Binary protocol generator
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Bin_prot - OCaml Type Converter for Binary Protocols

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 deployed 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), alignment requirements, or endianness. Endianness defines the byte order in which machine representations of integers (machine words) are stored in main memory.

Bin_prot 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 noticeably 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 data structure 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 data type to allow for tail-recursion. The limitation cannot be eliminated in this library without significant performance impact and increased complexity.


The API (.mli-files) in the bin_prot library directory (lib) is fully documented, and HTML-documentation can be built from it on installation. The documentation for the latest release can also be found online.

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. Some operations are most easily performed in C. If you only want to read or write single, basic, unstructured values, using this module is probably the most efficient and convenient way of doing this.

Otherwise you should annotate your type definitions to generate type converters automatically (see later sections for details). The preprocessor in syntax/ 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 optimal 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 representation 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 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) data type 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 data type (e.g. sets, maps, etc.).


Consider the following type definition:

type t = A | B with bin_io

This will generate the functions bin_size_t, bin_write_t, and bin_read_t, as well as 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.

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.

Basic OCaml-values are written out character-wise as described below. The specification uses hex codes to define 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 below:

CODE_NEG_INT8  ->  0xff
CODE_INT16     ->  0xfe
CODE_INT32     ->  0xfd
CODE_INT64     ->  0xfc


This type encodes natural numbers including zero. It is frequently used by Bin_prot itself for encoding size information for e.g. lists, arrays, etc., and hence defined first here. Developers can reuse this type in their code, too, of course.

If the value of the underlying integer is lower than a certain range, this implies a certain encoding as provided on the right hand side of the following definitions:

<  0x000000080  ->  lower 8 bits of the integer                     (1 byte)
<  0x000010000  ->  CODE_INT16 followed by lower 16 bits of integer (3 bytes)
<  0x100000000  ->  CODE_INT32 followed by lower 32 bits of integer (5 bytes)
>= 0x100000000  ->  CODE_INT64 followed by all 64 bits of integer   (9 bytes)

The last line in the definitions above is only supported on 64 bit platforms due to word size limitations.

Appropriate exceptions will be raised if there is an overflow, for example 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 of natural numbers on their respective platforms. The last kind of overflow is due to OCaml reserving one bit for GC-tagging and the sign bit being lost.

Unit values

()  ->  0x00


false  ->  0x00
true   ->  0x01


First the length of the string is written out as a Nat0.t. Then the contents of the string is copied verbatim.


Characters are written out verbatim.


This includes all integer types: int, int32, int64, nativeint. If the value is positive (including zero) and if it is:

<  0x00000080  ->  lower 8 bits of the integer                     (1 byte)
<  0x00008000  ->  CODE_INT16 followed by lower 16 bits of integer (3 bytes)
<  0x80000000  ->  CODE_INT32 followed by lower 32 bits of integer (5 bytes)
>= 0x80000000  ->  CODE_INT64 followed by all 64 bits of integer   (9 bytes)

If the value is negative and if it is:

>= -0x00000080  ->  CODE_NEG_INT8 followed by lower 8 bits of integer (2 bytes)
>= -0x00008000  ->  CODE_INT16 followed by lower 16 bits of integer   (3 bytes)
>= -0x80000000  ->  CODE_INT32 followed by lower 32 bits of integer   (5 bytes)
<  -0x80000000  ->  CODE_INT64 followed by all 64 bits of integer     (9 bytes)

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. An overflow can only happen with values of type int, because one bit is reserved by OCaml for the GC-tag again.

The reason for this peculiar encoding is of statistical nature. It was assumed that small or positive numbers are much more frequent in practice than big or negative ones. The code is biased accordingly to achieve good compression and decoding performance. For example, a positive integer in the range from 0 to 127 requires only a single byte on the wire and only a single branch to identify it.


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

References and lazy values

Same as the binary encoding of the value in the reference or of the lazily calculated value.

Option values

If the value is:

None    ->  0x00
Some v  ->  0x01 followed by the encoding of v

Tuples and records

Values in tuples and records are written out one after the other in the order specified in the type definition. Polymorphic record fields are supported unless a value of the type bound by the field were accessed, which would lead to an exception.

Sum types

Each of the n tags in a sum type is assigned an integer from 0 to n - 1 in exactly the same order as they occur in the type. If a value of this type needs to be written out, then if:

n <= 256    ->  write out lower 8 bits of n  (1 byte)
n <= 65536  ->  write out lower 16 bits of n (2 bytes)

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.

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.

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. E.g.:

type ab = [ `A | `B ] with bin_io
type cda = [ `C | `D | `A ] with bin_io
type abcda = [ ab | cda ] with bin_io

When reading type abcda, the reader associated with type ab rather than cda will be invoked if a value of type ```A`` can be read. This may not make a difference in this example, but is important to know if the user manually overrides converters. It is strongly recommended to not merge polymorphic variants if their readers might disagree about how to interpret a certain tag. This is inconsistent, confusing, and hard to debug.

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 data structure.

Hash tables

First the size of the hash table is written out as a Nat0.t. Then the writer iterates over each binding in the hash table and writes out the key followed by the value.

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. This has in fact already happened in practice with the release of OCaml 4.00. Users should take note of this and make sure that all of their serialization routines remain future-proof by defining wire formats that are independent of the implementation of abstract data types.

Bigarrays of doubles (type vec) and characters (type bigstring)

First the dimension(s) are written out as Nat0.t. Then the contents is copied verbatim.

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 int substituted for 'a on the right hand side.

Abstract data types

If you want to convert an abstract data type 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 data type.