[/
 / Copyright (c) 2003-2017 Christopher M. Kohlhoff (chris at kohlhoff dot com)
 /
 / Distributed under the Boost Software License, Version 1.0. (See accompanying
 / file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 /]

[section:history Revision History]

[heading Asio 1.11.0]

* Implemented changes to substantially reflect the Networking Library Proposal
  ([@www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4370.html N4370]).

  * New `Executor` type requirements and classes to support an executor
    framework, including the `execution_context` base class, the
    `executor_work` class for tracking outstanding work, and the `executor`
    polymorphic wrapper. Free functions `dispatch()`, `post()` and `defer()`
    have been added and are used to submit function objects to executors.

  * Completion handlers now have an associated executor and associated
    allocator. The free function `wrap()` is used to associate an executor with
    a handler or other object. The handler hooks for allocation, invocation and
    continuation have been deprecated.

  * A `system_executor` class has been added as a default executor.

  * The `io_service` class is now derived from `execution_context` and
    implements the executor type requirements in its nested `executor_type`
    class. The member functions `dispatch()`, `post()`, `defer()` and `wrap()`
    have been deprecated. The `io_service::work` class has been deprecated.

  * The `io_service` member function `reset()` has been renamed to `restart()`.
    The old name is retained for backward compatibility but has been
    deprecated.

  * The `make_service<>()` function is now used to add a new service to an
    execution context such as an `io_service`. The `add_service()` function has
    been deprecated.

  * A new `strand<>` template has been added to allow strand functionality to
    be used with generic executor types.

  * I/O objects (such as sockets and timers) now provide access to their
    associated `io_service` via a `context()` member function. The
    `get_io_service()` member function is deprecated.

  * All asynchronous operations and executor operations now support move-only
    handlers. However, the deprecated `io_service::post()`,
    `io_service::dispatch()`, `io_service::strand::post()` and
    `io_service::strand::dispatch()` functions still require copyable handlers.

  * Waitable timer objects are now movable.

  * Waitable timers, socket iostreams and socket streambufs now provide an
    `expiry()` member function for obtaining the expiry time. The accessors
    `expires_at()` and `expires_after()` have been deprecated, though those
    names are retained for the mutating members.

  * The `std::packaged_task` class template is now supported as a completion
    handler. The initiating operation automatically returns the future
    associated with the task. The `package()` function has been added as a
    convenient factory for packaged tasks.

  * Sockets, socket acceptors and descriptors now provide `wait()` and
    `async_wait()` operations that may be used to wait for readiness. The
    `null_buffers` type has been deprecated.

  * The proposed error code enum classes are simulated using namespaces.
    Existing asio error codes now have a correspondence with the standard error
    conditions.

  * Conversion between IP address types, and conversion from string to address,
    is now supported via the `address_cast<>()`, `make_address()`,
    `make_address_v4()` and `make_address_v6()` free functions. The
    `from_string()`, `to_v4()`, `to_v6()` and `v4_mapped()` member functions
    have been deprecated.

  * A default-constructed `ip::address` now represents an invalid address value
    that is neither IPv4 nor IPv6.

  * New `buffer()` overloads that generate mutable buffers for non-const
    `string` objects.

  * Support for dynamic buffer sequences that automatically grow and shrink to
    accomodate data as it is read or written. This is a generic facility
    similar to the existing `asio::streambuf` class. This support includes:

    * New `dynamic_string_buffer` and `dynamic_vector_buffer` adapter classes
      that meet the `DynamicBufferSequence` type requirements.

    * New `dynamic_buffer()` factory functions for creating a dynamic buffer
      adapter for a `vector` or `string`.

    * New overloads for the `read()`, `async_read()`, `write()` and
      `async_write()`, `read_until()` and `async_read_until()` free functions
      that directly support dynamic buffer sequences.

  * Support for networks and address ranges. Thanks go to Oliver Kowalke for
    contributing to the design and providing the implementation on which this
    facility is based. The following new classes have been added:

    * `address_iterator_v4` for iterating across IPv4 addresses
    * `address_iterator_v6` for iterating across IPv6 addresses
    * `address_range_v4` to represent a range of IPv4 addresses
    * `address_range_v6` to represent a range of IPv6 addresses
    * `network_v4` for manipulating IPv4 CIDR addresses, e.g. 1.2.3.0/24
    * `network_v6` for manipulating IPv6 CIDR addresses, e.g. ffe0:/120

  * New convenience headers in [^<asio/ts/*.hpp>] that correspond to the headers
    in the proposal.

* Added a new, executor-aware `thread_pool` class.

* Changed `spawn()` to be executor-aware.

* Added a new `spawn()` overload that takes only a function object.

* Changed `spawn()` and `yield_context` to permit nested calls to the
  completion handler.

* Removed previously deprecated functions.

* Added options for disabling TLS v1.1 and v1.2.

* Changed the SSL wrapper to call the password callback when loading an
  in-memory key.

* Changed the tutorial to use `std::endl` to ensure output is flushed.

* Fixed false SSL error reports by ensuring that the SSL error queue is cleared
  prior to each operation.

* Fixed an `ssl::stream<>` bug that may result in spurious 'short read' errors.

* Enabled perfect forwarding for the first `ssl::stream<>` constructor argument.

* Added standalone Asio support for Clang when used with libstdc++ and C++11.

* Fixed an unsigned integer overflow reported by Clang's integer sanitizer.

* Added support for move-only return types when using a `yield_context` object
  with asynchronous operations.

* Ensured errors generated by Windows' `ConnectEx` function are mapped to their
  portable equivalents.

* Changed multicast test to treat certain `join_group` failures as non-fatal.

