1. Brandon Mitchell
  2. ocaml-core


ocaml-core / base / bin-prot / README

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                          README: library "Bin_prot"
             Copyright   (C)   2008  Jane Street Holding, LLC (1)
                             Author: Markus Mottl 
                             New York, 2007-10-12

1  Directory contents

|     CHANGES       |                  History of code changes               
|    COPYRIGHT      |                    Notes on copyright                  
|     INSTALL       |               Short notes on compiling and             
|                   |                  installing the library                
|     LICENSE       |            "GNU LESSER GENERAL PUBLIC LICENSE"         
|  LICENSE.Tywith   |     License of Tywith, from which Sexplib is derived   
|     Makefile      |                       Top Makefile                     
|  OCamlMakefile    |            Generic Makefile for OCaml-projects         
|    OMakefile      |                     Ignore this file                   
|    README.txt     |                         This file                      
      |       lib/        |  OCaml-library for type-safe binary protocol
|    lib_test/      |        Test applications for the Bin_prot-library      

2  What is "Bin_prot"

  This library contains functionality for reading and writing OCaml-values in
a type-safe binary protocol. These functions are extremely efficient and
provide users with a convenient and safe way of performing I/O on any
extensionally defined data type. This means that functions, objects, and
values whose type is bound through a polymorphic record field are not
supported, but everything else is.
   As of now, there is no support for cyclic or shared values. Cyclic values
will lead to non-termination whereas shared values, besides requiring
significantly more space when encoded, may lead to a substantial increase in
memory footprint when they are read back in.
  Currently only little endian (2) computer architectures are supported. Some
architectures may potentially also suffer from data alignment issues with this
library. Only Intel architectures are currently well-tested. Both 32bit and
64bit architectures are fully supported.

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 below). 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 'unsafe_common', 'unsafe_read_c' and 'unsafe_write_c', which
handle the basic types with very low-level representations for efficiency.
   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.
   The modules 'Read_c' and 'Write_c' wrap the low-level converters for basic
values to ones accessible easily in OCaml and vice versa, and export functions
for wrapping user-defined converters. This should make it easy to add
user-defined converters that interact with a specific representation, but you
want to make them available to the other one quickly. The test applications in
the distribution make use of 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 hit. 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.
   The module 'Utils.ReadBuf' can be used to very efficiently read
size-prefixed values as written by 'bin_dump' with the 'header_size' flag.
This works even in the presence of partial data, e.g. when reading from
streaming data coming from sockets, etc. In most cases, when a whole value
fits into a buffer, this module will parse directly from the original buffer
and only copy data to an internal one on partial reads.

3.1  Examples
  E.g. given the following type definition:
<<  type t = A | B
    with bin_io
  The above will generate the functions 'bin_size_t', 'bin_write_t',
'bin_read_t', and the type class values 'bin_writer_t', 'bin_reader_t' and
'bin_rw_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. '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 written out 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 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 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). 
  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. 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.

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.

4.15  Bigarrays
  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, 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.

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 for
   integers and floating point numbers are written to 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