Question

In stdlib.h, there is a declaration of the function qsort() with prototype:

void qsort( void *ptr, size_t count, size_t size,
        int (*comp)(const void *, const void *) );

Obviously it is a generic programming.
I wonder how it implement, how to get the element from the void * type?

Answer 1

void * pointers are casted according to the size_t size (the 3rd argument in qsort)

First we typecast the void* to char* and then do pointer arithmetic according to the size (because char takes 1 byte so adding the size will give correct pointer arithmetic )

EDIT : (for in built data types)

char *ptr = (char*)vp; //here vp is void *p

*(ptr + (n-1)*size); //will give you nth element 

e.g.

size =1 and want 3rd element it means it will give you 3rd char
size =4 and want 3rd element it means it will give you 3rd int or float
size =2 and want 3rd element it means it will give you 3rd short int
size =8 and want 3rd element it means it will give you 3rd double

NOTE : size is implementation defined so it may vary to compilers