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Marshal(3) OCamldoc Marshal(3) NAME Marshal - Marshaling of data structures. Module Module Marshal Documentation Module Marshal : sig end Marshaling of data structures. This module provides functions to encode arbitrary data structures as sequences of bytes, which can then be written on a file or sent over a pipe or network connection. The bytes can then be read back later, possibly in another process, and decoded back into a data structure. The format for the byte sequences is compatible across all machines for a given version of OCaml. Warning: marshaling is currently not type-safe. The type of marshaled data is not transmitted along the value of the data, making it impossi- ble to check that the data read back possesses the type expected by the context. In particular, the result type of the Marshal.from_* functions is given as 'a , but this is misleading: the returned OCaml value does not possess type 'a for all 'a ; it has one, unique type which cannot be determined at compile-type. The programmer should explicitly give the expected type of the returned value, using the following syntax: - (Marshal.from_channel chan : type) . Anything can happen at run-time if the object in the file does not belong to the given type. Values of extensible variant types, for example exceptions (of extensi- ble type exn ), returned by the unmarhsaller should not be pat- tern-matched over through match ... with or try ... with , because un- marshalling does not preserve the information required for matching their constructors. Structural equalities with other extensible variant values does not work either. Most other uses such as Print- exc.to_string, will still work as expected. The representation of marshaled values is not human-readable, and uses bytes that are not printable characters. Therefore, input and output channels used in conjunction with Marshal.to_channel and Mar- shal.from_channel must be opened in binary mode, using e.g. open_out_bin or open_in_bin ; channels opened in text mode will cause unmarshaling errors on platforms where text channels behave differently than binary channels, e.g. Windows. type extern_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. val to_channel : Pervasives.out_channel -> 'a -> extern_flags list -> unit Marshal.to_channel chan v flags writes the representation of v on chan- nel 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 shar- ing 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 con- tains 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 be- tween different programs. If flags contains Marshal.Closures , func- tional 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 posi- tion.) The exact definition of which free variables are captured in a closure is not specified and can vary between bytecode and native code (and ac- cording to optimization flags). In particular, a function value ac- cessing a global reference may or may not include the reference in its closure. If it does, unmarshaling the corresponding closure will cre- ate a new reference, different from the global one. If flags contains Marshal.Compat_32 , marshaling fails when it encoun- ters an integer value outside the range [-2{^30}, 2{^30}-1] of integers that are representable on a 32-bit platform. This ensures that mar- shaled 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-marshal- ing 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. val to_bytes : 'a -> extern_flags list -> bytes Marshal.to_bytes v flags returns a byte sequence containing the repre- sentation of v . The flags argument has the same meaning as for Mar- shal.to_channel . Since 4.02.0 val to_string : 'a -> extern_flags list -> string Same as to_bytes but return the result as a string instead of a byte sequence. val to_buffer : bytes -> int -> int -> 'a -> extern_flags list -> int Marshal.to_buffer buff ofs len v flags 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. val from_channel : Pervasives.in_channel -> 'a Marshal.from_channel chan reads from channel chan the byte representa- tion of a structured value, as produced by one of the Marshal.to_* functions, and reconstructs and returns the corresponding value. It raises End_of_file if the function has already reached the end of file when starting to read from the channel, and raises Failure in- put_value: truncated object if it reaches the end of file later during the unmarshalling. val from_bytes : bytes -> int -> 'a Marshal.from_bytes buff ofs unmarshals a structured value like Mar- shal.from_channel does, except that the byte representation is not read from a channel, but taken from the byte sequence buff , starting at po- sition ofs . The byte sequence is not mutated. Since 4.02.0 val from_string : string -> int -> 'a Same as from_bytes but take a string as argument instead of a byte se- quence. val header_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_size buff ofs is the size, in bytes, of the data part, assuming a valid header is stored in buff starting at position ofs . Finally, Marshal.total_size buff ofs is the total size, in bytes, of the marshaled value. Both Marshal.data_size and Marshal.to- tal_size raise Failure if buff , ofs does not contain a valid header. To read the byte representation of a marshaled value into a byte se- quence, the program needs to read first Marshal.header_size bytes into the sequence, then determine the length of the remainder of the repre- sentation using Marshal.data_size , make sure the sequence is large enough to hold the remaining data, then read it, and finally call Mar- shal.from_bytes to unmarshal the value. val data_size : bytes -> int -> int See Marshal.header_size . val total_size : bytes -> int -> int See Marshal.header_size . 2021-03-01 source: Marshal(3)
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