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 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-time. 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 unmarshaller 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 : out_channel -> 'a -> extern_flags list -> unit
71 Marshal.to_channel chan v flags writes the representation of v on chan‐
74 nel chan . The flags argument is a possibly empty list of flags that
75 governs the marshaling behavior with respect to sharing, functional
76 values, and compatibility between 32- and 64-bit platforms.
77 If flags does not contain Marshal.No_sharing , circularities and shar‐
79 ing inside the value v are detected and preserved in the sequence of
80 bytes produced. In particular, this guarantees that marshaling always
81 terminates. Sharing between values marshaled by successive calls to
82 Marshal.to_channel is neither detected nor preserved, though. If flags
83 contains Marshal.No_sharing , sharing is ignored. This results in
84 faster marshaling if v contains no shared substructures, but may cause
85 slower marshaling and larger byte representations if v actually con‐
86 tains sharing, or even non-termination if v contains cycles.
87 If flags does not contain Marshal.Closures , marshaling fails when it
89 encounters a functional value inside v : only 'pure' data structures,
90 containing neither functions nor objects, can safely be transmitted
91 between different programs. If flags contains Marshal.Closures , func‐
92 tional values will be marshaled as a the position in the code of the
93 program together with the values corresponding to the free variables
94 captured in the closure. In this case, the output of marshaling can
95 only be read back in processes that run exactly the same program, with
96 exactly the same compiled code. (This is checked at un-marshaling time,
97 using an MD5 digest of the code transmitted along with the code posi‐
98 tion.)
99 The exact definition of which free variables are captured in a closure
101 is not specified and can vary between bytecode and native code (and
102 according to optimization flags). In particular, a function value
103 accessing a global reference may or may not include the reference in
104 its closure. If it does, unmarshaling the corresponding closure will
105 create a new reference, different from the global one.
106 If flags contains Marshal.Compat_32 , marshaling fails when it encoun‐
108 ters an integer value outside the range [-2{^30}, 2{^30}-1] of integers
109 that are representable on a 32-bit platform. This ensures that mar‐
110 shaled data generated on a 64-bit platform can be safely read back on a
111 32-bit platform. If flags does not contain Marshal.Compat_32 , integer
112 values outside the range [-2{^30}, 2{^30}-1] are marshaled, and can be
113 read back on a 64-bit platform, but will cause an error at un-marshal‐
114 ing time when read back on a 32-bit platform. The Mashal.Compat_32
115 flag only matters when marshaling is performed on a 64-bit platform; it
116 has no effect if marshaling is performed on a 32-bit platform.
117 val to_bytes : 'a -> extern_flags list -> bytes
121 Marshal.to_bytes v flags returns a byte sequence containing the repre‐
124 sentation of v . The flags argument has the same meaning as for Mar‐
125 shal.to_channel .
126 Since 4.02.0
129 val to_string : 'a -> extern_flags list -> string
133 Same as to_bytes but return the result as a string instead of a byte
135 sequence.
136 val to_buffer : bytes -> int -> int -> 'a -> extern_flags list -> int
140 Marshal.to_buffer buff ofs len v flags marshals the value v , storing
143 its byte representation in the sequence buff , starting at index ofs ,
144 and writing at most len bytes. It returns the number of bytes actually
145 written to the sequence. If the byte representation of v does not fit
146 in len characters, the exception Failure is raised.
147 val from_channel : in_channel -> 'a
151 Marshal.from_channel chan reads from channel chan the byte representa‐
154 tion of a structured value, as produced by one of the Marshal.to_*
155 functions, and reconstructs and returns the corresponding value.
156 It raises End_of_file if the function has already reached the end of
158 file when starting to read from the channel, and raises Failure
159 "input_value: truncated object" if it reaches the end of file later
160 during the unmarshalling.
161 val from_bytes : bytes -> int -> 'a
165 Marshal.from_bytes buff ofs unmarshals a structured value like Mar‐
168 shal.from_channel does, except that the byte representation is not read
169 from a channel, but taken from the byte sequence buff , starting at
170 position ofs . The byte sequence is not mutated.
171 Since 4.02.0
174 val from_string : string -> int -> 'a
178 Same as from_bytes but take a string as argument instead of a byte
180 sequence.
181 val header_size : int
185 The bytes representing a marshaled value are composed of a fixed-size
187 header and a variable-sized data part, whose size can be determined
188 from the header. Marshal.header_size is the size, in bytes, of the
189 header. Marshal.data_size buff ofs is the size, in bytes, of the data
190 part, assuming a valid header is stored in buff starting at position
191 ofs . Finally, Marshal.total_size buff ofs is the total size, in
192 bytes, of the marshaled value. Both Marshal.data_size and Mar‐
193 shal.total_size raise Failure if buff , ofs does not contain a valid
194 header.
195 To read the byte representation of a marshaled value into a byte
197 sequence, the program needs to read first Marshal.header_size bytes
198 into the sequence, then determine the length of the remainder of the
199 representation using Marshal.data_size , make sure the sequence is
200 large enough to hold the remaining data, then read it, and finally call
201 Marshal.from_bytes to unmarshal the value.
202 val data_size : bytes -> int -> int
206 See Marshal.header_size .
208 val total_size : bytes -> int -> int
212 See Marshal.header_size .
214OCamldoc 2020-09-01 Marshal(3)