Marshal(3) OCaml library Marshal(3)NAMEMarshal - Marshaling of data structures.
Module
Module MarshalDocumentation
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 impossi‐
ble 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 Mar‐
shal.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 chan‐
nel 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 shar‐
ing 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 con‐
tains 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 , func‐
tional values will be marshaled as a position in the code of the pro‐
gram. In this case, the output of marshaling can only be read back in
processes that run exactly the same program, with exactly the same com‐
piled 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 encoun‐
ters an integer value outside the range [-2{^30}, 2{^30}-1] of integers
that are representable on a 32-bit platform. This ensures that mar‐
shaled 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-marshal‐
ing 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_string : 'a -> extern_flags list -> string
Marshal.to_string v flags returns a string containing the representa‐
tion 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 num‐
ber 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 representa‐
tion 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 Mar‐
shal.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 Mar‐
shal.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 buf‐
fer, the program needs to read first Marshal.header_size characters
into the buffer, then determine the length of the remainder of the rep‐
resentation using Marshal.data_size , make sure the buffer is large
enough to hold the remaining data, then read it, and finally call Mar‐
shal.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 .
OCamldoc 2013-10-24 Marshal(3)
