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.input must be opened in binary mode, using e.g.
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 =
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
- parameter sharing
true(default value), circularities and sharing inside the value
vare 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
outputis not detected, though. If
false, sharing is ignored. This results in faster marshaling if
vcontains no shared substructures, but may cause slower marshaling and larger byte representations if
vactually contains sharing, or even non-termination if
- parameter closures
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.)
Marshal.to_bytes v flags returns a byte sequence containing the representation of
flags argument has the same meaning as for
- since 2.3.0
val to_string : 'a -> extern_flags list -> string
to_bytes but return the result as a string instead of a byte sequence.
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
from_bytes but take a string as argument instead of a byte sequence.
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.
buff ofs is the size, in bytes, of the data part, assuming a valid header is stored in
buff starting at position
buff ofs is the total size, in bytes, of the marshaled value. Both
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.