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.
OCaml exception values (of type
exn) returned by the unmarhsaller
should not be pattern-matched over through
match ... with or
try
... with, because unmarshalling does not preserve the information
required for matching their exception constructor. Structural
equalities with other exception values, or 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.
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, 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 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.)
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.
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.
: 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.