doesn't allow pointer arithmetic with those, but you can cast it to char* and then just add the number of bytes you want, e.g. mypointer + 16.

If you cast the pointer to a larger type (like int*), pointer arithmetic adds multiples of the size of that type. For example, if you add 16 to an int*, you're adding enough bytes for 16 int values (so 64 bytes, assuming sizeof(int) is 4).