1Marshal(3) OCamldoc 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 OCaml.
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 OCaml 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 Values of extensible variant types, for example exceptions (of extensi‐
38 ble type exn ), returned by the unmarhsaller should not be pat‐
39 tern-matched over through match ... with or try ... with , because
40 unmarshalling does not preserve the information required for matching
41 their constructors. Structural equalities with other extensible variant
42 values does not work either. Most other uses such as Print‐
43 exc.to_string, will still work as expected.
44 The representation of marshaled values is not human-readable, and uses
46 bytes that are not printable characters. Therefore, input and output
47 channels used in conjunction with Marshal.to_channel and Mar‐
48 shal.from_channel must be opened in binary mode, using e.g.
49 open_out_bin or open_in_bin ; channels opened in text mode will cause
50 unmarshaling errors on platforms where text channels behave differently
51 than binary channels, e.g. Windows.
52 type extern_flags =
58 | No_sharing (* Don't preserve sharing
59 *)
60 | Closures (* Send function closures
61 *)
62 | Compat_32 (* Ensure 32-bit compatibility
63 *)
64 The flags to the Marshal.to_* functions below.
67 val to_channel : Pervasives.out_channel -> 'a -> extern_flags list ->
71 unit
72 Marshal.to_channel chan v flags writes the representation of v on chan‐
75 nel chan . The flags argument is a possibly empty list of flags that
76 governs the marshaling behavior with respect to sharing, functional
77 values, and compatibility between 32- and 64-bit platforms.
78 If flags does not contain Marshal.No_sharing , circularities and shar‐
80 ing inside the value v are detected and preserved in the sequence of
81 bytes produced. In particular, this guarantees that marshaling always
82 terminates. Sharing between values marshaled by successive calls to
83 Marshal.to_channel is neither detected nor preserved, though. If flags
84 contains Marshal.No_sharing , sharing is ignored. This results in
85 faster marshaling if v contains no shared substructures, but may cause
86 slower marshaling and larger byte representations if v actually con‐
87 tains sharing, or even non-termination if v contains cycles.
88 If flags does not contain Marshal.Closures , marshaling fails when it
90 encounters a functional value inside v : only 'pure' data structures,
91 containing neither functions nor objects, can safely be transmitted
92 between different programs. If flags contains Marshal.Closures , func‐
93 tional values will be marshaled as a the position in the code of the
94 program together with the values corresponding to the free variables
95 captured in the closure. In this case, the output of marshaling can
96 only be read back in processes that run exactly the same program, with
97 exactly the same compiled code. (This is checked at un-marshaling time,
98 using an MD5 digest of the code transmitted along with the code posi‐
99 tion.)
100 The exact definition of which free variables are captured in a closure
102 is not specified and can vary between bytecode and native code (and
103 according to optimization flags). In particular, a function value
104 accessing a global reference may or may not include the reference in
105 its closure. If it does, unmarshaling the corresponding closure will
106 create a new reference, different from the global one.
107 If flags contains Marshal.Compat_32 , marshaling fails when it encoun‐
109 ters an integer value outside the range [-2{^30}, 2{^30}-1] of integers
110 that are representable on a 32-bit platform. This ensures that mar‐
111 shaled data generated on a 64-bit platform can be safely read back on a
112 32-bit platform. If flags does not contain Marshal.Compat_32 , integer
113 values outside the range [-2{^30}, 2{^30}-1] are marshaled, and can be
114 read back on a 64-bit platform, but will cause an error at un-marshal‐
115 ing time when read back on a 32-bit platform. The Mashal.Compat_32
116 flag only matters when marshaling is performed on a 64-bit platform; it
117 has no effect if marshaling is performed on a 32-bit platform.
118 val to_bytes : 'a -> extern_flags list -> bytes
122 Marshal.to_bytes v flags returns a byte sequence containing the repre‐
125 sentation of v . The flags argument has the same meaning as for Mar‐
126 shal.to_channel .
127 Since 4.02.0
130 val to_string : 'a -> extern_flags list -> string
134 Same as to_bytes but return the result as a string instead of a byte
136 sequence.
137 val to_buffer : bytes -> int -> int -> 'a -> extern_flags list -> int
141 Marshal.to_buffer buff ofs len v flags marshals the value v , storing
144 its byte representation in the sequence buff , starting at index ofs ,
145 and writing at most len bytes. It returns the number of bytes actually
146 written to the sequence. If the byte representation of v does not fit
147 in len characters, the exception Failure is raised.
148 val from_channel : Pervasives.in_channel -> 'a
152 Marshal.from_channel chan reads from channel chan the byte representa‐
155 tion of a structured value, as produced by one of the Marshal.to_*
156 functions, and reconstructs and returns the corresponding value.
157 It raises End_of_file if the function has already reached the end of
159 file when starting to read from the channel, and raises Failure
160 input_value: truncated object if it reaches the end of file later dur‐
161 ing the unmarshalling.
162 val from_bytes : bytes -> int -> 'a
166 Marshal.from_bytes buff ofs unmarshals a structured value like Mar‐
169 shal.from_channel does, except that the byte representation is not read
170 from a channel, but taken from the byte sequence buff , starting at
171 position ofs . The byte sequence is not mutated.
172 Since 4.02.0
175 val from_string : string -> int -> 'a
179 Same as from_bytes but take a string as argument instead of a byte
181 sequence.
182 val header_size : int
186 The bytes representing a marshaled value are composed of a fixed-size
188 header and a variable-sized data part, whose size can be determined
189 from the header. Marshal.header_size is the size, in bytes, of the
190 header. Marshal.data_size buff ofs is the size, in bytes, of the data
191 part, assuming a valid header is stored in buff starting at position
192 ofs . Finally, Marshal.total_size buff ofs is the total size, in
193 bytes, of the marshaled value. Both Marshal.data_size and Mar‐
194 shal.total_size raise Failure if buff , ofs does not contain a valid
195 header.
196 To read the byte representation of a marshaled value into a byte
198 sequence, the program needs to read first Marshal.header_size bytes
199 into the sequence, then determine the length of the remainder of the
200 representation using Marshal.data_size , make sure the sequence is
201 large enough to hold the remaining data, then read it, and finally call
202 Marshal.from_bytes to unmarshal the value.
203 val data_size : bytes -> int -> int
207 See Marshal.header_size .
209 val total_size : bytes -> int -> int
213 See Marshal.header_size .
2152018-04-14 source: Marshal(3)