[heading Asio 1.10.5]

* Fixed the [^kqueue] reactor so that it works on FreeBSD.

* Fixed an issue in the [^kqueue] reactor which resulted in spinning when using
  serial ports on Mac OS.

* Fixed [^kqueue] reactor support for read-only file descriptors.

* Fixed a compile error when using the [^/dev/poll] reactor.

* Changed the Windows backend to use `WSASocketW`, as `WSASocketA` has been
  deprecated.

* Fixed some warnings reported by Visual C++ 2013.

* Fixed integer type used in the WinRT version of the byte-order conversion
  functions.

* Changed documentation to indicate that `use_future` and `spawn()` are not
  made available when including the `asio.hpp` convenience header.

* Explicitly marked `asio::strand` as deprecated. Use
  `asio::io_service::strand` instead.

[heading Asio 1.10.4]

* Stopped using certain Winsock functions that are marked as deprecated in the
  latest Visual C++ and Windows SDK.

* Fixed a shadow variable warning on Windows.

* Fixed a regression in the [^kqueue] backend that was introduced in Asio
  1.10.2.

* Added a workaround for building the unit tests with [^gcc] on AIX.

[heading Asio 1.10.3]

* Worked around a [^gcc] problem to do with anonymous enums.

* Reverted the Windows `HANDLE` backend change to ignore `ERROR_MORE_DATA`.
  Instead, the error will be propagated as with any other (i.e. in an
  `error_code` or thrown as a `system_error`), and the number of bytes
  transferred will be returned. For code that needs to handle partial messages,
  the `error_code` overload should be used.

* Fixed an off-by-one error in the `signal_set` implementation's signal
  number check.

* Changed the Windows IOCP backend to not assume that
  `SO_UPDATE_CONNECT_CONTEXT` is defined.

* Fixed a Windows-specific issue, introduced in Asio 1.10.2, by using
  `VerifyVersionInfo` rather than `GetVersionEx`, as `GetVersionEx` has been
  deprecated.

* Changed to use SSE2 intrinsics rather than inline assembly, to allow the
  Cray compiler to work.

[heading Asio 1.10.2]

* Fixed `asio::spawn()` to work correctly with new Boost.Coroutine interface.

* Ensured that incomplete `asio::spawn()` coroutines are correctly unwound when
  cleaned up by the `io_service` destructor.

* Fixed delegation of continuation hook for handlers produced by
  `io_service::wrap()` and `strand::wrap()`.

* Changed the Windows I/O completion port backend to use `ConnectEx`, if
  available, for connection-oriented IP sockets.

* Changed the `io_service` backend for non-Windows (and non-IOCP Windows)
  platforms to use a single condition variable per `io_service` instance.
  This addresses a potential race condition when `run_one()` is used from
  multiple threads.

* Prevented integer overflow when computing timeouts based on some
  `boost::chrono` and `std::chrono` clocks.

* Made further changes to `EV_CLEAR` handling in the kqueue backend, to address
  other cases where the `close()` system call may hang on Mac OS X.

* Fixed infinite recursion in implementation of
  `resolver_query_base::flags::operator~`.

* Made the `select` reactor more efficient on Windows for large numbers of
  sockets.

* Fixed a Windows-specific type-aliasing issue reported by [^gcc].

* Prevented execution of compile-time-only buffer test to avoid triggering an
  address sanitiser warning.

* Disabled the `GetQueuedCompletionStatus` timeout workaround on recent
  versions of Windows.

* Changed implementation for Windows Runtime to use `FormatMessageW` rather
  than `FormatMessageA`, as the Windows store does not permit the latter.

* Added support for string-based scope IDs when using link-local multicast
  addresses.

* Changed IPv6 multicast group join to use the address's scope ID as the
  interface, if an interface is not explicitly specified.

* Fixed multicast test failure on Mac OS X and the BSDs by using a link-local
  multicast address.

* Various minor documentation improvements.

[heading Asio 1.10.1]

* Implemented a limited port to Windows Runtime. This support requires that the
  language extensions be enabled. Due to the restricted facilities exposed by
  the Windows Runtime API, the port also comes with the following caveats:

  * The core facilities such as the `io_service`, `strand`, buffers, composed
    operations, timers, etc., should all work as normal.

  * For sockets, only client-side TCP is supported.

  * Explicit binding of a client-side TCP socket is not supported.

  * The `cancel()` function is not supported for sockets. Asynchronous
    operations may only be cancelled by closing the socket.

  * Operations that use `null_buffers` are not supported.

  * Only `tcp::no_delay` and `socket_base::keep_alive` options are supported.

  * Resolvers do not support service names, only numbers. I.e. you must
    use "80" rather than "http".

  * Most resolver query flags have no effect.

* Extended the ability to use Asio without Boost to include Microsoft Visual
  Studio 2012. When using a C++11 compiler, most of Asio may now be used
  without a dependency on Boost header files or libraries. To use Asio in this
  way, define `ASIO_STANDALONE` on your compiler command line or as part of the
  project options. This standalone configuration has been tested for the
  following platforms and compilers:

  * Microsoft Visual Studio 2012

  * Linux with g++ 4.7 or 4.8 (requires [^-std=c++11])

  * Mac OS X with clang++ / Xcode 4.6 (requires [^-std=c++11 -stdlib=libc++])

* Fixed a regression (introduced in 1.10.0) where, on some platforms, errors
  from `async_connect` were not correctly propagated through to the completion
  handler.

* Fixed a Windows-specific regression (introduced in 1.10.0) that occurs when
  multiple threads are running an `io_service`. When the bug occurs, the result
  of an asynchronous operation (error and bytes tranferred) is incorrectly
  discarded and zero values used instead. For TCP sockets this results in
  spurious end-of-file notifications.

