package batteries

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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.

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.output and Marshal.input must be opened in binary mode, using e.g. BatPervasives.open_out_bin or BatPervasives.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.

  • author Xavier Leroy (base module)
  • author David Teller
type extern_flags = Marshal.extern_flags =
  1. | No_sharing

    Don't preserve sharing

  2. | Closures

    Send function closures

  3. | Compat_32

    Ensure 32-bit compatibility


The flags to the Marshal.to_* functions below.

val output : _ BatInnerIO.output -> ?sharing:bool -> ?closures:bool -> 'a -> unit

output out v writes the representation of v on chan.

  • parameter sharing

    If true (default value), 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 output is not detected, though. If false, 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.

  • parameter closures

    If false (default value) 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 true, 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_bytes : 'a -> extern_flags list -> Bytes.t

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

  • since 2.3.0
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.t -> 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 input : BatInnerIO.input -> 'a

input inp reads from inp 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_bytes : Bytes.t -> 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 2.3.0
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.t -> int -> int
val total_size : Bytes.t -> int -> int
val to_channel : _ BatInnerIO.output -> 'a -> extern_flags list -> unit
val from_channel : BatInnerIO.input -> 'a
  • deprecated

    Use input instead