void remove_if(node ** head, remove_fn rm)
{   
 for (node** curr = head; *curr; )     
{         
   node * entry = *curr;         
if (rm(entry))         
{            
 *curr = entry->next;            
free(entry);         
}         
else           
 curr = &entry->next;   
 }
}

同过指针不就可以通过free释放内存，为什么还要用多级指针？