[Joker_vD]

    if (scanf("%9lu",&len) < 1) barf();  /* >999999999 bytes is bad */
    if (getchar() != ':') barf();
    buf = malloc(len + 1);       /* malloc(0) is not portable */
    if (!buf) barf();
    if (fread(buf,1,len,stdin) < len) barf();
    if (getchar() != ',') barf();

Ah, the wonders of error-handling in C. Also, I wonder what's wrong with

    buf = malloc(len ? len : 1);

[kazinator]

Or even:

  if (len == 0)
    return null_buffer_singleton; /* special shared representation for 0: */

[repiret]

Because then the caller needs to treat it differently from non-empty buffers.

[kazinator]

No, it doesn't. It calls:

  void buffer_free(void *buf)
  {
     if (buf != null_buffer_singleton)
       free(buf);
  }

[Joker_vD]

...which treats it differently from non-empty buffers :)

But frankly, having a 1-byte buffer, pointer to which can serve as a sentinel value à la NULL (but dereferenceable!), and which you can pass to free() without it being deallocated is indeed rather useful.