What's New In Libevent 2.0 so far:

1. Meta-issues

1.1. About this document

  This document describes the key differences between Libevent 1.4 and

  Libevent 2.0, from a user's point of view.  It was most recently

  updated based on features in git master as of August 2010.

  NOTE: I am very sure that I missed some thing on this list.  Caveat

  haxxor.

1.2. Better documentation

  There is now a book-in-progress that explains how to use Libevent and its

  growing pile of APIs.  As of this writing, it covers everything except the

  http and rpc code.  Check out the latest draft at

  http://www.wangafu.net/~nickm/libevent-book/ .

2. New and Improved Event APIs

  Many APIs are improved, refactored, or deprecated in Libevent 2.0.

  COMPATIBILITY:

  Nearly all existing code that worked with Libevent 1.4 should still

  work correctly with Libevent 2.0.  However, if you are writing new code,

  or if you want to port old code, we strongly recommend using the new APIs

  and avoiding deprecated APIs as much as possible.

  Binaries linked against Libevent 1.4 will need to be recompiled to link

  against Libevent 2.0.  This is nothing new; we have never been good at

  preserving binary compatibility between releases.  We'll try harder in the

  future, though: see 2.1 below.

2.1. New header layout for improved forward-compatibility

  Libevent 2.0 has a new header layout to make it easier for programmers to

  write good, well-supported libevent code.  The new headers are divided

  into three types.

  There are *regular headers*, like event2/event.h.  These headers contain

  the functions that most programmers will want to use.

  There are *backward compatibility headers*, like event2/event_compat.h.

  These headers contain declarations for deprecated functions from older

  versions of Libevent.  Documentation in these headers should suggest what's

  wrong with the old functions, and what functions you want to start using

  instead of the old ones.  Some of these functions might be removed in a

  future release.  New programs should generally not include these headers.

  Finally, there are *structure headers*, like event2/event_struct.h.

  These headers contain definitions of some structures that Libevent has

  historically exposed.  Exposing them caused problems in the past,

  since programs that were compiled to work with one version of Libevent

  would often stop working with another version that changed the size or

  layout of some object.  We've moving them into separate headers so

  that programmers can know that their code is not depending on any

  unstable aspect of the Libvent ABI.  New programs should generally not

  include these headers unless they really know what they are doing, are

  willing to rebuild their software whenever they want to link it

  against a new version of Libevent, and are willing to risk their code

  breaking if and when data structures change.

  Functionality that once was located in event.h is now more subdivided.

  The core event logic is now in event2/event.h.  The "evbuffer" functions

  for low-level buffer manipulation are in event2/buffer.h.  The

  "bufferevent" functions for higher-level buffered IO are in

  event2/bufferevent.h.

  COMPATIBILITY:

  All of the old headers (event.h, evdns.h, evhttp.h, evrpc.h, and

  evutil.h) will continue to work by including the corresponding new

  headers.  Old code should not be broken by this change.

2.2. New thread-safe, binary-compatible, harder-to-mess-up APIs

  Some aspects of the historical Libevent API have encouraged

  non-threadsafe code, or forced code built against one version of Libevent

  to no longer build with another.  The problems with now-deprecated APIs

  fell into two categories:

     1) Dependence on the "current" event_base.  In an application with

        multiple event_bases, Libevent previously had a notion of the

        "current" event_base.  New events were linked to this base, and

        the caller needed to explicitly reattach them to another base.

        This was horribly error-prone.

        Functions like "event_set" that worked with the "current" event_base

        are now deprecated but still available (see 2.1).  There are new

        functions like "event_assign" that take an explicit event_base

        argument when setting up a structure.  Using these functions will help

        prevent errors in your applications, and to be more threadsafe.

     2) Structure dependence.  Applications needed to allocate 'struct

        event' themselves, since there was no function in Libevent to do it

        for them.  But since the size and contents of struct event can

        change between libevent versions, this created binary-compatibility

        nightmares.  All structures of this kind are now isolated in

        _struct.h header (see 2.1), and there are new allocate-and-

        initialize functions you can use instead of the old initialize-only

        functions.  For example, instead of malloc and event_set, you

        can use event_new().

        (For people who do really want to allocate a struct event on the

        stack, or put one inside another structure, you can still use

        event2/event_compat.h.)

