1Stdlib.Marshal(3) OCaml library Stdlib.Marshal(3)
2
6 Stdlib.Marshal - no description
7
9 Module Stdlib.Marshal
10
12 Module Marshal
13 : (module Stdlib__marshal)
14 type extern_flags =
22 | No_sharing (* Don't preserve sharing
23 *)
24 | Closures (* Send function closures
25 *)
26 | Compat_32 (* Ensure 32-bit compatibility
27 *)
28 The flags to the Marshal.to_* functions below.
31 val to_channel : out_channel -> 'a -> extern_flags list -> unit
35 Marshal.to_channel chan v flags writes the representation of v on chan‐
38 nel chan . The flags argument is a possibly empty list of flags that
39 governs the marshaling behavior with respect to sharing, functional
40 values, and compatibility between 32- and 64-bit platforms.
41 If flags does not contain Marshal.No_sharing , circularities and shar‐
43 ing inside the value v are detected and preserved in the sequence of
44 bytes produced. In particular, this guarantees that marshaling always
45 terminates. Sharing between values marshaled by successive calls to
46 Marshal.to_channel is neither detected nor preserved, though. If flags
47 contains Marshal.No_sharing , sharing is ignored. This results in
48 faster marshaling if v contains no shared substructures, but may cause
49 slower marshaling and larger byte representations if v actually con‐
50 tains sharing, or even non-termination if v contains cycles.
51 If flags does not contain Marshal.Closures , marshaling fails when it
53 encounters a functional value inside v : only 'pure' data structures,
54 containing neither functions nor objects, can safely be transmitted be‐
55 tween different programs. If flags contains Marshal.Closures , func‐
56 tional values will be marshaled as a the position in the code of the
57 program together with the values corresponding to the free variables
58 captured in the closure. In this case, the output of marshaling can
59 only be read back in processes that run exactly the same program, with
60 exactly the same compiled code. (This is checked at un-marshaling time,
61 using an MD5 digest of the code transmitted along with the code posi‐
62 tion.)
63 The exact definition of which free variables are captured in a closure
65 is not specified and can vary between bytecode and native code (and ac‐
66 cording to optimization flags). In particular, a function value ac‐
67 cessing a global reference may or may not include the reference in its
68 closure. If it does, unmarshaling the corresponding closure will cre‐
69 ate a new reference, different from the global one.
70 If flags contains Marshal.Compat_32 , marshaling fails when it encoun‐
72 ters an integer value outside the range [-2{^30}, 2{^30}-1] of integers
73 that are representable on a 32-bit platform. This ensures that mar‐
74 shaled data generated on a 64-bit platform can be safely read back on a
75 32-bit platform. If flags does not contain Marshal.Compat_32 , integer
76 values outside the range [-2{^30}, 2{^30}-1] are marshaled, and can be
77 read back on a 64-bit platform, but will cause an error at un-marshal‐
78 ing time when read back on a 32-bit platform. The Mashal.Compat_32
79 flag only matters when marshaling is performed on a 64-bit platform; it
80 has no effect if marshaling is performed on a 32-bit platform.
81 val to_bytes : 'a -> extern_flags list -> bytes
85 Marshal.to_bytes v flags returns a byte sequence containing the repre‐
88 sentation of v . The flags argument has the same meaning as for Mar‐
89 shal.to_channel .
90 Since 4.02.0
93 val to_string : 'a -> extern_flags list -> string
97 Same as to_bytes but return the result as a string instead of a byte
99 sequence.
100 val to_buffer : bytes -> int -> int -> 'a -> extern_flags list -> int
104 Marshal.to_buffer buff ofs len v flags marshals the value v , storing
107 its byte representation in the sequence buff , starting at index ofs ,
108 and writing at most len bytes. It returns the number of bytes actually
109 written to the sequence. If the byte representation of v does not fit
110 in len characters, the exception Failure is raised.
111 val from_channel : 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 It raises End_of_file if the function has already reached the end of
122 file when starting to read from the channel, and raises Failure "in‐
123 put_value: truncated object" if it reaches the end of file later during
124 the unmarshalling.
125 val from_bytes : bytes -> int -> 'a
129 Marshal.from_bytes buff ofs unmarshals a structured value like Mar‐
132 shal.from_channel does, except that the byte representation is not read
133 from a channel, but taken from the byte sequence buff , starting at po‐
134 sition ofs . The byte sequence is not mutated.
135 Since 4.02.0
138 val from_string : string -> int -> 'a
142 Same as from_bytes but take a string as argument instead of a byte se‐
144 quence.
145 val header_size : int
149 The bytes representing a marshaled value are composed of a fixed-size
151 header and a variable-sized data part, whose size can be determined
152 from the header. Marshal.header_size is the size, in bytes, of the
153 header. Marshal.data_size buff ofs is the size, in bytes, of the data
154 part, assuming a valid header is stored in buff starting at position
155 ofs . Finally, Marshal.total_size buff ofs is the total size, in
156 bytes, of the marshaled value. Both Marshal.data_size and Marshal.to‐
157 tal_size raise Failure if buff , ofs does not contain a valid header.
158 To read the byte representation of a marshaled value into a byte se‐
160 quence, the program needs to read first Marshal.header_size bytes into
161 the sequence, then determine the length of the remainder of the repre‐
162 sentation using Marshal.data_size , make sure the sequence is large
163 enough to hold the remaining data, then read it, and finally call Mar‐
164 shal.from_bytes to unmarshal the value.
165 val data_size : bytes -> int -> int
169 See Marshal.header_size .
171 val total_size : bytes -> int -> int
175 See Marshal.header_size .
177OCamldoc 2021-07-22 Stdlib.Marshal(3)