module Marshal:Marshaling of data structures.sig
..end
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.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 | *) |
Marshal.to_*
functions below.val to_channel : out_channel -> 'a -> 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 -> 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 -> 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 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 -> 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
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.
val data_size : string -> int -> int
Marshal.header_size
.val total_size : string -> int -> int
Marshal.header_size
.