* Fixed a bug in handler tracking, where it was not correctly printing out some
  handler IDs.

* Fixed the comparison used to test for successful synchronous accept
  operations so that it works correctly with unsigned socket descriptors.

* Ensured the signal number is correctly passed to the completion handler when
  starting an `async_wait` on a signal that is already raised.

* Suppressed a g++ 4.8+ warning about unused typedefs.

* Enabled the move optimisation for handlers that use the default invocation
  hook.

* Clarified that programs must not issue overlapping `async_write_at`
  operations.

* Changed the Windows `HANDLE` backend to treat `ERROR_MORE_DATA` as a
  non-fatal error when returned by `GetOverlappedResult` for a synchronous
  read.

* Visual C++ language extensions use `generic` as a keyword. Added a
  workaround that renames the namespace to `cpp_generic` when those language
  extensions are in effect.

* Fixed some asynchronous operations that missed out on getting `async_result`
  support in 1.10.0. In particular, the buffered stream templates have been
  updated so that they adhere to current handler patterns.

* Enabled move support for Microsoft Visual Studio 2012.

* Added `use_future` support for Microsoft Visual Studio 2012.

* Removed a use of `std::min` in the Windows IOCP backend to avoid a
  dependency on the `<algorithm>` header.

* Eliminated some unnecessary handler copies.

* Fixed support for older versions of OpenSSL that do not provide the
  `SSL_CTX_clear_options` function.

* Fixed various minor and cosmetic issues in code and documentation.

[heading Asio 1.10.0]

* Added new traits classes, `handler_type` and `async_result`, that allow the
  customisation of the return type of an initiating function.

* Added the `asio::spawn()` function, a high-level wrapper for running
  stackful coroutines, based on the Boost.Coroutine library. The `spawn()`
  function enables programs to implement asynchronous logic in a synchronous
  manner. For example: `size_t n = my_socket.async_read_some(my_buffer, yield);`.
  For further information, see [link asio.overview.core.spawn Stackful
  Coroutines].

* Added the `asio::use_future` special value, which provides first-class
  support for returning a C++11 `std::future` from an asynchronous
  operation's initiating function. For example:
  `future<size_t> = my_socket.async_read_some(my_buffer, asio::use_future);`.
  For further information, see [link asio.overview.cpp2011.futures C++
  2011 Support - Futures].

* Promoted the stackless coroutine class and macros to be part of Asio's
  documented interface, rather than part of the HTTP server 4 example.
  For further information, see [link asio.overview.core.coroutine
  Stackless Coroutines].

* Added a new handler hook called `asio_handler_is_continuation`.
  Asynchronous operations may represent a continuation of the asynchronous
  control flow associated with the current executing handler. The
  `asio_handler_is_continuation` hook can be customised to return `true` if
  this is the case, and Asio's implementation can use this knowledge to
  optimise scheduling of the new handler. To cover common cases, Asio
  customises the hook for strands, `spawn()` and composed asynchronous
  operations.

* Added four new generic protocol classes, `generic::datagram_protocol`,
  `generic::raw_protocol`, `generic::seq_packet_protocol` and
  `generic::stream_protocol`, which implement the `Protocol` type
  requirements, but allow the user to specify the address family (e.g.
  `AF_INET`) and protocol type (e.g. `IPPROTO_TCP`) at runtime.
  For further information, see [link
  asio.overview.networking.other_protocols Support for Other Protocols].

* Added C++11 move constructors that allow the conversion of a socket (or
  acceptor) into a more generic type. For example, an `ip::tcp::socket` can
  be converted into a `generic::stream_protocol::socket` via move
  construction.
  For further information, see [link
  asio.overview.networking.other_protocols Support for Other Protocols].

* Extended the `basic_socket_acceptor<>`'s `accept()` and `async_accept()`
  functions to allow a new connection to be accepted directly into a socket
  of a more generic type. For example, an `ip::tcp::acceptor` can be used to
  accept into a `generic::stream_protocol::socket` object.
  For further information, see [link
  asio.overview.networking.other_protocols Support for Other Protocols].

* Moved existing examples into a C++03-specific directory, and added a new
  directory for C++11-specific examples. A limited subset of the C++03
  examples have been converted to their C++11 equivalents.

* Add the ability to use Asio without Boost, for a limited set of platforms.
  When using a C++11 compiler, most of Asio may now be used without a
  dependency on Boost header files or libraries. To use Asio in this way,
  define `ASIO_STANDALONE` on your compiler command line or as part of the
  project options. This standalone configuration has currently been tested for
  the following platforms and compilers:

  * Linux with g++ 4.7 (requires [^-std=c++11])

  * Mac OS X with clang++ / Xcode 4.6 (requires [^-std=c++11 -stdlib=libc++])

* Various SSL enhancements. Thanks go to Nick Jones, on whose work these changes
  are based.

  * Added support for SSL handshakes with re-use of data already read from
    the wire. New overloads of the `ssl::stream<>` class's `handshake()` and
    `async_handshake()` functions have been added. These accept a
    `ConstBufferSequence` to be used as initial input to the ssl engine for
    the handshake procedure.

  * Added support for creation of TLSv1.1 and TLSv1.2 `ssl::context` objects.

  * Added a `set_verify_depth()` function to the `ssl::context` and
    `ssl::stream<>` classes.

  * Added the ability to load SSL certificate and key data from memory
    buffers. New functions, `add_certificate_authority()`,
    `use_certificate()`, `use_certificate_chain()`, `use_private_key()`,
    `use_rsa_private_key()` and `use_tmp_dh()`, have been added to the
    `ssl::context` class.

