1Marshal(3) OCaml library Marshal(3)
2
6 Marshal - Marshaling of data structures.
7
9 Module Marshal
10
12 Module Marshal
13 : sig end
14 Marshaling of data structures.
17 This module provides functions to encode arbitrary data structures as
19 sequences of bytes, which can then be written on a file or sent over a
20 pipe or network connection. The bytes can then be read back later,
21 possibly in another process, and decoded back into a data structure.
22 The format for the byte sequences is compatible across all machines for
23 a given version of Objective Caml.
24 Warning: marshaling is currently not type-safe. The type of marshaled
26 data is not transmitted along the value of the data, making it impossi‐
27 ble to check that the data read back possesses the type expected by the
28 context. In particular, the result type of the Marshal.from_* functions
29 is given as 'a , but this is misleading: the returned Caml value does
30 not possess type 'a for all 'a ; it has one, unique type which cannot
31 be determined at compile-type. The programmer should explicitly give
32 the expected type of the returned value, using the following syntax:
33 - (Marshal.from_channel chan : type) . Anything can happen at run-time
35 if the object in the file does not belong to the given type.
36 The representation of marshaled values is not human-readable, and uses
38 bytes that are not printable characters. Therefore, input and output
39 channels used in conjunction with Marshal.to_channel and Mar‐
40 shal.from_channel must be opened in binary mode, using e.g.
41 open_out_bin or open_in_bin ; channels opened in text mode will cause
42 unmarshaling errors on platforms where text channels behave differently
43 than binary channels, e.g. Windows.
44 type extern_flags =
51 | No_sharing (* Don't preserve sharing *)
52 | Closures (* Send function closures *)
53 The flags to the Marshal.to_* functions below.
56 val to_channel : Pervasives.out_channel -> 'a -> extern_flags list ->
61 unit
62 Marshal.to_channel chan v flags writes the representation of v on chan‐
65 nel chan . The flags argument is a possibly empty list of flags that
66 governs the marshaling behavior with respect to sharing and functional
67 values.
68 If flags does not contain Marshal.No_sharing , circularities and shar‐
70 ing inside the value v are detected and preserved in the sequence of
71 bytes produced. In particular, this guarantees that marshaling always
72 terminates. Sharing between values marshaled by successive calls to
73 Marshal.to_channel is not detected, though. If flags contains Mar‐
74 shal.No_sharing , sharing is ignored. This results in faster marshal‐
75 ing if v contains no shared substructures, but may cause slower mar‐
76 shaling and larger byte representations if v actually contains sharing,
77 or even non-termination if v contains cycles.
78 If flags does not contain Marshal.Closures , marshaling fails when it
80 encounters a functional value inside v : only ``pure'' data structures,
81 containing neither functions nor objects, can safely be transmitted
82 between different programs. If flags contains Marshal.Closures , func‐
83 tional values will be marshaled as a position in the code of the pro‐
84 gram. In this case, the output of marshaling can only be read back in
85 processes that run exactly the same program, with exactly the same com‐
86 piled code. (This is checked at un-marshaling time, using an MD5 digest
87 of the code transmitted along with the code position.)
88 val to_string : 'a -> extern_flags list -> string
93 Marshal.to_string v flags returns a string containing the representa‐
96 tion of v as a sequence of bytes. The flags argument has the same
97 meaning as for Marshal.to_channel .
98 val to_buffer : string -> int -> int -> 'a -> extern_flags list -> int
103 Marshal.to_buffer buff ofs len v flags marshals the value v , storing
106 its byte representation in the string buff , starting at character num‐
107 ber ofs , and writing at most len characters. It returns the number of
108 characters actually written to the string. If the byte representation
109 of v does not fit in len characters, the exception Failure is raised.
110 val from_channel : Pervasives.in_channel -> 'a
115 Marshal.from_channel chan reads from channel chan the byte representa‐
118 tion of a structured value, as produced by one of the Marshal.to_*
119 functions, and reconstructs and returns the corresponding value.
120 val from_string : string -> int -> 'a
125 Marshal.from_string buff ofs unmarshals a structured value like Mar‐
128 shal.from_channel does, except that the byte representation is not read
129 from a channel, but taken from the string buff , starting at position
130 ofs .
131 val header_size : int
136 The bytes representing a marshaled value are composed of a fixed-size
138 header and a variable-sized data part, whose size can be determined
139 from the header. Marshal.header_size is the size, in characters, of
140 the header. Marshal.data_size buff ofs is the size, in characters, of
141 the data part, assuming a valid header is stored in buff starting at
142 position ofs . Finally, Marshal.total_size buff ofs is the total size,
143 in characters, of the marshaled value. Both Marshal.data_size and Mar‐
144 shal.total_size raise Failure if buff , ofs does not contain a valid
145 header.
146 To read the byte representation of a marshaled value into a string buf‐
148 fer, the program needs to read first Marshal.header_size characters
149 into the buffer, then determine the length of the remainder of the rep‐
150 resentation using Marshal.data_size , make sure the buffer is large
151 enough to hold the remaining data, then read it, and finally call Mar‐
152 shal.from_string to unmarshal the value.
153 val data_size : string -> int -> int
158 See Marshal.header_size .
160 val total_size : string -> int -> int
165 See Marshal.header_size .
167OCamldoc 2017-03-22 Marshal(3)