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 -> 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 Mars