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 transmitted 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 Marshal.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.

       OCaml  exception  values  (of  type exn ) returned by the unmarhsaller should not be pattern-matched over
       through match ... with or try ... with , because unmarshalling does not preserve the information required
       for matching their exception constructor. Structural equalities with  other  exception  values,  or  most
       other uses such as Printexc.to_string, will still work as expected.

       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  Marshal.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 *)
        | Compat_32  (* Ensure 32-bit compatibility *)

       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,  functional
       values, and compatibility between 32- and 64-bit platforms.

       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 guarantees  that  marshaling  always
       terminates.  Sharing  between  values  marshaled  by  successive  calls  to Marshal.to_channel is neither
       detected nor preserved, though.  If flags contains Marshal.No_sharing , sharing is ignored.  This results
       in faster marshaling if v contains no shared substructures, but may cause slower  marshaling  and  larger
       byte representations if v actually contains sharing, or even non-termination if v contains cycles.

       If  flags  does  not  contain  Marshal.Closures  , marshaling fails when it encounters a functional value
       inside v : only 'pure'  data  structures,  containing  neither  functions  nor  objects,  can  safely  be
       transmitted  between  different  programs. If flags contains Marshal.Closures , functional values will be
       marshaled as a position in the code of the program. 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-marshaling time, using an MD5 digest of the code transmitted along with the code position.)

       If flags contains Marshal.Compat_32 , marshaling fails when it encounters an integer  value  outside  the
       range  [-2{^30},  2{^30}-1]  of  integers that are representable on a 32-bit platform.  This ensures that
       marshaled data generated on a 64-bit platform can be safely read back on a  32-bit  platform.   If  flags
       does  not contain Marshal.Compat_32 , integer values outside the range [-2{^30}, 2{^30}-1] are marshaled,
       and can be read back on a 64-bit platform, but will cause an error at un-marshaling time when  read  back
       on  a  32-bit  platform.  The Mashal.Compat_32 flag only matters when marshaling is performed on a 64-bit
       platform; it has no effect if marshaling is performed on a 32-bit platform.

       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 representation in the
       string buff , starting at character number ofs , and writing at most  len  characters.   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  variable-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 position 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 buffer is large enough to hold the remaining data,
       then read it, and finally call Marshal.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                                           2016-05-05                                        Marshal(3o)