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 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-time. 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 unmarshaller 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 : 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 -230, 230-1 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 -230, 230-1 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.

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 : 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_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.

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
val total_size : bytes -> int -> int

Marshal and domain safety

Care must be taken when marshaling a mutable value that may be modified by a different domain. Mutating a value that is being marshaled (i.e., turned into a sequence of bytes) is a programming error and might result in suprising values (when unmarshaling) due to tearing, since marshaling involves byte-per-byte copy.