  * Changed `ssl::context` to automatically disable SSL compression by
    default. To enable, use the new `ssl::context::clear_options()` function,
    as in `my_context.clear_options(ssl::context::no_compression)`.

* Fixed a potential deadlock in `signal_set` implementation.

* Fixed an error in acceptor example in documentation.

* Fixed copy-paste errors in waitable timer documentation.

* Added assertions to satisfy some code analysis tools.

* Fixed a malformed `#warning` directive.

* Fixed a potential data race in the Linux `epoll` implementation.

* Fixed a Windows-specific bug, where certain operations might generate an
  `error_code` with an invalid (i.e. `NULL`) `error_category`.

* Fixed `basic_waitable_timer`'s underlying implementation so that it can
  handle any `time_point` value without overflowing the intermediate duration
  objects.

* Fixed a problem with lost thread wakeups that can occur when making
  concurrent calls to `run()` and `poll()` on the same `io_service` object.

* Fixed implementation of asynchronous connect operation so that it can cope
  with spurious readiness notifications from the reactor.

* Fixed a memory leak in the `ssl::rfc2818_verification` class.

* Added a mechanism for disabling automatic Winsock initialisation. See the
  header file [^asio/detail/winsock_init.hpp] for details.

[heading Asio 1.8.3]

* Fixed some 64-to-32-bit conversion warnings.

* Fixed various small errors in documentation and comments.

* Fixed an error in the example embedded in `basic_socket::get_option`'s
  documentation.

* Changed to use `long` rather than `int` for SSL_CTX options, to match OpenSSL.

* Changed to use `_snwprintf` to address a compile error due to the changed
  `swprintf` signature in recent versions of MinGW.

* Fixed a deadlock that can occur on Windows when shutting down a pool of
  `io_service` threads due to running out of work.

* Changed UNIX domain socket example to treat errors from `accept` as non-fatal.

* Added a small block recycling optimisation to improve default memory
  allocation behaviour.

[heading Asio 1.8.2]

* Fixed an incompatibility between `ip::tcp::iostream` and C++11.

* Decorated GCC attribute names with underscores to prevent interaction
  with user-defined macros.

* Added missing `#include <cctype>`, needed for some versions of MinGW.

* Changed to use [^gcc]'s atomic builtins on ARM CPUs, when available.

* Changed strand destruction to be a no-op, to allow strand objects to be
  destroyed after their associated `io_service` has been destroyed.

* Added support for some newer versions of glibc which provide the
  `epoll_create1()` function but always fail with ENOSYS.

* Changed the SSL implementation to throw an exception if SSL engine
  initialisation fails.

* Fixed another regression in `buffered_write_stream`.

* Implemented various minor performance improvements, primarily targeted at
  Linux x86 and x86-64 platforms.

[heading Asio 1.8.1]

* Changed the `epoll_reactor` backend to do lazy registration for `EPOLLOUT`
  events.

* Fixed the `epoll_reactor` handling of out-of-band data, which was broken by
  an incomplete fix in the last release.

* Changed Asio's SSL wrapper to respect OpenSSL's `OPENSSL_NO_ENGINE` feature
  test `#define`.

* Fixed `windows::object_handle` so that it works with Windows compilers that
  support C++11 move semantics (such as [^g++]).

* Improved the performance of strand rescheduling.

* Added support for [^g++] 4.7 when compiling in C++11 mode.

* Fixed a problem where `signal_set` handlers were not being delivered when
  the `io_service` was constructed with a `concurrency_hint` of 1.

[heading Asio 1.8.0]

* Added a new class template `basic_waitable_timer` based around the C++11 clock
  type requirements. It may be used with the clocks from the C++11 `<chrono>`
  library facility or, if those are not available, Boost.Chrono. The typedefs
  `high_resolution_timer`, `steady_timer` and `system_timer` may be used to
  create timer objects for the standard clock types.

* Added a new `windows::object_handle` class for performing waits on Windows
  kernel objects. Thanks go to Boris Schaeling for contributing substantially
  to the development of this feature.

* On Linux, `connect()` can return EAGAIN in certain circumstances. Remapped
  this to another error so that it doesn't look like a non-blocking operation.

* Fixed a compile error on NetBSD.

* Fixed deadlock on Mac OS X.

* Fixed a regression in `buffered_write_stream`.

* Fixed a non-paged pool "leak" on Windows when an `io_service` is repeatedly
  run without anything to do.

* Reverted earlier change to allow some speculative operations to be performed
  without holding the lock, as it introduced a race condition in some
  multithreaded scenarios.

* Fixed a bug where the second buffer in an array of two buffers may be ignored
  if the first buffer is empty.

[heading Asio 1.6.1]

* Implemented various performance improvements, including:

  * Using thread-local operation queues in single-threaded use cases (i.e. when
    `concurrency_hint` is 1) to eliminate a lock/unlock pair.

  * Allowing some `epoll_reactor` speculative operations to be performed
    without holding the lock.

  * Improving locality of reference by performing an `epoll_reactor`'s I/O
    operation immediately before the corresponding handler is called. This also
    improves scalability across CPUs when multiple threads are running the
    `io_service`.

  * Specialising asynchronous read and write operations for buffer sequences
    that are arrays (`boost::array` or `std::array`) of exactly two buffers.

* Fixed a compile error in the regex overload of `async_read_until`.

* Fixed a Windows-specific compile error by explicitly specifying the
  `signal()` function from the global namespace.

* Changed the `deadline_timer` implementation so that it does not read the
  clock unless the timer heap is non-empty.

* Changed the SSL stream's buffers' sizes so that they are large enough to hold
  a complete TLS record.

* Fixed the behaviour of the synchronous `null_buffers` operations so that they
  obey the user's non-blocking setting.

