A pointer you get from calloc() is guaranteed to be properly aligned to not cause SIGBUS on RISCs and some such. You cannot just add random sizeof(Header) to it and call it a general purpose allocation. struct Header needs to be a bit more elaborate than what they have. In fact, it shouldn't be a struct, but a union -

  union Header 
  {
      var type;
      max_align_t foo;
  };

whereby you get max_align_t from your standard libraries or do something like this -

  typedef union max_align
  {
    int i;
    long l;
    double d;
    void * p;
    ...
  } max_align_t;