Endian Buffer Types

Introduction

The internal byte order of arithmetic types is traditionally called endianness. See the Wikipedia for a full exploration of endianness, including definitions of big endian and little endian.

Header boost/endian/buffers.hpp provides endian_buffer, a portable endian integer binary buffer class template with control over byte order, value type, size, and alignment independent of the platform's native endianness. Typedefs provide easy-to-use names for common configurations.

Use cases primarily involve data portability, either via files or network connections, but these byte-holders may also be used to reduce memory use, file size, or network activity since they provide binary numeric sizes not otherwise available.

Class endian_buffer is aimed at users who wish explicit control over when endianness conversions occur. It also serves as the base class for the endian_arithmetic class template, which is aimed at users who wish fully automatic endianness conversion and direct support for all normal arithmetic operations.

Example

The example/endian_example.cpp program writes a binary file containing four-byte, big-endian and little-endian integers:

#include <iostream>
#include <cstdio>
#include <boost/endian/buffers.hpp>  // see Synopsis below
#include <boost/static_assert.hpp>

using namespace boost::endian;

namespace 
{
  //  This is an extract from a very widely used GIS file format.
  //  Why the designer decided to mix big and little endians in
  //  the same file is not known. But this is a real-world format
  //  and users wishing to write low level code manipulating these
  //  files have to deal with the mixed endianness.

  struct header
  {
    big_int32_buf_t     file_code;
    big_int32_buf_t     file_length;
    little_int32_buf_t  version;
    little_int32_buf_t  shape_type;
  };

  const char* filename = "test.dat";
}

int main(int, char* [])
{
  header h;

  BOOST_STATIC_ASSERT(sizeof(h) == 16U);  // reality check
  
  h.file_code   = 0x01020304;
  h.file_length = sizeof(header);
  h.version     = 1;
  h.shape_type  = 0x01020304;

  //  Low-level I/O such as POSIX read/write or <cstdio>
  //  fread/fwrite is sometimes used for binary file operations
  //  when ultimate efficiency is important. Such I/O is often
  //  performed in some C++ wrapper class, but to drive home the
  //  point that endian integers are often used in fairly
  //  low-level code that does bulk I/O operations, <cstdio>
  //  fopen/fwrite is used for I/O in this example.

  std::FILE* fi = std::fopen(filename, "wb");  // MUST BE BINARY
  
  if (!fi)
  {
    std::cout << "could not open " << filename << '\n';
    return 1;
  }

  if (std::fwrite(&h, sizeof(header), 1, fi)!= 1)
  {
    std::cout << "write failure for " << filename << '\n';
    return 1;
  }

  std::fclose(fi);

  std::cout << "created file " << filename << '\n';

  return 0;
}

After compiling and executing example/endian_example.cpp, a hex dump of test.dat shows:

01020304 00000010 01000000 04030201

Notice that the first two 32-bit integers are big endian while the second two are little endian, even though the machine this was compiled and run on was little endian.

Limitations

Requires <climits> CHAR_BIT == 8. If CHAR_BIT is some other value, compilation will result in an #error. This restriction is in place because the design, implementation, testing, and documentation has only considered issues related to 8-bit bytes, and there have been no real-world use cases presented for other sizes.

In C++03, endian_buffer does not meet the requirements for POD types because it has constructors, private data members, and a base class. This means that common use cases are relying on unspecified behavior in that the C++ Standard does not guarantee memory layout for non-POD types. This has not been a problem in practice since all known C++ compilers lay out memory as if endian were a POD type. In C++11, it is possible to specify the default constructor as trivial, and private data members and base classes no longer disqualify a type from being a POD type. Thus under C++11, endian_buffer will no longer be relying on unspecified behavior.

Feature set

• Big endian| little endian | native endian byte ordering.
• Signed | unsigned
• Unaligned | aligned
• 1-8 byte (unaligned) | 1, 2, 4, 8 byte (aligned)
• Choice of value type

Enums and typedefs

Two scoped enums are provided:

enum class order {big, little, native};

enum class align {no, yes}; 

One class template is provided:

template <order Order, typename T, std::size_t Nbits,
  align Align = align::no>
class endian_buffer;

Typedefs, such as big_int32_buf_t, provide convenient naming conventions for common use cases:

Name	Alignment	Endianness	Sign	Sizes in bits (n)
big_intn_buf_t	no	big	signed	8,16,24,32,40,48,56,64
big_uintn_buf_t	no	big	unsigned	8,16,24,32,40,48,56,64
little_intn_buf_t	no	little	signed	8,16,24,32,40,48,56,64
little_uintn_buf_t	no	little	unsigned	8,16,24,32,40,48,56,64
native_intn_buf_t	no	native	signed	8,16,24,32,40,48,56,64
native_uintn_buf_t	no	native	unsigned	8,16,24,32,40,48,56,64
big_intn_buf_at	yes	big	signed	8,16,32,64
big_uintn_buf_at	yes	big	unsigned	8,16,32,64
little_intn_buf_at	yes	little	signed	8,16,32,64
little_uintn_buf_at	yes	little	unsigned	8,16,32,64

The unaligned types do not cause compilers to insert padding bytes in classes and structs. This is an important characteristic that can be exploited to minimize wasted space in memory, files, and network transmissions.