* Changed to set the size of the select `fd_set` at runtime when using Windows.

* Disabled an MSVC warning due to const qualifier being applied to function type.

* Fixed a crash that occurs when using the Intel C++ compiler.

* Changed the initialisation of the OpenSSL library so that it supports all
  available algorithms.

* Fixed the SSL error mapping used when the session is gracefully shut down.

* Added some latency test programs.

* Clarified that a read operation ends when the buffer is full.

* Fixed an exception safety issue in `epoll_reactor` initialisation.

* Made the number of strand implementations configurable by defining
  `(BOOST_)ASIO_STRAND_IMPLEMENTATIONS` to the desired number.

* Added support for a new `(BOOST_)ASIO_ENABLE_SEQUENTIAL_STRAND_ALLOCATION` flag
  which switches the allocation of strand implementations to use a round-robin
  approach rather than hashing.

* Fixed potential strand starvation issue that can occur when `strand.post()`
  is used.

[heading Asio 1.6.0]

* Improved support for C++0x move construction to further reduce copying of
  handler objects. In certain designs it is possible to eliminate virtually all
  copies. Move support is now enabled when compiling in [^-std=c++0x] mode on
  g++ 4.5 or higher.

* Added build support for platforms that don't provide either of `signal()` or
  `sigaction()`.

* Changed to use C++0x variadic templates when they are available, rather than
  generating function overloads using the Boost.Preprocessor library.

* Ensured the value of `errno` is preserved across the implementation's signal
  handler.

* On Windows, ensured the count of outstanding work is decremented for
  abandoned operations (i.e. operations that are being cleaned up within the
  `io_service` destructor).

* Fixed behaviour of zero-length reads and writes in the new SSL implementation.

* Added support for building with OpenSSL 1.0 when `OPENSSL_NO_SSL2` is defined.

* Changed most examples to treat a failure by an accept operation as non-fatal.

* Fixed an error in the [^tick_count_timer] example by making the duration type
  signed. Previously, a wait on an already-passed deadline would not return for
  a very long time.

[heading Asio 1.5.3]

* Added a new, completely rewritten SSL implementation. The new implementation
  compiles faster, shows substantially improved performance, and supports
  custom memory allocation and handler invocation. It includes new API features
  such as certificate verification callbacks and has improved error reporting.
  The new implementation is source-compatible with the old for most uses.
  However, if necessary, the old implementation may still be used by defining
  `(BOOST_)ASIO_ENABLE_OLD_SSL`.

* Added new `asio::buffer()` overloads for `std::array`, when available. The
  support is automatically enabled when compiling in [^-std=c++0x] mode on g++
  4.3 or higher, or when using MSVC 10. The support may be explicitly enabled
  by defining `(BOOST_)ASIO_HAS_STD_ARRAY`, or disabled by defining
  `(BOOST_)ASIO_DISABLE_STD_ARRAY`.

* Changed to use the C++0x standard library templates `array`, `shared_ptr`,
  `weak_ptr` and `atomic` when they are available, rather than the Boost
  equivalents.

* Support for `std::error_code` and `std::system_error` is no longer enabled by
  default for g++ 4.5, as that compiler's standard library does not implement
  `std::system_error::what()` correctly.

[heading Asio 1.5.2]

* Added support for C++0x move construction and assignment to sockets, serial
  ports, POSIX descriptors and Windows handles.

* Added support for the `fork()` system call. Programs that use `fork()` must
  call `io_service.notify_fork()` at the appropriate times. Two new examples
  have been added showing how to use this feature.

* Cleaned up the handling of errors reported by the `close()` system call. In
  particular, assume that most operating systems won't have `close()` fail with
  `EWOULDBLOCK`, but if it does then set the blocking mode and restart the call.
  If any other error occurs, assume the descriptor is closed.

* The kqueue flag `EV_ONESHOT` seems to cause problems on some versions of Mac
  OS X, with the `io_service` destructor getting stuck inside the `close()`
  system call. Changed the kqueue backend to use `EV_CLEAR` instead.

* Changed exception reporting to include the function name in exception `what()`
  messages.

* Fixed an insufficient initialisers warning with MinGW.

* Changed the `shutdown_service()` member functions to be private.

* Added archetypes for testing socket option functions.

* Added a missing lock in `signal_set_service::cancel()`.

* Fixed a copy/paste error in `SignalHandler` example.

* Added the inclusion of the signal header to `signal_set_service.hpp` so that
  constants like `NSIG` may be used.

* Changed the `signal_set_service` implementation so that it doesn't assume
  that `SIGRTMAX` is a compile-time constant.

* Changed the Boost.Asio examples so that they don't use Boost.Thread's
  convenience header. Use the header file that is specifically for the
  boost::thread class instead.

[heading Asio 1.5.1]

* Added support for signal handling, using a new class called `signal_set`.
  Programs may add one or more signals to the set, and then perform an
  `async_wait()` operation. The specified handler will be called when one of
  the signals occurs. The same signal number may registered with multiple
  `signal_set` objects, however the signal number must be used only with Asio.

* Added handler tracking, a new debugging aid. When enabled by defining
  `(BOOST_)ASIO_ENABLE_HANDLER_TRACKING`, Asio writes debugging output to the
  standard error stream. The output records asynchronous operations and the
  relationships between their handlers. It may be post-processed using the
  included [^handlerviz.pl] tool to create a visual representation of the
  handlers (requires GraphViz).

* Fixed a bug in `asio::streambuf` where the `consume()` function did not
  always update the internal buffer pointers correctly. The problem may occur
  when the `asio::streambuf` is filled with data using the standard C++ member
  functions such as `sputn()`. (Note: the problem does not manifest when the
  streambuf is populated by the Asio free functions `read()`,
  `async_read()`, `read_until()` or `async_read_until()`.)

