Module Marshal
: (moduleStdlib__Marshal)typeextern_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.
valto_channel : out_channel->'a->extern_flagslist->unitMarshal.to_channelchanvflags 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 -2 ^ 30 , 2 ^ 30-1 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 -2 ^ 30 , 2 ^ 30-1 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.
RaisesFailure if chan is not in binary mode.
valto_bytes : 'a->extern_flagslist->bytesMarshal.to_bytesvflags returns a byte sequence containing the representation of v . The flags argument
has the same meaning as for Marshal.to_channel .
Since 4.02
valto_string : 'a->extern_flagslist->string
Same as to_bytes but return the result as a string instead of a byte sequence.
valto_buffer : bytes->int->int->'a->extern_flagslist->intMarshal.to_bufferbuffofslenvflags 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.
valfrom_channel : in_channel->'aMarshal.from_channelchan 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.
RaisesEnd_of_file if chan is already at the end of the file.
RaisesFailure if the end of the file is reached during unmarshalling itself or if chan is not in binary
mode.
valfrom_bytes : bytes->int->'aMarshal.from_bytesbuffofs 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.
Since 4.02
valfrom_string : string->int->'a
Same as from_bytes but take a string as argument instead of a byte sequence.
valheader_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_sizebuffofs is the size, in bytes, of the data part, assuming a valid header is
stored in buff starting at position ofs . Finally, Marshal.total_sizebuffofs 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.
valdata_size : bytes->int->int
See Marshal.header_size .
valtotal_size : bytes->int->int
See Marshal.header_size .
Marshalanddomainsafety
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 surprising values (when unmarshaling) due to tearing, since marshaling involves byte-per-byte
copy.
OCamldoc 2025-06-12 Stdlib.Marshal(3o)
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