Provided by: ocaml-nox_4.05.0-10ubuntu1_amd64 
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 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.
Values of extensible variant types, for example exceptions (of extensible 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 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 : 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 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.
val to_bytes : 'a -> extern_flags list -> bytes
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 .
Since 4.02.0
val to_string : 'a -> extern_flags list -> 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 -> int
Marshal.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 : 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.
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 -> 'a
Marshal.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.
Since 4.02.0
val from_string : string -> int -> 'a
Same as from_bytes but take a string as argument instead of a byte sequence.
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 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_size buff 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.
val data_size : bytes -> int -> int
See Marshal.header_size .
val total_size : bytes -> int -> int
See Marshal.header_size .