How to traverse an array parameter as void pointer

Question

I have similar "generic" procedure like qsort, which has a void pointer (pointing at an array) and also a function pointer parameter. This function should work on any type of array.

Example:

void do_something(void * array, int count, int size, void (*test)(const void*)){
    int i;
    for(i=0; i<count; i++){
        test(array + (index * size));
    }
}

This however gives me the following warning (gcc test.c -pedantic-errors):

error: pointer of type ‘void *’ used in arithmetic [-Wpedantic]

And after some research I found out it's a bad practice to use void pointers like this. (E.g. Pointer arithmetic for void pointer in C)

So how does the standard library do this kind of stuff for like qsort? Looking at this code: (http://aturing.umcs.maine.edu/~sudarshan.chawathe/200801/capstone/n/qsort.c), I see the following:

void
_quicksort (void *const pbase, size_t total_elems, size_t size,
        __compar_fn_t cmp)
{
  register char *base_ptr = (char *) pbase;
  ....
  char *lo = base_ptr;
  char *hi = &lo[size * (total_elems - 1)];
  ...
}

Are they casting to (char *) regardless of the actual type?

Well using char is not more regardless of type than using void :) Arthmetic on char do the job (move with size steps). — hexasoft

pm100 pm100 · Accepted Answer · 2015-10-15T17:04:38

i asked similar question Can I do arithmetic on void * pointers in C?.

Void * arithmetic is not defined. What does it mean to add 1 to a void pointer ? Most compilers (if they allow it) treat it as incrementing by sizeof(char) ("the next byte") but warn you.

So the right thing to do is to explicitly make it do what you want -> cast to char* and increment that

How to traverse an array parameter as void pointer

3 Answers