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 .