   So in the case where old code would look like this:

      #include <event.h>

      ...

      struct event *ev = malloc(sizeof(struct event));

      /* This call will cause a buffer overrun if you compile with one version

         of Libevent and link dynamically against another. */

      event_set(ev, fd, EV_READ, cb, NULL);

      /* If you forget this call, your code will break in hard-to-diagnose

         ways in the presence of multiple event bases. */

      event_set_base(ev, base);

   New code will look more like this:

     #include <event2/event.h>

     ...

     struct event *ev;

     ev = event_new(base, fd, EV_READ, cb, NULL);

2.3. Overrideable allocation functions

  If you want to override the allocation functions used by libevent

  (for example, to use a specialized allocator, or debug memory

  issues, or so on), you can replace them by calling

  event_set_mem_functions.  It takes replacements for malloc(),

  free(), and realloc().

  If you're going to use this facility, you need to call it _before_

  Libevent does any memory allocation; otherwise, Libevent may allocate some

  memory with malloc(), and free it with the free() function you provide.

  You can disable this feature when you are building Libevent by passing

  the --disable-malloc-replacement argument to configure.

2.4. Configurable event_base creation

  Older versions of Libevent would always got the fastest backend

  available, unless you reconfigured their behavior with the environment

  variables EVENT_NOSELECT, EVENT_NOPOLL, and so forth.  This was annoying

  to programmers who wanted to pick a backend explicitly without messing

  with the environment.

  Also, despite our best efforts, not every backend supports every

  operation we might like.  Some features (like edge-triggered events, or

  working with non-socket file descriptors) only work with some operating

  systems' fast backends.  Previously, programmers who cared about this

  needed to know which backends supported what.  This tended to get quite

  ungainly.

  There is now an API to choose backends, either by name or by feature.

  Here is an example:

      struct event_config_t *config;

      struct event_base *base;

      /* Create a new configuration object. */

      config = event_config_new();

      /* We don't want to use the "select" method. */

      event_config_avoid_method(config, "select");

      /* We want a method that can work with non-socket file descriptors */

      event_config_require_features(config, EV_FEATURE_FDS);

      base = event_base_new_with_config(config);

      if (!base) {

         /* There is no backend method that does what we want. */

         exit(1);

      }

      event_config_free(config);

  Supported features are documented in event2/event.h

2.5. Socket is now an abstract type

  All APIs that formerly accepted int as a socket type now accept

  "evutil_socket_t".  On Unix, this is just an alias for "int" as

  before.  On Windows, however, it's an alias for SOCKET, which can

  be wider than int on 64-bit platforms.

2.6. Timeouts and persistent events work together.

  Previously, it wasn't useful to set a timeout on a persistent event:

  the timeout would trigger once, and never again.  This is not what

  applications tend to want.  Instead, applications tend to want every

  triggering of the event to re-set the timeout.  So now, if you set

  up an event like this:

       struct event *ev;

       struct timeval tv;

       ev = event_new(base, fd, EV_READ|EV_PERSIST, cb, NULL);

       tv.tv_sec = 1;

       tv.tv_usec = 0;

       event_add(ev, &tv;;

  The callback 'cb' will be invoked whenever fd is ready to read, OR whenever

  a second has passed since the last invocation of cb.

2.7. Multiple events allowed per fd

  Older versions of Libevent allowed at most one EV_READ event and at most

  one EV_WRITE event per socket, per event base.  This restriction is no

  longer present.

2.8. evthread_* functions for thread-safe structures.

  Libevent structures can now be built with locking support.  This code

  makes it safe to add, remove, and activate events on an event base from a

  different thread.  (Previously, if you wanted to write multithreaded code

  with Libevent, you could only an event_base or its events in one thread at

  a time.)

  If you want threading support and you're using pthreads, you can just

  call evthread_use_pthreads().  (You'll need to link against the

  libevent_pthreads library in addition to libevent_core.  These functions are

  not in libevent_core.)

  If you want threading support and you're using Windows, you can just

  call evthread_use_windows_threads().

  If you are using some locking system besides Windows and pthreads, You

  can enable this on a per-event-base level by writing functions to

  implement mutexes, conditions, and thread IDs, and passing them to

  evthread_set_lock_callbacks and related functions in event2/thread.h.

