ocaml-core / base / bin_prot /

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                          README: library "Bin_prot"
             Copyright (C) 2007-2011  Jane Street Holding, LLC (1)
                             Author: Markus Mottl 
                             New York, 2011-07-29

1  Directory contents

      |    Changelog      |           History of code changes          |
      |    COPYRIGHT      |              Notes on copyright            |
      |     INSTALL       |         Short notes on compiling and       |
      |                   |            installing the library          |
      |     LICENSE       |           License of this library          |
      |  LICENSE.Tywith   |              License of Tywith             |
      |     Makefile      |                 Top Makefile               |
      |    README.txt     |          This text in ASCII format         |
      |       doc/        |         LaTeX sources for this text        |
      |       lib/        |               Library sources              |
      |    lib_test/      |  Test applications for the Bin_prot-library|
      |     syntax/       |      Preprocessor for type definitions     |

2  What is "Bin_prot"

  This library contains functionality for reading and writing OCaml-values in
a type-safe binary protocol. It is extremely efficient, typically supporting
type-safe marshalling and unmarshalling of even highly structured values at
speeds sufficient to saturate a gigabit connection. The protocol is also
heavily optimized for size, making it ideal for long-term storage of large
amounts of data.
   The library is highly dependable and safe to use: a rigorous test suite has
to date guaranteed that this library has never exhibited a bug in production
systems in several years of use. "Bin_prot" has been successfully employed in
mission-critical financial applications, storing many terabytes of structured
data derived from thousands of type definitions, and typically processing
millions of messages a day in realtime for low-latency applications that must
not crash.
   Since version two this library should work with all CPU architectures
currently supported by OCaml, no matter the word size (32 or 64 bit),
endianness (2), or alignment requirements. It provides users with a convenient
and safe way of performing I/O on any extensionally defined OCaml type (see
later sections for details). Functions, objects, first-class modules, as well
as values whose type is bound through a polymorphic record field are hence not
supported. This is hardly ever a limitation in practice.
   As of now, there is no support for cyclic or shared values. Cyclic values
will lead to non-termination whereas shared values, besides requiring more
space when encoded, may lead to a substantial increase in memory footprint
when they are read back. It would not be trivial to support these kinds of
values in a type-safe way without noticably sacrificing performance. If these
kinds of values are needed, the user may want to use the as of today still
unsafe marshalling functions provided by OCaml.
   This library uses the machine stack for efficiency reasons. This can
potentially lead to a crash if the stack limit is reached. Note that this is
also a limitation of the (unsafe) standard marshalling functions shipped with
OCaml. This problem can happen for large amounts of data if recursion in the
type definition of the datastructure is not limited to the last element. Only
in the latter case will tail-recursion allow for (practically) unlimited
amounts of data. If this exceedingly rare limitation ever turned out to be an
issue in a user application, it can often be solved by redefining the datatype
to allow for tail-recursion. The limitation cannot be eliminated in this
library without significant performance impact and increased complexity.

3  How can you use it?

  The API (.mli-files) in the library directory is fully documented. Module
'Common' defines some globally used types, functions, exceptions, and values.
'Nat0' implements natural numbers including zero.
   Modules 'Read_ml' and 'Write_ml' contain read and write functions
respectively for all basic types and are implemented in OCaml as far as
reasonable. If you only want to read or write single, basic, unstructured
values, this module is probably the most efficient and convenient for doing
   Otherwise you should annotate your type definitions to generate type
converters automatically (see later sections for details). The preprocessor in
'pa_bin_prot.ml' will then generate highly optimized functions for converting
your OCaml-values to and from the binary representation. This automatically
generated code will use functions in modules 'Unsafe_common', 'Unsafe_read_c'
and 'Unsafe_write_c', which employ unsafe internal representations to achieve
this performance. The auto-generated code is extremely well-tested and should
use these unsafe representations correctly. Developers who want to make manual
use of these unsafe calling conventions for efficiency are strongly encouraged
to test their code carefully.
   The module 'Size' allows you to compute the size of basic OCaml-values in
the binary representations before writing them to a buffer. The code generator
will also provide you with functions for your user-defined types.
   Module 'Std' predefines converters for most standard OCaml types. If you
use the preprocessor macros to generate code from type definitions, make sure
that the contents of this module is visible by e.g. adding the following at
the top of files using this library:
<<  open Bin_prot.Std
  Note that you can shadow the definitions in the above module in the unlikely
event that the predefined ways of converting data are unsatisfactory to you.
   The modules 'Read_c' and 'Write_c' wrap the unsafe low-level converters for
basic values to ones accessible safely from within OCaml and vice versa. They
also export functions for wrapping user-defined converters. This should help
developers make their converters available in the respective other
representation (low- or high-level). The test applications in the distribution
use these wrappers to verify the correctness of implementations for low-level
(C) and high-level (OCaml) representations.
   The module 'Type_class' contains some extra definitions for type classes of
basic values. These definitions can be passed to the function 'bin_dump' in
module 'Utils' to marshal values into buffers of exact size using the binary
protocol. However, if bounds on the size of values are known, it is usually
more efficient to write them directly into preallocated buffers and just catch
exceptions if the buffer limits are unexpectedly violated. Doing so should
never cause a crash. That way one does not have to compute the size of the
value, which can sometimes be almost as expensive as writing the value in the
first place.
   In module 'Utils' the function 'bin_read_stream' can be used to efficiently
read size-prefixed values as written by 'bin_dump' there with the 'header'
flag set to 'true'. This module also offers several useful functors. The ones
for 'binable' types help users create readers and writers if a type needs to
be converted to or from some intermediate representation before marshalling or
after unmarshalling respectively. The functors for 'iterable' types are
helpful if some (usually abstract) datatype offers iteration over elements and
if the series of iterated-over elements alone is sufficient to reconstruct the
original value. This allows for a more compact protocol and for robustness
against changes to the internal representation of the datatype (e.g. sets,
maps, etc.).

