module Netpagebuffer:sig..end
    The idea of this buffer is that input data is added to the last
    page of the buffer only (with add_inplace). If then all previous
    input was already a multiple of the page size, it is ensured that
    the new input is added at a page boundary. This kind of input operation
    can often be accelerated by the OS ("zero copy network I/O").
type 
val create : int -> tblocksize: creates new buffer with this blocksize, which must
      be a whole multiple of the page size of the OS
      The blocksize is normally either
val contents : t -> stringval length : t -> intval sub : t -> int -> int -> stringval blit_to_string : t -> int -> string -> int -> int -> unitval blit_to_memory : t -> int -> Netsys_mem.memory -> int -> int -> unitval add_string : t -> string -> unitval add_sub_string : t -> string -> int -> int -> unitval add_sub_memory : t -> Netsys_mem.memory -> int -> int -> unitval add_inplace : t -> (Netsys_mem.memory -> int -> int -> int) -> intadd_inplace b f: Calls f m pos len where m is the last page
      of the buffer, and pos is the first free byte on the page, and
      len is the number of free bytes on the page. The function f is
      expected to store new data in m from pos to pos+n-1 and to
      return n. The number n is also returned as final result.
      It is ensured that f is called with a value of len>=1.
val page_for_additions : t -> Netsys_mem.memory * int * intlet (m,pos,len) = page_for_additions b: Returns the last page 
      in m, the first free byte on the page in pos, and 
      the number of free bytes on the page in len. (The same values
      the function f would have got as arguments in add_inplace.)val advance : t -> int -> unitadvance b n: Marks further n bytes in the last page of the
      buffer as used. These bytes are not modified in any way - it is
      expected that the user calls page_for_additions first, and sets
      these n bytes to new values directly.val page_for_consumption : t -> Netsys_mem.memory * int * intlet (m,pos,len) = page_for_consumption b: Returns the first page 
      in m, the first used byte on the page in pos, and 
      the number of used bytes on the page in len.val delete_hd : t -> int -> unitdelete_hd b n: Deletes n bytes from the beginning of the bufferval clear : t -> unitval index_from : t -> int -> char -> intindex_from nb k c: Searches the character c in the buffer beginning
 at position k. If found, the position of the left-most occurence is
 returned. Otherwise, Not_found is raised.