  Once locking functions are enabled, every new event_base is created with a

  lock.  You can prevent a single event_base from being built with a lock

  disabled by using the EVENT_BASE_FLAG_NOLOCK flag in its

  event_config.  If an event_base is created with a lock, it is safe to call

  event_del, event_add, and event_active on its events from any thread.  The

  event callbacks themselves are still all executed from the thread running

  the event loop.

  To make an evbuffer or a bufferevent object threadsafe, call its

  *_enable_locking() function.

  The HTTP api is not currently threadsafe.

  To build Libevent with threading support disabled, pass

  --disable-thread-support to the configure script.

2.9. Edge-triggered events on some backends.

  With some backends, it's now possible to add the EV_ET flag to an event

  in order to request that the event's semantics be edge-triggered.  Right

  now, epoll and kqueue support this.

  The corresponding event_config feature is EV_FEATURE_ET; see 2.4 for more

  information.

2.10. Better support for huge numbers of timeouts

  The heap-based priority queue timer implementation for Libevent 1.4 is good

  for randomly distributed timeouts, but suboptimal if you have huge numbers

  of timeouts that all expire in the same amount of time after their

  creation.  The new event_base_init_common_timeout() logic lets you signal

  that a given timeout interval will be very common, and should use a linked

  list implementation instead of a priority queue.

2.11. Improved debugging support

  It's been pretty easy to forget to delete all your events before you

  re-initialize them, or otherwise put Libevent in an internally inconsistent

  state.  You can tell libevent to catch these and other common errors with

  the new event_enable_debug_mode() call.  Just invoke it before you do

  any calls to other libevent functions, and it'll catch many common

  event-level errors in your code.

2.12. Functions to access all event fields

  So that you don't have to access the struct event fields directly, Libevent

  now provides accessor functions to retrieve everything from an event that

  you set during event_new() or event_assign().

3. Backend-specific and performance improvements.

3.1. Change-minimization on O(1) backends

  With previous versions of Libevent, if you called event_del() and

  event_add() repeatedly on a single event between trips to the backend's

  dispatch function, the backend might wind up making unnecessary calls or

  passing unnecessary data to the kernel.  The new backend logic batches up

  redundant adds and deletes, and performs no more operations than necessary

  at the kernel level.

  This logic is on for the kqueue backend, and available (but off by

  default) for the epoll backend.  To turn it on for the epoll backend,

  set the EVENT_BASE_FLAG_EPOLL_USE_CHANGELIST flag in the

  event_base_cofig, or set the EVENT_EPOLL_USE_CHANGELIST environment

  variable.  Doing this with epoll may result in weird bugs if you give

  any fds closed by dup() or its variants.

3.2. Improved notification on Linux

  When we need to wake the event loop up from another thread, we use

  an epollfd to do so, instead of a socketpair.  This is supposed to be

  faster.

3.3. Windows: better support for everything

  Bufferevents on Windows can use a new mechanism (off-by-default; see below)

  to send their data via Windows overlapped IO and get their notifications

  via the IOCP API.  This should be much faster than using event-based

  notification.

  Other functions throughout the code have been fixed to work more

  consistently with Windows.  Libevent now builds on Windows using either

  mingw, or using MSVC (with nmake).  Libevent works fine with UNICODE

  defined, or not.

  Data structures are a little smarter: our lookups from socket to pending

  event are now done with O(1) hash tables rather than O(lg n) red-black

  trees.

  Unfortunately, the main Windows backend is still select()-based: from

  testing the IOCP backends on the mailing list, it seems that there isn't

  actually a way to tell for certain whether a socket is writable with IOCP.

  Libevent 2.1 may add a multithreaded WaitForMultipleEvents-based

  backend for better performance with many inactive sockets and better

  integration with Windows events.

4. Improvements to evbuffers

  Libevent has long had an "evbuffer" implementation to wrap access to an

  input or output memory buffer.  In previous versions, the implementation

  was very inefficient and lacked some desirable features.  We've made many

  improvements in Libevent 2.0.

4.1. Chunked-memory internal representation

  Previously, each evbuffer was a huge chunk of memory.  When we ran out of

  space in an evbuffer, we used realloc() to grow the chunk of memory.  When

  data was misaligned, we used memmove to move the data back to the front

  of the buffer.

