NAME¶
Marshal - Marshaling of data structures.
Module¶
Module Marshal
Documentation¶
Module
Marshal
:
sig end
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 :
Pervasives.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 :
Pervasives.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
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 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
See
Marshal.header_size .
val total_size :
string -> int -> int
See
Marshal.header_size .