19.18 | Module `Marshal`: 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 Objective Caml.
> 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 Caml 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 `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 *)

> The flags to the `Marshal.to_*` functions below.

    val to_channel: out_channel -> 'a -> mode: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 guarantees 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 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.)

    val to_string: 'a -> mode: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 -> pos:int -> len:int ->
                   'a -> mode: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: 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 -> pos: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
    val data_size : string -> pos:int -> int
    val total_size : string -> pos:int -> 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.