126 lines
6.7 KiB
OCaml
126 lines
6.7 KiB
OCaml
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(***********************************************************************)
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(* *)
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(* Objective Caml *)
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(* *)
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(* Xavier Leroy, projet Cristal, INRIA Rocquencourt *)
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(* *)
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(* Copyright 1996 Institut National de Recherche en Informatique et *)
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(* Automatique. Distributed only by permission. *)
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(* *)
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(***********************************************************************)
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(* $Id$ *)
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(* Module [Marshal]: marshaling of data structures *)
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(* This module provides functions to encode arbitrary data structures
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as sequences of bytes, which can then be written on a file or
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sent over a pipe or network connection. The bytes can then
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be read back later, possibly in another process, and decoded back
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into a data structure. The format for the byte sequences
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is compatible across all machines for a given version of Objective Caml.
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Warning: marshaling is currently not type-safe. The type
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of marshaled data is not transmitted along the value of the data,
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making it impossible to check that the data read back possesses the
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type expected by the context. In particular, the result type of
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the [Marshal.from_*] functions is given as ['a], but this is
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misleading: the returned Caml value does not possess type ['a]
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for all ['a]; it has one, unique type which cannot be determined
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at compile-type. The programmer should explicitly give the expected
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type of the returned value, using the following syntax:
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[(Marshal.from_channel chan : type)].
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The behavior is unspecified if the object in the file does not
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belong to the given type.
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The representation of marshaled values is not human-readable,
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and uses bytes that are not printable characters. Therefore,
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input and output channels used in conjunction with [Marshal.to_channel]
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and [Marshal.from_channel] must be opened in binary mode, using e.g.
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[open_out_bin] or [open_in_bin]; channels opened in text mode will
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cause unmarshaling errors on platforms where text channels behave
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differently than binary channels, e.g. Windows. *)
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type extern_flags =
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No_sharing (* Don't preserve sharing *)
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| Closures (* Send function closures *)
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(* The flags to the [Marshal.to_*] functions below. *)
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external to_channel: out_channel -> 'a -> extern_flags list -> unit
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= "output_value"
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(* [Marshal.to_channel chan v flags] writes the representation
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of [v] on channel [chan]. The [flags] argument is a
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possibly empty list of flags that governs the marshaling
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behavior with respect to sharing and functional values.
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If [flags] does not contain [Marshal.No_sharing], circularities
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and sharing inside the value [v] are detected and preserved
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in the sequence of bytes produced. In particular, this
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guarantees that marshaling always terminates. Sharing
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between values marshaled by successive calls to
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[Marshal.to_channel] is not detected, though.
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If [flags] contains [Marshal.No_sharing], sharing is ignored.
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This results in faster marshaling if [v] contains no shared
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substructures, but may cause slower marshaling and larger
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byte representations if [v] actually contains sharing,
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or even non-termination if [v] contains cycles.
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If [flags] does not contain [Marshal.Closures],
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marshaling fails when it encounters a functional value
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inside [v]: only ``pure'' data structures, containing neither
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functions nor objects, can safely be transmitted between
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different programs. If [flags] contains [Marshal.Closures],
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functional values will be marshaled as a position in the code
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of the program. In this case, the output of marshaling can
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only be read back in processes that run exactly the same program,
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with exactly the same compiled code. (This is checked
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at un-marshaling time, using an MD5 digest of the code
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transmitted along with the code position.) *)
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external to_string: 'a -> extern_flags list -> string
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= "output_value_to_string"
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(* [Marshal.to_string v flags] returns a string containing
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the representation of [v] as a sequence of bytes.
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The [flags] argument has the same meaning as for
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[Marshal.to_channel]. *)
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val to_buffer: string -> int -> int -> 'a -> extern_flags list -> unit
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(* [Marshal.to_buffer v buff ofs len flags] marshals the value [v],
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storing its byte representation in the string [buff],
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starting at character number [ofs], and writing at most
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[len] characters. If the byte representation of [v]
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does not fit in [len] characters, the exception [Failure]
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is raised. *)
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external from_channel: in_channel -> 'a = "input_value"
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(* [Marshal.from_channel chan] reads from channel [chan] the
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byte representation of a structured value, as produced by
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one of the [Marshal.to_*] functions, and reconstructs and
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returns the corresponding value.*)
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val from_string: string -> int -> 'a
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(* [Marshal.from_string buff ofs] unmarshals a structured value
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like [Marshal.from_channel] does, except that the byte
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representation is not read from a channel, but taken from
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the string [buff], starting at position [ofs]. *)
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val header_size : int
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val data_size : string -> int -> int
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(* The bytes representing a marshaled value are composed of
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a fixed-size header and a variable-sized data part,
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whose size can be determined from the header.
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[Marshal.header_size] is the size, in characters, of the header.
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[Marshal.data_size buff ofs] is the size, in characters,
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of the data part, assuming a valid header is stored in
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[buff] starting at position [ofs]. It raises [Failure]
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if [buff], [ofs] does not contain a valid header.
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To read the byte representation of a marshaled value into
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a string buffer, one needs to read first [Marshal.header_size]
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characters into the buffer, then determine the length of the
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remainder of the representation using [Marshal.data_size],
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make sure the buffer is large enough to hold the variable
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size, then read it, and finally call [Marshal.from_string]
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to unmarshal the value. *)
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