If free() knows the length of my array, why can't I ask for it in my own code?

Question

I know that it\'s a common convention to pass the length of dynamically allocated arrays to functions that manipulate them:

void initializeAndFree(int* anArr         


        

Answer 1

You can allocate more memory to store size:

void my_malloc(size_t n,size_t size ) 
{
void *p = malloc( (n * size) + sizeof(size_t) );
if( p == NULL ) return NULL;
*( (size_t*)p) = n;
return (char*)p + sizeof(size_t);
}
void my_free(void *p)
{
     free( (char*)p - sizeof(size_t) );
}
void my_realloc(void *oldp,size_t new_size)
{
     ...
}
int main(void)
{
   char *p = my_malloc( 20, 1 );
    printf("%lu\n",(long int) ((size_t*)p)[-1] );
   return 0;
}