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-time.  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.

       Values of extensible variant types, for example exceptions (of extensible type exn  ),  returned  by  the
       unmarshaller  should  not  be  pattern-matched  over  through  match  ...  with or try ... with , because
       unmarshalling does not preserve the information required  for  matching  their  constructors.  Structural
       equalities  with  other  extensible  variant  values  does  not  work  either.   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 : 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 the position in the code of the program together with the values corresponding to the free
       variables  captured  in  the  closure.   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.)

       The  exact  definition  of  which  free variables are captured in a closure is not specified and can vary
       between bytecode and native code (and according to optimization flags).  In particular, a function  value
       accessing  a  global  reference  may  or  may  not  include  the  reference  in its closure.  If it does,
       unmarshaling the corresponding closure will create a new reference, different from the global one.

       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.

       Raises Failure if chan is not in binary mode.

       val to_bytes : 'a -> extern_flags list -> bytes

       Marshal.to_bytes v flags returns a byte sequence containing the representation of v .  The flags argument
       has the same meaning as for Marshal.to_channel .

       Since 4.02.0

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

       Same as to_bytes but return the result as a string instead of a byte sequence.

       val to_buffer : bytes -> 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
       sequence buff , starting at index ofs , and writing at most len bytes.  It returns the  number  of  bytes
       actually  written  to  the  sequence. If the byte representation of v does not fit in len characters, the
       exception Failure is raised.

       val from_channel : 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.

       Raises End_of_file if chan is already at the end of the file.

       Raises  Failure if the end of the file is reached during unmarshalling itself or if chan is not in binary
       mode.

       val from_bytes : bytes -> int -> 'a

       Marshal.from_bytes 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 byte sequence buff , starting at
       position ofs .  The byte sequence is not mutated.

       Since 4.02.0

       val from_string : string -> int -> 'a

       Same as from_bytes but take a string as argument instead of a byte sequence.

       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 bytes, of the
       header.  Marshal.data_size buff ofs is the size, in bytes, 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
       bytes, 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 byte sequence, the program needs to read
       first Marshal.header_size bytes into the sequence, then determine the length  of  the  remainder  of  the
       representation  using  Marshal.data_size  ,  make sure the sequence is large enough to hold the remaining
       data, then read it, and finally call Marshal.from_bytes to unmarshal the value.

       val data_size : bytes -> int -> int

       See Marshal.header_size .

       val total_size : bytes -> int -> int

       See Marshal.header_size .