Warning: Code that uses aligned types is possibly non-portable because alignment requirements vary between hardware architectures and because alignment may be affected by compiler switches or pragmas. For example, alignment of an 64-bit integer may be to a 32-bit boundary on a 32-bit machine and to a 64-bit boundary on a 64-bit machine. Furthermore, aligned types are only available on architectures with 8, 16, 32, and 64-bit integer types.

Recommendation: Prefer unaligned buffer types.

Recommendation: Protect yourself against alignment ills. For example:

static_assert(sizeof(containing_struct) == 12, "sizeof(containing_struct) is wrong"); 

Note: One-byte big and little buffer types have identical layout on all platforms, so they never actually reverse endianness. They are provided to enable generic code, and to improve code readability and searchability.

Class template endian_buffer

An endian_buffer is a byte-holder for arithmetic types with user-specified endianness, value type, size, and alignment.

Synopsis

#include <boost/endian/conversion.hpp

namespace boost
{
  namespace endian
  {
    //  C++11 features emulated if not available
   
    enum class align {no, yes};            

    template <order Order, class T, std::size_t Nbits,
      align Align = align::no>
    class endian_buffer
    {
    public:
      typedef T value_type;

      endian_buffer() noexcept = default;
      explicit endian_buffer(T v) noexcept;

      endian_buffer& operator=(T v) noexcept;
      value_type     value() const noexcept;
      const char*    data() const noexcept;
    protected:
      implementaton-defined  endian_value;  // for exposition only
    };
    
    //  stream inserter
    template <class charT, class traits, order Order, class T,
      std::size_t n_bits, align Align>
    std::basic_ostream<charT, traits>&
      operator<<(std::basic_ostream<charT, traits>& os,
        const endian_buffer<Order, T, n_bits, Align>& x);

    //  stream extractor 
    template <class charT, class traits, order Order, class T,
      std::size_t n_bits, align A>
    std::basic_istream<charT, traits>&
      operator>>(std::basic_istream<charT, traits>& is,
        endian_buffer<Order, T, n_bits, Align>& x);

    // typedefs  

    // unaligned big endian signed integer buffers
    typedef endian_buffer<order::big, int_least8_t, 8>        big_int8_buf_t;
    typedef endian_buffer<order::big, int_least16_t, 16>      big_int16_buf_t;
    typedef endian_buffer<order::big, int_least32_t, 24>      big_int24_buf_t;
    typedef endian_buffer<order::big, int_least32_t, 32>      big_int32_buf_t;
    typedef endian_buffer<order::big, int_least64_t, 40>      big_int40_buf_t;
    typedef endian_buffer<order::big, int_least64_t, 48>      big_int48_buf_t;
    typedef endian_buffer<order::big, int_least64_t, 56>      big_int56_buf_t;
    typedef endian_buffer<order::big, int_least64_t, 64>      big_int64_buf_t;
  
    // unaligned big endian unsigned integer buffers
    typedef endian_buffer<order::big, uint_least8_t, 8>       big_uint8_buf_t;
    typedef endian_buffer<order::big, uint_least16_t, 16>     big_uint16_buf_t;
    typedef endian_buffer<order::big, uint_least32_t, 24>     big_uint24_buf_t;
    typedef endian_buffer<order::big, uint_least32_t, 32>     big_uint32_buf_t;
    typedef endian_buffer<order::big, uint_least64_t, 40>     big_uint40_buf_t;
    typedef endian_buffer<order::big, uint_least64_t, 48>     big_uint48_buf_t;
    typedef endian_buffer<order::big, uint_least64_t, 56>     big_uint56_buf_t;
    typedef endian_buffer<order::big, uint_least64_t, 64>     big_uint64_buf_t;
  
    // unaligned little endian signed integer buffers
    typedef endian_buffer<order::little, int_least8_t, 8>     little_int8_buf_t;
    typedef endian_buffer<order::little, int_least16_t, 16>   little_int16_buf_t;
    typedef endian_buffer<order::little, int_least32_t, 24>   little_int24_buf_t;
    typedef endian_buffer<order::little, int_least32_t, 32>   little_int32_buf_t;
    typedef endian_buffer<order::little, int_least64_t, 40>   little_int40_buf_t;
    typedef endian_buffer<order::little, int_least64_t, 48>   little_int48_buf_t;
    typedef endian_buffer<order::little, int_least64_t, 56>   little_int56_buf_t;
    typedef endian_buffer<order::little, int_least64_t, 64>   little_int64_buf_t;
  
