void* ptr = buf_ptr[20]; //----------->error

what i want to say is "buf_ptr[20]" is not a pointer,it's
the value of the 20th in buf_ptr

// fill in the array is not shown here
qsort(array, NumStrings, sizeof(array[0]), compare);

here,"sizeof(array[0])" is 20bytes;

// fill in the array is not shown here
qsort(array,...