* Fixed a bug on kqueue-based platforms, where reactor read operations that
  return false from their `perform()` function are not correctly re-registered
  with kqueue.

* Support for `std::error_code` and `std::system_error` is no longer enabled by
  default for MSVC10, as that compiler's standard library does not implement
  `std::system_error::what()` correctly.

* Modified the `buffers_iterator<>` and `ip::basic_resolver_iterator` classes
  so that the value_type typedefs are non-const byte types.

[heading Asio 1.5.0]

* Added support for timeouts on socket iostreams, such as `ip::tcp::iostream`.
  A timeout is set by calling `expires_at()` or `expires_from_now()` to
  establish a deadline. Any socket operations which occur past the deadline
  will put the iostream into a bad state.

* Added a new `error()` member function to socket iostreams, for retrieving the
  error code from the most recent system call.

* Added a new `basic_deadline_timer::cancel_one()` function. This function lets
  you cancel a single waiting handler on a timer. Handlers are cancelled in
  FIFO order.

* Added a new `transfer_exactly()` completion condition. This can be used to
  send or receive a specified number of bytes even if the total size of the
  buffer (or buffer sequence) is larger.

* Added new free functions `connect()` and `async_connect()`. These operations
  try each endpoint in a list until the socket is successfully connected.

* Extended the `buffer_size()` function so that it works for buffer sequences
  in addition to individual buffers.

* Added a new `buffer_copy()` function that can be used to copy the raw bytes
  between individual buffers and buffer sequences.

* Added new non-throwing overloads of `read()`, `read_at()`, `write()` and
  `write_at()` that do not require a completion condition.

* Added friendlier compiler errors for when a completion handler does not meet
  the necessary type requirements. When C++0x is available (currently supported
  for g++ 4.5 or later, and MSVC 10), `static_assert` is also used to generate
  an informative error message. This checking may be disabled by defining
  `(BOOST_)ASIO_DISABLE_HANDLER_TYPE_REQUIREMENTS`.

* Added support for using `std::error_code` and `std::system_error`, when
  available. The support is automatically enabled when compiling in
  [^-std=c++0x] mode on g++ 4.5 or higher, or when using MSVC 10. The support
  may be explicitly enabled by defining `ASIO_HAS_STD_SYSTEM_ERROR`, or
  disabled by defining `ASIO_DISABLE_STD_SYSTEM_ERROR`. (Available in non-Boost
  version of Asio only.)

* Made the `is_loopback()`, `is_unspecified()` and `is_multicast()` functions
  consistently available across the `ip::address`, `ip::address_v4` and
  `ip::address_v6` classes.

* Added new `non_blocking()` functions for managing the non-blocking behaviour
  of a socket or descriptor. The `io_control()` commands named `non_blocking_io`
  are now deprecated in favour of these new functions.

* Added new `native_non_blocking()` functions for managing the non-blocking
  mode of the underlying socket or descriptor. These functions are intended to
  allow the encapsulation of arbitrary non-blocking system calls as
  asynchronous operations, in a way that is transparent to the user of the
  socket object. The functions have no effect on the behaviour of the
  synchronous operations of the socket or descriptor.

* Added the `io_control()` member function for socket acceptors.

* For consistency with the C++0x standard library, deprecated the `native_type`
  typedefs in favour of `native_handle_type`, and the `native()` member
  functions in favour of `native_handle()`.

* Added a `release()` member function to posix descriptors. This function
  releases ownership of the underlying native descriptor to the caller.

* Added support for sequenced packet sockets (`SOCK_SEQPACKET`).

* Added a new `io_service::stopped()` function that can be used to determine
  whether the `io_service` has stopped (i.e. a `reset()` call is needed prior
  to any further calls to `run()`, `run_one()`, `poll()` or `poll_one()`).

* Reduced the copying of handler function objects.

* Added support for C++0x move construction to further reduce copying of
  handler objects. Move support is enabled when compiling in [^-std=c++0x] mode
  on g++ 4.5 or higher, or when using MSVC10.

* Removed the dependency on OS-provided macros for the well-known IPv4 and IPv6
  addresses. This should eliminate the annoying "missing braces around
  initializer" warnings.

* Reduced the size of `ip::basic_endpoint<>` objects (such as
  `ip::tcp::endpoint` and `ip::udp::endpoint`).

* Changed the reactor backends to assume that any descriptors or sockets added
  using `assign()` may have been `dup()`-ed, and so require explicit
  deregistration from the reactor.

* Changed the SSL error category to return error strings from the OpenSSL
  library.

* Changed the separate compilation support such that, to use Asio's SSL
  capabilities, you should also include 'asio/ssl/impl/src.hpp` in one source
  file in your program.

* Removed the deprecated member functions named `io_service()`. The
  `get_io_service()` member functions should be used instead.

* Removed the deprecated typedefs `resolver_query` and `resolver_iterator` from
  the `ip::tcp`, `ip::udp` and `ip::icmp` classes.

* Fixed a compile error on some versions of g++ due to anonymous enums.

* Added an explicit cast to the `FIONBIO` constant to int to suppress a compiler
  warning on some platforms.

* Fixed warnings reported by g++'s [^-Wshadow] compiler option.

[heading Asio 1.4.8]

* Fixed an integer overflow problem that occurs when
  `ip::address_v4::broadcast()` is used on 64-bit platforms.

* Fixed a problem on older Linux kernels (where epoll is used without `timerfd`
  support) that prevents timely delivery of `deadline_timer` handlers, after the
  program has been running for some time.

