Module Marshal

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
val total_size : string -> int -> int