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## Revision History

#### Asio 1.6.1 / Boost 1.48

- 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` (#5688).
- Fixed a Windows-specific compile error by explicitly specifying the `signal()` function from the global namespace (#5722).
- 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 (#5854).
- Fixed the behaviour of the synchronous `null_buffers` operations so that they obey the user's non-blocking setting (#5756).
- 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 due that occurs when using the Intel C++ compiler (#5763).
- 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 (#5999).
- Fixed an exception safety issue in `epoll_reactor` initialisation (#6006).
- 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.

#### Asio 1.6.0 / Boost 1.47

- 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 be registered with multiple `signal_set` objects, however the signal number must be used only with Asio. Addresses #2879.
- 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).
- 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, and are useful for creating TCP clients that work with both IPv4 and IPv6.
- 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 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`. Addresses #3702, #3958.
- Changed the separate compilation support such that, to use Asio's SSL capabilities, you should also include `boost/asio/ssl/impl/src.hpp` in one source file in your program.
- Changed the SSL implementation to support build environments where SSL v2 is explicitly disabled (#5453).
- Made the `is_loopback()`, `is_unspecified()` and `is_multicast()` functions consistently available across the `ip::address`, `ip::address_v4` and `ip::address_v6` classes (#3939).
- 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 (#3297).
- Added a `release()` member function to posix descriptors. This function releases ownership of the underlying native descriptor to the caller. Addresses #3900.
- 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()`).
- 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 support for C++0x move construction and assignment to sockets, serial ports, POSIX descriptors and Windows handles.
- Reduced the copying of handler function objects.
- Added support for C++0x move construction to further reduce (and in some cases eliminate) copying of handler objects.
- 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. Addresses #3238, #4162.
- 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. Addresses #3307.
- Added new `asio::buffer()` overloads for `std::array`, when available.
- Changed the implementation to use the C++0x standard library templates `array`, `shared_ptr`, `weak_ptr` and `atomic` when they are available, rather than the Boost equivalents.
- Use C++0x variadic templates when available, rather than generating function overloads using Boost.Preprocessor.
- Changed exception reporting to include the function name in exception `what()` messages.
- Fixed insufficient initialisers warning with MinGW.
- Changed the `shutdown_service()` member functions to be private.
- Added archetypes for testing socket option functions.
- 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.
- Removed the dependency on OS-provided macros for the well-known IPv4 and IPv6 addresses. This should eliminate annoying "missing braces around initializer" warnings (#3741).
- 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 (#4971).
- 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.
- Modified the `buffers_iterator<>` and `ip::basic_resolver_iterator` classes so that the value_type typedefs are non-const byte types.
- Fixed warnings reported by g++'s `-Wshadow` compiler option (#3905).
- Added an explicit cast to convert the `FIONBIO` constant to int, to suppress a compiler warning on some platforms (#5128).
- Changed most examples to treat a failure by an accept operation as non-fatal (#5124).
- 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 (#5418).

#### Asio 1.4.9 / Boost 1.46.1

- `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 (#5021).
- Fixed compile failures with some versions of `g++` due to the use of anonymous enums (#4883).
- Fixed a bug on kqueue-based platforms, where some system calls that repeatedly fail with `EWOULDBLOCK` are not correctly re-registered with kqueue.
- Changed `asio::streambuf` to ensure that its internal pointers are updated correctly after the data has been modified using `std::streambuf` member functions.
- Fixed a bug that prevented the linger socket option from working on platforms other than Windows.

#### Asio 1.4.8 / Boost 1.46

- 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 (#5045).