    // unaligned little endian unsigned integer buffers
    typedef endian_buffer<order::little, uint_least8_t, 8>    little_uint8_buf_t;
    typedef endian_buffer<order::little, uint_least16_t, 16>  little_uint16_buf_t;
    typedef endian_buffer<order::little, uint_least32_t, 24>  little_uint24_buf_t;
    typedef endian_buffer<order::little, uint_least32_t, 32>  little_uint32_buf_t;
    typedef endian_buffer<order::little, uint_least64_t, 40>  little_uint40_buf_t;
    typedef endian_buffer<order::little, uint_least64_t, 48>  little_uint48_buf_t;
    typedef endian_buffer<order::little, uint_least64_t, 56>  little_uint56_buf_t;
    typedef endian_buffer<order::little, uint_least64_t, 64>  little_uint64_buf_t;
  
    // unaligned native endian signed integer types
    typedef implementation-defined_int8_buf_t   native_int8_buf_t;
    typedef implementation-defined_int16_buf_t  native_int16_buf_t;
    typedef implementation-defined_int24_buf_t  native_int24_buf_t;
    typedef implementation-defined_int32_buf_t  native_int32_buf_t;
    typedef implementation-defined_int40_buf_t  native_int40_buf_t;
    typedef implementation-defined_int48_buf_t  native_int48_buf_t;
    typedef implementation-defined_int56_buf_t  native_int56_buf_t;
    typedef implementation-defined_int64_buf_t  native_int64_buf_t;

    // unaligned native endian unsigned integer types
    typedef implementation-defined_uint8_buf_t   native_uint8_buf_t;
    typedef implementation-defined_uint16_buf_t  native_uint16_buf_t;
    typedef implementation-defined_uint24_buf_t  native_uint24_buf_t;
    typedef implementation-defined_uint32_buf_t  native_uint32_buf_t;
    typedef implementation-defined_uint40_buf_t  native_uint40_buf_t;
    typedef implementation-defined_uint48_buf_t  native_uint48_buf_t;
    typedef implementation-defined_uint56_buf_t  native_uint56_buf_t;
    typedef implementation-defined_uint64_buf_t  native_uint64_buf_t;
    
    // aligned big endian signed integer buffers
    typedef endian_buffer<order::big, int8_t, 8, align::yes>       big_int8_buf_at;
    typedef endian_buffer<order::big, int16_t, 16, align::yes>     big_int16_buf_at;
    typedef endian_buffer<order::big, int32_t, 32, align::yes>     big_int32_buf_at;
    typedef endian_buffer<order::big, int64_t, 64, align::yes>     big_int64_buf_at;
  
    // aligned big endian unsigned integer buffers
    typedef endian_buffer<order::big, uint8_t, 8, align::yes>      big_uint8_buf_at;
    typedef endian_buffer<order::big, uint16_t, 16, align::yes>    big_uint16_buf_at;
    typedef endian_buffer<order::big, uint32_t, 32, align::yes>    big_uint32_buf_at;
    typedef endian_buffer<order::big, uint64_t, 64, align::yes>    big_uint64_buf_at;
  
    // aligned little endian signed integer buffers
    typedef endian_buffer<order::little, int8_t, 8, align::yes>    little_int8_buf_at;
    typedef endian_buffer<order::little, int16_t, 16, align::yes>  little_int16_buf_at;
    typedef endian_buffer<order::little, int32_t, 32, align::yes>  little_int32_buf_at;
    typedef endian_buffer<order::little, int64_t, 64, align::yes>  little_int64_buf_at;
  
    // aligned little endian unsigned integer buffers
    typedef endian_buffer<order::little, uint8_t, 8, align::yes>   little_uint8_buf_at;
    typedef endian_buffer<order::little, uint16_t, 16, align::yes> little_uint16_buf_at;
    typedef endian_buffer<order::little, uint32_t, 32, align::yes> little_uint32_buf_at;
    typedef endian_buffer<order::little, uint64_t, 64, align::yes> little_uint64_buf_at;

    // aligned native endian typedefs are not provided because
    // <cstdint> types are superior for this use case
  
  } // namespace endian
} // namespace boost

The implementation-defined text in typedefs above is either big or little according to the native endianness of the platform.

The expository data member endian_value stores the current value of an endian_value object as a sequence of bytes ordered as specified by the Order template parameter.  The implementation-defined type of endian_value is a type such as char[Nbits/CHAR_BIT] or T that meets the requirements imposed by the Nbits and Align template parameters.  The CHAR_BIT macro is defined in <climits>. The only value of CHAR_BIT that is required to be supported is 8.