3.1  Examples
  E.g. given the following type definition:
<<  type t = A | B
    with bin_io
  This will generate functions 'bin_size_t', 'bin_write_t', and 'bin_read_t'.
Furthermore the type class values 'bin_writer_t', 'bin_reader_t' and 'bin_t'.
If you use the annotation 'bin_write' instead of 'bin_io', then only the write
and size functions and their type class will be generated. Specifying
'bin_read' will generate the read functions and associated type class only.
The annotation 'bin_type_class' will generate the combined type class only,
thus allowing the user to easily define their own reader and writer type
classes. The code generator may also generate low-level entry points used for
efficiency or backtracking.
  The preprocessor can also generate signatures for conversion functions. Just
add the wanted annotation to the type in a module signature for that purpose.

4  Specification of the Binary Protocol

  The binary protocol does not contain any data other than the minimum needed
to decode values. This means that the user is responsible for e.g. writing out
the size of messages themselves if they want to be able to preallocate
sufficiently sized buffers before reading. The 'Utils' module provides some
simple functions for that matter, though users may obtain optimum efficiency
by managing buffers themselves.
   The basic OCaml-values are written out character-wise as described below
using hex codes for the character encoding. Some of these values require
size/length information to be written out before the value (e.g. for lists,
hash tables, strings, etc.). Size information is always encoded as natural
numbers ('Nat0.t'). The little-endian format is used in the protocol for the
contents of integers on all platforms.
   The following definitions will be used in the encoding specifications
   - 'CODE_NEG_INT8' -> '0xff' 
   - 'CODE_INT16' -> '0xfe' 
   - 'CODE_INT32' -> '0xfd' 
   - 'CODE_INT64' -> '0xfc' 

4.1  Nat0.t
  If the value is:
   - < '0x000000080' -> lower 8 bit of the integer. 
   - < '0x000010000' -> 'CODE_INT16' followed by lower 16 bits of integer. 
   - < '0x100000000' -> 'CODE_INT32' followed by lower 32 bits of integer. 
   - >= '0x100000000' -> 'CODE_INT64' followed by all 64 bits of integer (3). 
  Appropriate exceptions will be raised if there is an overflow, e.g. if a 64
bit encoding is read on a 32 bit platform, or if the 32 bit or 64 bit encoding
overflowed the 30 bit or 62 bit capacity (4) of natural numbers on their
respective platforms.

4.2  Unit values
   - '()' -> '0x00' 

4.3  Booleans
   - 'false' -> '0x00' 
   - 'true' -> '0x01' 

4.4  Strings
  First the length of the string is written out as a 'Nat0.t'. Then the
contents of the string is copied verbatim.