[heading Asio 1.4.7]

* Fixed a problem on kqueue-based platforms where a `deadline_timer` may
  never fire if the `io_service` is running in a background thread.

* Fixed a const-correctness issue that prevented valid uses of
  `has_service<>` from compiling.

* Fixed MinGW cross-compilation.

* Removed dependency on deprecated Boost.System functions (Boost.Asio only).

* Ensured `close()`/`closesocket()` failures are correctly propagated.

* Added a check for errors returned by `InitializeCriticalSectionAndSpinCount`.

* Added support for hardware flow control on QNX.

* Always use `pselect()` on HP-UX, if it is available.

* Ensured handler arguments are passed as lvalues.

* Fixed Windows build when thread support is disabled.

* Fixed a Windows-specific problem where `deadline_timer` objects with expiry
  times set more than 5 minutes in the future may never expire.

* Fixed the resolver backend on BSD platforms so that an empty service name
  resolves to port number `0`, as per the documentation.

* Fixed read operations so that they do not accept buffer sequences of type
  `const_buffers_1`.

* Redefined `Protocol` and `id` to avoid clashing with Objective-C++ keywords.

* Fixed a `vector` reallocation performance issue that can occur when there are
  many active `deadline_timer` objects.

* Fixed the kqueue backend so that it compiles on NetBSD.

* Fixed the socket `io_control()` implementation on 64-bit Mac OS X and BSD
  platforms.

* Fixed a Windows-specific problem where failures from `accept()` are
  incorrectly treated as successes.

* Deprecated the separate compilation header `asio/impl/src.cpp` in
  favour of `asio/impl/src.hpp`.

[heading Asio 1.4.6]

* Reduced compile times. (Note that some programs may need to add additional
  `#include`s, e.g. if the program uses boost::array but does not explicitly
  include `<boost/array.hpp>`.)

* Reduced the size of generated code.

* Refactored `deadline_timer` implementation to improve performance.

* Improved multiprocessor scalability on Windows by using a dedicated hidden
  thread to wait for timers.

* Improved performance of `asio::streambuf` with `async_read()` and
  `async_read_until()`. These read operations now use the existing capacity of
  the `streambuf` when reading, rather than limiting the read to 512 bytes.

* Added optional separate compilation. To enable, include
  `asio/impl/src.cpp` in one source file in a program, then
  build the program with `(BOOST_)ASIO_SEPARATE_COMPILATION` defined in the
  project\/compiler settings. Alternatively, `(BOOST_)ASIO_DYN_LINK` may be
  defined to build a separately-compiled Asio as part of a shared library.

* Added new macro `(BOOST_)ASIO_DISABLE_FENCED_BLOCK` to permit the disabling of
  memory fences around completion handlers, even if thread support is enabled.

* Reworked timeout examples to better illustrate typical use cases.

* Ensured that handler arguments are passed as const types.

* Fixed incorrect parameter order in `null_buffers` variant of `async_send_to`.

* Ensured unsigned char is used with `isdigit` in `getaddrinfo` emulation.

* Fixed handling of very small but non-zero timeouts.

* Fixed crash that occurred when an empty buffer sequence was passed to a
  composed read or write operation.

* Added missing operator+ overload in `buffers_iterator`.

* Implemented cancellation of `null_buffers` operations on Windows.

[heading Asio 1.4.5]

* Improved performance.

* Reduced compile times.

* Reduced the size of generated code.

* Extended the guarantee that background threads don't call user code to
  all asynchronous operations.

* Changed to use edge-triggered epoll on Linux.

* Changed to use `timerfd` for dispatching timers on Linux, when
  available.

* Changed to use one-shot notifications with kqueue on Mac OS X and BSD
  platforms.

* Added a bitmask type `ip::resolver_query_base::flags` as per the TR2
  proposal. This type prevents implicit conversion from `int` to
  `flags`, allowing the compiler to catch cases where users incorrectly
  pass a numeric port number as the service name.

* Added `#define NOMINMAX` for all Windows compilers. Users can define
  `(BOOST_)ASIO_NO_NOMINMAX` to suppress this definition.

* Fixed a bug where 0-byte asynchronous reads were incorrectly passing
  an `error::eof` result to the completion handler.

* Changed the `io_control()` member functions to always call `ioctl` on
  the underlying descriptor when modifying blocking mode.

* Changed the resolver implementation so that it no longer requires the
  typedefs `InternetProtocol::resolver_query` and
  `InternetProtocol::resolver_iterator`, as neither typedef is part of
  the documented `InternetProtocol` requirements. The corresponding
  typedefs in the `ip::tcp`, `ip::udp` and `ip::icmp` classes have been
  deprecated.

* Fixed out-of-band handling for reactors not based on `select()`.

* Added new `(BOOST_)ASIO_DISABLE_THREADS` macro that allows Asio's
  threading support to be independently disabled.

* Minor documentation improvements.

[heading Asio 1.4.4]

* Added a new HTTP Server 4 example illustrating the use of stackless
  coroutines with Asio.

* Changed handler allocation and invocation to use `boost::addressof` to
  get the address of handler objects, rather than applying `operator&`
  directly.

* Restricted MSVC buffer debugging workaround to 2008, as it causes a
  crash with 2010 beta 2.

* Fixed a problem with the lifetime of handler memory, where Windows
  needs the `OVERLAPPED` structure to be valid until both the initiating
  function call has returned and the completion packet has been
  delivered.

* Don't block signals while performing system calls, but instead restart
  the calls if they are interrupted.

* Documented the guarantee made by strand objects with respect to order
  of handler invocation.

* Changed strands to use a pool of implementations, to make copying of
  strands cheaper.

* Ensured that kqueue support is enabled for BSD platforms.