Template parameter T is required to be a standard integer type (C++std, 3.9.1) and sizeof(T)*CHAR_BIT is required to be greater or equal to Nbits.

Members

endian_buffer() noexcept = default;

Effects: Constructs an uninitialized object of type endian_buffer<Order, T, Nbits, Align>.

explicit endian_buffer(T v) noexcept;

Effects: Constructs an object of type endian_buffer<Order, T, Nbits, Align>.

Postcondition: value() == v & mask, where mask is a constant of type value_type with Nbits low-order bits set to one.

Remarks: If Align is align::yes then endianness conversion, if required, is performed by boost::endian::endian_reverse.

endian_buffer& operator=(T v) noexcept;

Postcondition: value() == v & mask, where mask is a constant of type value_type with Nbits low-order bits set to one.

Returns: *this.

Remarks: If Align is align::yes then endianness conversion, if required, is performed by boost::endian::endian_reverse.

value_type value() const noexcept;

Returns: endian_value, converted to value_type, if required, and having the endianness of the native platform.

Remarks: If Align is align::yes then endianness conversion, if required, is performed by boost::endian::endian_reverse.

const char* data() const noexcept;

Returns: A pointer to the first byte of endian_value.

Non-member functions

template <class charT, class traits, order Order, class T,
  std::size_t n_bits, align Align>
std::basic_ostream<charT, traits>& operator<<(std::basic_ostream<charT, traits>& os,
  const endian_buffer<Order, T, n_bits, Align>& x);

Returns: os << x.value().

template <class charT, class traits, order Order, class T,
  std::size_t n_bits, align A>
std::basic_istream<charT, traits>& operator>>(std::basic_istream<charT, traits>& is,
  endian_buffer<Order, T, n_bits, Align>& x);

Effects: As if:

T i;
if (is >> i)
  x = i;

Returns: is.

FAQ

See the Endian home page FAQ for a library-wide FAQ.

Why not just use Boost.Serialization? Serialization involves a conversion for every object involved in I/O. Endian integers require no conversion or copying. They are already in the desired format for binary I/O. Thus they can be read or written in bulk.

Are endian types PODs? Yes for C++11. No for C++03, although several macros are available to force PODness in all cases.

What are the implications of endian integer types not being PODs with C++03 compilers? They can't be used in unions. Also, compilers aren't required to align or lay out storage in portable ways, although this potential problem hasn't prevented use of Boost.Endian with real compilers.

What good is native endianness? It provides alignment and size guarantees not available from the built-in types. It eases generic programming.

Why bother with the aligned endian types? Aligned integer operations may be faster (as much as 10 to 20 times faster) if the endianness and alignment of the type matches the endianness and alignment requirements of the machine. The code, however, is likely to be somewhat less portable than with the unaligned types.

Design considerations for Boost.Endian buffers

• Must be suitable for I/O - in other words, must be memcpyable.
• Must provide exactly the size and internal byte ordering specified.
• Must work correctly when the internal integer representation has more bits that the sum of the bits in the external byte representation. Sign extension must work correctly when the internal integer representation type has more bits than the sum of the bits in the external bytes. For example, using a 64-bit integer internally to represent 40-bit (5 byte) numbers must work for both positive and negative values.
• Must work correctly (including using the same defined external representation) regardless of whether a compiler treats char as signed or unsigned.
• Unaligned types must not cause compilers to insert padding bytes.
• The implementation should supply optimizations with great care. Experience has shown that optimizations of endian integers often become pessimizations when changing machines or compilers. Pessimizations can also happen when changing compiler switches, compiler versions, or CPU models of the same architecture.

C++11

The availability of the C++11 Defaulted Functions feature is detected automatically, and will be used if present to ensure that objects of class endian_buffer are trivial, and thus PODs.

Compilation

Boost.Endian is implemented entirely within headers, with no need to link to any Boost object libraries.

Several macros allow user control over features:

• BOOST_ENDIAN_NO_CTORS causes class endian_buffer to have no constructors. The intended use is for compiling user code that must be portable between compilers regardless of C++11 Defaulted Functions support. Use of constructors will always fail,
   
• BOOST_ENDIAN_FORCE_PODNESS causes BOOST_ENDIAN_NO_CTORS to be defined if the compiler does not support C++11 Defaulted Functions. This is ensures that objects of class endian_buffer are PODs, and so can be used in C++03 unions. In C++11, class endian_buffer objects are PODs, even though they have constructors, so can always be used in unions.

Last revised: 14 October, 2015

© Copyright Beman Dawes, 2006-2009, 2013

Distributed under the Boost Software License, Version 1.0. See www.boost.org/ LICENSE_1_0.txt