c - getting at pointers in an array of struct passed as a void* -

i have function signature qsort:

const char* get_str(int i, const void *base, size_t nmemb, size_t size); 

i passed arrays of pointers const char*, or arrays of structs first field pointer const char*.

what casts need extract pointer in array element i?

i have tried casting base array of char itself, can advance right element:

return *(const char**)(((const char*)base)[i * size]); 

but compiler complaining: warning: cast pointer integer of different size [-wint-to-pointer-cast]

it looks want implement type or identification sytem structs so:

#include <stdlib.h> #include <stdio.h>  struct {     const char *id;     int x; };  struct b {     const char *id;     double d; };  union {     struct a;     struct b b; };  int main() {     struct a[] = {{"one", 1}, {"two", 2}, {"three", 3}};     struct b b[] = {{"pi", 3.1415}, {"e", 2.71}};     union any[3];      any[0].a = a[0];     any[1].b = b[0];     any[2].a = a[1];      puts(get_str(1, a, 3, sizeof(*a)));     puts(get_str(1, b, 2, sizeof(*b)));     puts(get_str(1, any, 3, sizeof(*any)));      return 0; } 

in case, following works:

const char* get_str(int i, const void *base, size_t nmemb, size_t size) {     const char *p = base;     const char **pp = (const char**) (p + * size);      return *pp; } 

this can written in 1 line as:

const char* get_str(int i, const void *base, size_t nmemb, size_t size) {     return *(const char**) ((const char *) base + * size); } 

but think detour via void pointers not necessary. can address calculations typed array:

puts(*(const char **) (&a[1])); puts(*(const char **) (&b[1])); puts(*(const char **) (&any[1])); 

if wrap in function:

const char *get_str(const void *str) {     return *(const char **) str; } 

you get:

puts(get_str(&a[1])); puts(get_str(&b[1])); puts(get_str(any + 1)); 

which more readable qsortish syntax in opinion.

this works, because acces 1 element @ known position. functions bsort , qsort, however, can't use technique, because have access array @ several indices , hence must able index calculation themselves.