fillitoff, strcpy_v7 and strlen_v6 functions

#include <stdio.h>
#include <stdlib.h>
unsigned int strlen_v6 (const char*);
char *strcpy_v7 (char*, const char*);
void *fillitoff (void*, const unsigned int, unsigned int);
int main ()
{
   char c;
   char *str;
   unsigned int a;

   c = 'C';
   a = 99;

   str = (char*) malloc (25);

   fillitoff (str, '\0', sizeof (str));
   strcpy_v7 (str, "C Programming Language.");
   printf ("[%s]\n", str);

   str = (char*) realloc (str, 35);

   printf ("[This is the ", fillitoff (str, '\0', sizeof (str)));
   printf ("%s\n", strcpy_v7 (str, "C Programming Language tutorial.]"));

   printf ("[%s]\n", fillitoff (str, 'C', strlen_v6 (str)));
   printf ("[%s]\n", fillitoff (str, c, strlen_v6 (str)));
   printf ("[%s]\n", fillitoff (str, a, strlen_v6 (str)));
   printf ("[%s]\n", fillitoff (str, a = c, strlen_v6 (str)));
   printf ("[%s]\n", fillitoff (str, 67, strlen_v6 (str)));

   fillitoff (str, '\0', sizeof (str));
   printf ("[%s]\n", str);

   free (str);

   return 0;
}

void *fillitoff (void *str, const unsigned int c, unsigned int length)
{
   char *ch_str_ptr;
   ch_str_ptr = str;
   while (length > 0)
      *ch_str_ptr++ = c, length--;
   return (str);
}

char *strcpy_v7 (char *str1, const char *str2)
{
   char *str1_ptr = str1;
   while (*str1++ = *str2++)
      ;
   return (str1_ptr);
}

unsigned int strlen_v6 (const char *str)
{
   unsigned int i = 0;
   while (*str++)
      i++;
   return i;
}