  Needless to say, this is a terrible interface for networked IO.

  Now, evbuffers are implemented as a linked list of memory chunks, like

  most Unix kernels use for network IO.  (See Linux's skbuf interfaces,

  or *BSD's mbufs).  Data is added at the end of the linked list and

  removed from the front, so that we don't ever need realloc huge chunks

  or memmove the whole buffer contents.

  To avoid excessive calls to read and write, we use the readv/writev

  interfaces (or WSASend/WSARecv on Windows) to do IO on multiple chunks at

  once with a single system call.

  COMPATIBILITY NOTE:

  The evbuffer struct is no longer exposed in a header.  The code here is

  too volatile to expose an official evbuffer structure, and there was never

  any means provided to create an evbuffer except via evbuffer_new which

  heap-allocated the buffer.

  If you need access to the whole bufer as a linear chunk of memory, the

  EVBUFFER_DATA() function still works.  Watch out, though: it needs to copy

  the buffer's contents in a linear chunk before you can use it.

4.2. More flexible readline support

  The old evbuffer_readline() function (which accepted any sequence of

  CR and LF characters as a newline, and which couldn't handle lines

  containing NUL characters), is now deprecated.  The preferred

  function is evbuffer_readln(), which supports a variety of

  line-ending styles, and which can return the number of characters in

  the line returned.

  You can also call evbuffer_search_eol() to find the end of a line

  in an evbuffer without ever extracting the line.

4.3. Support for file-based IO in evbuffers.

  You can now add chunks of a file into a evbuffer, and Libevent will have

  your OS use mapped-memory functionality, sendfile, or splice to transfer

  the data without ever copying it to userspace.  On OSs where this is not

  supported, Libevent just loads the data.

  There are probably some bugs remaining in this code.  On some platforms

  (like Windows), it just reads the relevant parts of the file into RAM.

4.4. Support for zero-copy ("scatter/gather") writes in evbuffers.

  You can add a piece of memory to an evbuffer without copying it.

  Instead, Libevent adds a new element to the evbuffer's linked list of

  chunks with a pointer to the memory you supplied.  You can do this

  either with a reference-counted chunk (via evbuffer_add_reference), or

  by asking Libevent for a pointer to its internal vectors (via

  evbuffer_reserve_space or evbuffer_peek()).

4.5. Multiple callbacks per evbuffer

  Previously, you could only have one callback active on an evbuffer at a

  time.  In practice, this meant that if one part of Libevent was using an

  evbuffer callback to notice when an internal evbuffer was reading or

  writing data, you couldn't have your own callback on that evbuffer.

  Now, you can now use the evbuffer_add_cb() function to add a callback that

  does not interfere with any other callbacks.

  The evbuffer_setcb() function is now deprecated.

4.6. New callback interface

  Previously, evbuffer callbacks were invoked with the old size of the

  buffer and the new size of the buffer.  This interface could not capture

  operations that simultaneously filled _and_ drained a buffer, or handle

  cases where we needed to postpone callbacks until multiple operations were

  complete.

  Callbacks that are set with evbuffer_setcb still use the old API.

  Callbacks added with evbuffer_add_cb() use a new interface that takes a

  pointer to a struct holding the total number of bytes drained read and the

  total number of bytes written.  See event2/buffer.h for full details.

4.7. Misc new evbuffer features

   You can use evbuffer_remove() to move a given number of bytes from one

   buffer to another.

   The evbuffer_search() function lets you search for repeated instances of

   a pattern inside an evbuffer.

   You can use evbuffer_freeze() to temporarily suspend drains from or adds

   to a given evbuffer.  This is useful for code that exposes an evbuffer as

   part of its public API, but wants users to treat it as a pure source or

   sink.

   There's an evbuffer_copyout() that looks at the data at the start of an

   evbuffer without doing a drain.

   You can have an evbuffer defer all of its callbacks, so that rather than

   being invoked immediately when the evbuffer's length changes, they are

   invoked from within the event_loop.  This is useful when you have a

   complex set of callbacks that can change the length of other evbuffers,

   and you want to avoid having them recurse and overflow your stack.

