void* vs. char* pointer arithmetic

Question

I'm looking through my textbook and I'm a little confused about some of the code that is in there. In one part, they are performing pointer arithmetic in the following way:

void* bp;
...
bp = (void*)((char*)(bp)+16);
...

but later on, they do the following:

void* bp;
...
bp = bp+16;
...

I feel like they should be two different things but they are treating it like the same thing. I feel this way because, for example, if you were to do an array access (for an integer array for example),you would do the following

int* a = malloc(n*sizeof(int));
...
q = *(a+1);
...

in this case, aren't I accessing the next 4 bytes in the integer array and not the next byte? Similarly, I feel that if I have void* a, then *(a+1) should be the next 4 bytes... Or is that not the case? Thank you.

Answer 1

It's a slip-up. Arithmetic on void * is not defined by the standard, but some compilers offer it as an extension, behaving the same as char * for arithmetic. The second is formally not valid C, but slipped through presumably out of (bad) habit.