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 : Stdlib.out_channel -> 'a -> extern_flags list -> unitMarshal.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.
external to_bytes : 'a -> extern_flags list -> bytes = "caml_output_value_to_string" 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.
external to_string : 'a -> extern_flags list -> string = "caml_output_value_to_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 -> intMarshal.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 : Stdlib.in_channel -> 'aMarshal.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.
It raises End_of_file if the function has already reached the
end of file when starting to read from the channel, and raises
Failure "input_value: truncated object" if it reaches the end
of file later during the unmarshalling.
val from_bytes : bytes -> int -> 'aMarshal.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.
val from_string : string -> int -> 'aSame as from_bytes but take a string as argument instead of a
byte sequence.
val header_size : intThe 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_sizebuff 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.