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Marshal(3)				  OCaml library 			       Marshal(3)

NAME
       Marshal - Marshaling of data structures.

Module
       Module	Marshal

Documentation
       Module Marshal
	: sig end

       Marshaling of data structures.

       This  module provides functions to encode arbitrary data structures as sequences of bytes,
       which can then be written on a file or sent over a pipe or network connection.  The  bytes
       can  then  be  read  back later, possibly in another process, and decoded back into a data
       structure. The format for the byte sequences is compatible across all machines for a given
       version of OCaml.

       Warning:  marshaling  is currently not type-safe. The type of marshaled data is not trans-
       mitted along the value of the data, making it impossible to check that the data read  back
       possesses  the  type  expected  by the context. In particular, the result type of the Mar-
       shal.from_* functions is given as 'a , but this is misleading: the  returned  OCaml  value
       does  not  possess type 'a for all 'a ; it has one, unique type which cannot be determined
       at compile-type.  The programmer should explicitly give the expected type of the  returned
       value, using the following syntax:

       -  (Marshal.from_channel  chan : type) .  Anything can happen at run-time if the object in
       the file does not belong to the given type.

       The representation of marshaled values is not human-readable, and uses bytes that are  not
       printable  characters.  Therefore, input and output channels used in conjunction with Mar-
       shal.to_channel and Marshal.from_channel  must  be  opened  in  binary  mode,  using  e.g.
       open_out_bin  or open_in_bin ; channels opened in text mode will cause unmarshaling errors
       on platforms where text channels behave differently than binary channels, e.g. Windows.

       type extern_flags =
	| No_sharing  (* Don't preserve sharing *)
	| Closures  (* Send function closures *)

       The flags to the Marshal.to_* functions below.

       val to_channel : Pervasives.out_channel -> 'a -> extern_flags list -> unit

       Marshal.to_channel chan v flags writes the representation of v on channel chan . The flags
       argument  is  a	possibly  empty  list  of flags that governs the marshaling behavior with
       respect to sharing and functional values.

       If flags does not contain Marshal.No_sharing , circularities and sharing inside the  value
       v  are detected and preserved in the sequence of bytes produced. In particular, this guar-
       antees that marshaling always terminates. Sharing between values marshaled  by  successive
       calls to Marshal.to_channel is not detected, though.  If flags contains Marshal.No_sharing
       , sharing is ignored.  This results in faster marshaling if v contains no shared substruc-
       tures,  but may cause slower marshaling and larger byte representations if v actually con-
       tains sharing, or even non-termination if v contains cycles.

       If flags does not contain Marshal.Closures , marshaling fails when it encounters  a  func-
       tional  value  inside  v : only ``pure'' data structures, containing neither functions nor
       objects, can safely be transmitted between different  programs.	If  flags  contains  Mar-
       shal.Closures  , functional values will be marshaled as a position in the code of the pro-
       gram. In this case, the output of marshaling can only be read back in processes	that  run
       exactly the same program, with exactly the same compiled code. (This is checked at un-mar-
       shaling time, using an MD5 digest of the code transmitted along with the code position.)

       val to_string : 'a -> extern_flags list -> string

       Marshal.to_string v flags returns a  string  containing	the  representation  of  v  as	a
       sequence of bytes.  The flags argument has the same meaning as for Marshal.to_channel .

       val to_buffer : string -> int -> int -> 'a -> extern_flags list -> int

       Marshal.to_buffer buff ofs len v flags marshals the value v , storing its byte representa-
       tion in the string buff , starting at character number ofs , and writing at most len char-
       acters.	 It  returns the number of characters actually written to the string. If the byte
       representation of v does not fit in len characters, the exception Failure is raised.

       val from_channel : Pervasives.in_channel -> 'a

       Marshal.from_channel chan reads from channel chan the byte representation of a  structured
       value,  as produced by one of the Marshal.to_* functions, and reconstructs and returns the
       corresponding value.

       val from_string : string -> int -> 'a

       Marshal.from_string buff ofs unmarshals a structured value like Marshal.from_channel does,
       except  that the byte representation is not read from a channel, but taken from the string
       buff , starting at position ofs .

       val header_size : int

       The bytes representing a marshaled value are composed of a fixed-size header and  a  vari-
       able-sized  data  part, whose size can be determined from the header.  Marshal.header_size
       is the size, in characters, of the header.  Marshal.data_size buff ofs  is  the	size,  in
       characters,  of the data part, assuming a valid header is stored in buff starting at posi-
       tion ofs .  Finally, Marshal.total_size buff ofs is the total size, in characters, of  the
       marshaled  value.   Both  Marshal.data_size and Marshal.total_size raise Failure if buff ,
       ofs does not contain a valid header.

       To read the byte representation of a marshaled value into a  string  buffer,  the  program
       needs  to  read	first  Marshal.header_size characters into the buffer, then determine the
       length of the remainder of the representation using Marshal.data_size , make sure the buf-
       fer  is	large  enough  to  hold  the  remaining data, then read it, and finally call Mar-
       shal.from_string to unmarshal the value.

       val data_size : string -> int -> int

       See Marshal.header_size .

       val total_size : string -> int -> int

       See Marshal.header_size .

OCamldoc				    2014-06-09				       Marshal(3)
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