4.5  Characters
  Characters are written out verbatim.

4.6  Integers
  This includes all integer types: 'int, int32, int64, nativeint'. If the
value is positive (including zero) and if it is:
   - < '0x00000080' -> lower 8 bit of the integer. 
   - < '0x00008000' -> 'CODE_INT16' followed by lower 16 bits of integer. 
   - < '0x80000000' -> 'CODE_INT32' followed by lower 32 bits of integer. 
   - >= '0x80000000' -> 'CODE_INT64' followed by all 64 bits of integer. 
  If the value is negative and if it is:
   - >= '-0x00000080' -> 'CODE_NEG_INT8' followed by lower 8 bit of integer. 
   - >= '-0x00008000' -> 'CODE_INT16' followed by lower 16 bits of integer. 
   - >= '-0x80000000' -> 'CODE_INT32' followed by lower 32 bits of integer. 
   - < '-0x80000000' -> 'CODE_INT64' followed by all 64 bits of integer. 
  All of the above branches will be considered when converting values of type
'int64'. The case for 'CODE_INT64' will only be considered with types 'int'
and 'nativeint' if the architecture supports it. 'int32' will never be encoded
as a 'CODE_INT64'. Appropriate exceptions will be raised if the architecture
of or the type requested by the reader does not support some encoding, or if
there is an overflow (5).

4.7  Floats
  Floats are written out according to the 64 bit IEEE 754 floating point
standard, i.e. their memory representation is copied verbatim.

4.8  References and lazy values
  Same as the binary encoding of the value in the reference or of the value
calculated lazily.

4.9  Option values
  If the value is:
   - 'None' -> '0x00' 
   - 'Some v' -> '0x01' followed by the encoding of 'v'. 

4.10  Tuples and records
  Values in tuples and records are written out one after the other in the
order as specified in the type specification without any extra information.
Polymorphic record fields are supported unless a value of the type bound by
the field were accessed, which would lead to an exception.

4.11  Sum types
  Each tag is assigned an integer from 0 to n - 1 in exactly the same order as
they occur in the type, where n is the number of sum tags in the type. If a
value of this type needs to be written out, then if:
   - n <= 256 -> the integer associated with the tag is written out as one
   character (lower 8 bits). 
   - n <= 65536 -> the integer associated with the tag is written out as two
   characters (lower 16 bits). 
  Sum types with more tags are currently not supported and highly unlikely to
occur in practice. Arguments to the tag are written out in the order of
occurrence without any extra information.

4.12  Polymorphic variants
  The tags of these values are written out as four characters, more precisely
as the 32 bit hash value computed by OCaml for the given tag in little-endian
format. Any arguments associated with the tag are written out afterwards in
the order of occurrence without any extra information.
   When polymorphic variants are being read, they will be matched in order of
occurrence (left-to-right) in the type and depth-first in the case of included
polymorphic types. The first type containing a match for the variant will be
used for reading. Note that this only matters if semantic invariants are used
that may impose constraints on whether a value is legal. It is strongly
suggested to not use different constraints for polymorphic variants with the
same representation if it is used within the same type, since this is
inconsistent and hence confusing.

4.13  Lists and arrays
  For lists and arrays the length is written out as a 'Nat0.t' first, followed
by all values in the same order as in the datastructure without any extra

4.14  Hash tables
  First the size of the hash table is written out as 'Nat0.t'. Then the writer
iterates over each binding in the hash table and writes out the key followed
by the value without any extra information. Note that this makes reading
somewhat slower than if we used the internal (extensional) representation of
the hash table, because all values have to be rehashed. On the other hand, the
format becomes more robust in case the hash table implementation changes.

4.15  Bigarrays of doubles and characters
  First the dimension(s) are written out as 'Nat0.t'. Then the contents is
copied verbatim.

4.16  Polymorphic values
  There is nothing special about polymorphic values as long as there are
conversion functions for the type parameters. E.g.:
<<  type 'a t = A | B of 'a with bin_io
    type foo = int t with bin_io
  In the above case the conversion functions will behave as if 'foo' had been
defined as a monomorphic version of 't' with ''a' replaced by 'int' on the
right hand side.

4.17  Abstract datatypes
  If you want to convert an abstract datatype that may impose constraints on
the well-formedness of values, you will have to roll your own conversion
functions. Use the functions in module 'Read_c' and 'Write_c' to map between
low-level and high-level representations, or implement those manually for
maximum efficiency. The 'Utils' module may also come in handy as described in
earlier sections, e.g. if the value can be converted to and from an
intermediate representation that does not impose constraints, or if some sort
of iteration is supported by the datatype.

5  Contact information

  In the case of bugs, feature requests and similar, you can contact us here:
   Up-to-date information concerning this library should be available here:
   This document was translated from LaTeX by HeVeA (6).
 (1) http://www.janestreet.com
 (2) Endianness defines the byte order in which machine representations of
   integers are stored in main memory.
 (3) Only supported on 64 bit platforms.
 (4) One bit is reserved by OCaml for GC-tagging, and the sign bit is lost.
 (5) An overflow can only happen with int values: one bit is reserved by OCaml
   for the GC-tag.
 (6) http://hevea.inria.fr/index.html
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