* Added a `boost_` prefix to the `extern "C"` thread entry point
  function.

* In `getaddrinfo` emulation, only check the socket type (`SOCK_STREAM`
  or `SOCK_DGRAM`) if a service name has been specified. This should
  allow the emulation to work with raw sockets.

* Added a workaround for some broken Windows firewalls that make a
  socket appear bound to 0.0.0.0 when it is in fact bound to 127.0.0.1.

* Applied a fix for reported excessive CPU usage under Solaris.

* Added some support for platforms that use older compilers such as g++
  2.95.

[heading Asio 1.4.3]

* Added a new ping example to illustrate the use of ICMP sockets.

* Changed the `buffered*_stream<>` templates to treat 0-byte reads and
  writes as no-ops, to comply with the documented type requirements for
  `SyncReadStream`, `AsyncReadStream`, `SyncWriteStream` and
  `AsyncWriteStream`.

* Changed some instances of the `throw` keyword to
  `boost::throw_exception()` to allow Asio to be used when exception
  support is disabled. Note that the SSL wrappers still require
  exception support.

* Made Asio compatible with the OpenSSL 1.0 beta.

* Eliminated a redundant system call in the Solaris /dev/poll backend.

* Fixed a bug in resizing of the bucket array in the internal hash maps.

* Ensured correct propagation of the error code when a synchronous
  accept fails.

* Ensured correct propagation of the error code when a synchronous read
  or write on a Windows `HANDLE` fails.

* Fixed failures reported when `_GLIBCXX_DEBUG` is defined.

* Fixed custom memory allocation support for timers.

* Tidied up various warnings reported by g++.

* Various documentation improvements, including more obvious hyperlinks
  to function overloads, header file information, examples for the
  handler type requirements, and adding enum values to the index.

[heading Asio 1.4.2]

* Implement automatic resizing of the bucket array in the internal hash
  maps. This is to improve performance for very large numbers of
  asynchronous operations and also to reduce memory usage for very small
  numbers. A new macro `(BOOST_)ASIO_HASH_MAP_BUCKETS` may be used to
  tweak the sizes used for the bucket arrays. (N.B. this feature
  introduced a bug which was fixed in Asio 1.4.3 / Boost 1.40.)

* Add performance optimisation for the Windows IOCP backend for when no
  timers are used.

* Prevent locale settings from affecting formatting of TCP and UDP
  endpoints.

* Fix a memory leak that occurred when an asynchronous SSL operation's
  completion handler threw an exception.

* Fix the implementation of `io_control()` so that it adheres to the
  documented type requirements for IoControlCommand.

* Fix incompatibility between Asio and ncurses.h.

* On Windows, specifically handle the case when an overlapped `ReadFile`
  call fails with `ERROR_MORE_DATA`. This enables a hack where a
  `windows::stream_handle` can be used with a message-oriented named
  pipe.

* Fix system call wrappers to always clear the error on success, as
  POSIX allows successful system calls to modify errno.

* Don't include termios.h if `(BOOST_)ASIO_DISABLE_SERIAL_PORT` is
  defined.

* Cleaned up some more MSVC level 4 warnings.

* Various documentation fixes.

[heading Asio 1.4.1]

* Improved compatibility with some Windows firewall software.

* Ensured arguments to `windows::overlapped_ptr::complete()` are correctly
  passed to the completion handler.

* Fixed a link problem and multicast failure on QNX.

* Fixed a compile error in SSL support on MinGW / g++ 3.4.5.

* Drop back to using a pipe for notification if eventfd is not available
  at runtime on Linux.

* Various minor bug and documentation fixes.

[heading Asio 1.4.0]

* Enhanced CompletionCondition concept with the signature
  `size_t CompletionCondition(error_code ec, size_t total)`, where the return
  value indicates the maximum number of bytes to be transferred on the next
  read or write operation. (The old CompletionCondition signature is still
  supported for backwards compatibility).

* New `windows::overlapped_ptr` class to allow arbitrary overlapped I/O
  functions (such as `TransmitFile`) to be used with Asio.

* On recent versions of Linux, an `eventfd` descriptor is now used (rather than
  a pipe) to interrupt a blocked select/epoll reactor.

* Added const overloads of `lowest_layer()`.

* Synchronous read, write, accept and connect operations are now thread safe
  (meaning that it is now permitted to perform concurrent synchronous
  operations on an individual socket, if supported by the OS).

* Reactor-based `io_service` implementations now use lazy initialisation to
  reduce the memory usage of an `io_service` object used only as a message
  queue.

[heading Asio 1.2.0]

* Added support for serial ports.

* Added support for UNIX domain sockets.

* Added support for raw sockets and ICMP.

* Added wrappers for POSIX stream-oriented file descriptors (excluding regular
  files).

* Added wrappers for Windows stream-oriented `HANDLE`s such as named pipes
  (requires `HANDLE`s that work with I/O completion ports).

* Added wrappers for Windows random-access `HANDLE`s such as files (requires
  `HANDLE`s that work with I/O completion ports).

* Added support for reactor-style operations (i.e. they report readiness but
  perform no I/O) using a new `null_buffers` type.

* Added an iterator type for bytewise traversal of buffer sequences.

* Added new `read_until()` and `async_read_until()` overloads that take a
  user-defined function object for locating message boundaries.

* Added an experimental two-lock queue (enabled by defining
  `(BOOST_)ASIO_ENABLE_TWO_LOCK_QUEUE`) that may provide better `io_service`
  scalability across many processors.

* Various fixes, performance improvements, and more complete coverage of the
  custom memory allocation support.

[heading Asio 1.0.0]

First stable release of Asio.

[endsect]