#### Asio 1.4.7 / Boost 1.45

- Fixed a problem on kqueue-based platforms where a `deadline_timer` may never fire if the `io_service` is running in a background thread (#4568).
- Fixed a const-correctness issue that prevented valid uses of `has_service<>` from compiling (#4638).
- Fixed MinGW cross-compilation (#4491).
- Removed dependency on deprecated Boost.System functions (#4672).
- Ensured `close()`/`closesocket()` failures are correctly propagated (#4573).
- Added a check for errors returned by `InitializeCriticalSectionAndSpinCount` (#4574).
- Added support for hardware flow control on QNX (#4625).
- Always use `pselect()` on HP-UX, if it is available (#4578).
- Ensured handler arguments are passed as lvalues (#4744).
- Fixed Windows build when thread support is disabled (#4680).
- Fixed a Windows-specific problem where `deadline_timer` objects with expiry times set more than 5 minutes in the future may never expire (#4745).
- Fixed the `resolver` backend on BSD platforms so that an empty service name resolves to port number `0`, as per the documentation (#4690).
- Fixed read operations so that they do not accept buffer sequences of type `const_buffers_1` (#4746).
- Redefined `Protocol` and `id` to avoid clashing with Objective-C++ keywords (#4191).
- Fixed a `vector` reallocation performance issue that can occur when there are many active `deadline_timer` objects (#4780).
- Fixed the kqueue backend so that it compiles on NetBSD (#4662).
- Fixed the socket `io_control()` implementation on 64-bit Mac OS X and BSD platforms (#4782).
- Fixed a Windows-specific problem where failures from `accept()` are incorrectly treated as successes (#4859).
- Deprecated the separate compilation header `<boost/asio/impl/src.cpp>` in favour of `<boost/asio/impl/src.hpp>` (#4560).

#### Asio 1.4.6 / Boost 1.44

- 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, add `#include <boost/asio/impl/src.cpp>` to 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` (#4170).
- Ensured `unsigned char` is used with `isdigit` in `getaddrinfo` emulation (#4201).
- Fixed handling of very small but non-zero timeouts (#4205).
- 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` (#4382).
- Implemented cancellation of `null_buffers` operations on Windows.

#### Asio 1.4.5 / Boost 1.43

- 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 (#3923).
- 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 (#3901).
- Fixed a bug where 0-byte asynchronous reads were incorrectly passing an `error::eof` result to the completion handler (#4023).
- Changed the `io_control()` member functions to always call `ioctl` on the underlying descriptor when modifying blocking mode (#3307).
- Changed the resolver implementation to longer require 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.

#### Asio 1.4.4 / Boost 1.42

- 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 (#2977).
- Restricted MSVC buffer debugging workaround to 2008, as it causes a crash with 2010 beta 2 (#3796, #3822).
- 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 (#3626).
- Added a `boost_` prefix to the `extern "C"` thread entry point function (#3809).
- 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 (#3670).
- Added some support for platforms that use older compilers such as g++ 2.95 (#3743).

#### Asio 1.4.3 / Boost 1.40

- 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 (#2754).
- Made Asio compatible with the OpenSSL 1.0 beta (#3256).
- 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 (#3095).
- Ensured correct propagation of the error code when a synchronous accept fails (#3216).
- 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 (#3098).
- Fixed custom memory allocation support for timers (#3107).
- Tidied up various warnings reported by g++ (#1341, #2618).
- 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 (#3157, #2620).

#### Asio 1.4.2 / Boost 1.39

- 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 (#2682).
- Fix a memory leak that occurred when an asynchronous SSL operation's completion handler threw an exception (#2910).
- Fix the implementation of `io_control()` so that it adheres to the documented type requirements for IoControlCommand (#2820).
- Fix incompatibility between Asio and ncurses.h (#2156).
- 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 (#2936).
- Fix system call wrappers to always clear the error on success, as POSIX allows successful system calls to modify errno (#2953).
- Don't include termios.h if `BOOST_ASIO_DISABLE_SERIAL_PORT` is defined (#2917).
- Cleaned up some more MSVC level 4 warnings (#2828).
- Various documentation fixes (#2871).

#### Asio 1.4.1 / Boost 1.38

- Improved compatibility with some Windows firewall software.
- Ensured arguments to `windows::overlapped_ptr::complete()` are correctly passed to the completion handler (#2614).
- Fixed a link problem and multicast failure on QNX (#2504, #2530).
- 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 (#2683).
- Various minor bug and documentation fixes (#2534, #2541, #2607, #2617, #2619).

#### Asio 1.4.0 / Boost 1.37

- 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.

#### Asio 1.2.0 / Boost 1.36

- 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.

#### Asio 1.0.0 / Boost 1.35

First release of Asio as part of Boost.