5. Bufferevents improvements

   Libevent has long included a "bufferevents" structure and related

   functions that were useful for generic buffered IO on a TCP connection.

   This is what Libevent uses for its HTTP implementation.  In addition to

   the improvements that they get for free from the underlying evbuffer

   implementation above, there are many new features in Libevent 2.0's

   evbuffers.

5.1. New OO implementations

   The "bufferevent" structure is now an abstract base type with multiple

   implementations.  This should not break existing code, which always

   allocated bufferevents with bufferevent_new().

   Current implementations of the bufferevent interface are described below.

5.2. bufferevent_socket_new() replaces bufferevent_new()

   Since bufferevents that use a socket are not the only kind,

   bufferevent_new() is now deprecated.  Use bufferevent_socket_new()

   instead.

5.3. Filtered bufferevent IO

   You can use bufferevent_filter_new() to create a bufferevent that wraps

   around another bufferevent and transforms data it is sending and

   receiving.  See test/regress_zlib.c for a toy example that uses zlib to

   compress data before sending it over a bufferevent.

5.3. Linked pairs of bufferevents

   You can use bufferevent_pair_new() to produce two linked

   bufferevents.  This is like using socketpair, but doesn't require

   system-calls.

5.4. SSL support for bufferevents with OpenSSL

   There is now a bufferevent type that supports SSL/TLS using the

   OpenSSL library.  The code for this is build in a separate

   library, libevent_openssl, so that your programs don't need to

   link against OpenSSL unless they actually want SSL support.

   There are two ways to construct one of these bufferevents, both

   declared in <event2/bufferevent_ssl.h>.  If you want to wrap an

   SSL layer around an existing bufferevent, you would call the

   bufferevent_openssl_filter_new() function.  If you want to do SSL

   on a socket directly, call bufferevent_openssl_socket_new().

5.5. IOCP support for bufferevents on Windows

   There is now a bufferevents backend that supports IOCP on Windows.

   Supposedly, this will eventually make Windows IO much faster for

   programs using bufferevents.  We'll have to see; the code is not

   currently optimized at all.  To try it out, call the

   event_base_start_iocp() method on an event_base before contructing

   bufferevents.

   This is tricky code; there are probably some bugs hiding here.

5.6. Improved connect support for bufferevents.

   You can now create a bufferevent that is not yet connected to any

   host, and tell it to connect, either by address or by hostname.

   The functions to do this are bufferevent_socket_connect and

   bufferevent_socket_connect_hostname.

5.7. Rate-limiting for bufferevents

   If you need to limit the number of bytes read/written by a single

   bufferevent, or by a group of them, you can do this with a new set of

   bufferevent rate-limiting calls.

6. Other improvements

6.1. DNS improvements

6.1.1. DNS: IPv6 nameservers

   The evdns code now lets you have nameservers whose addresses are IPv6.

6.1.2. DNS: Better security

   Libevent 2.0 tries harder to resist DNS answer-sniping attacks than

   earlier versions of evdns.  See comments in the code for full details.

   Notably, evdns now supports the "0x20 hack" to make it harder to

   impersonate a DNS server.  Additionally, Libevent now uses a strong

   internal RNG to generate DNS transaction IDs, so you don't need to supply

   your own.

6.1.3. DNS: Getaddrinfo support

   There's now an asynchronous getaddrinfo clone, evdns_getaddrinfo(),

   to make the results of the evdns functions more usable.  It doesn't

   support every feature of a typical platform getaddrinfo() yet, but it

   is quite close.

   There is also a blocking evutil_getaddrinfo() declared in

   event2/util.h, to provide a getaddrinfo() implementation for

   platforms that don't have one, and smooth over the differences in

   various platforms implementations of RFC3493.

   Bufferevents provide bufferevent_connect_hostname(), which combines

   the name lookup and connect operations.

6.1.4. DNS: No more evdns globals

   Like an event base, evdns operations are now supposed to use an evdns_base

   argument.  This makes them easier to wrap for other (more OO) languages,

   and easier to control the lifetime of.  The old evdns functions will

   still, of course, continue working.

6.2. Listener support

   You can now more easily automate setting up a bound socket to listen for

   TCP connections.  Just use the evconnlistener_*() functions in the

   event2/listener.h header.

   The listener code supports IOCP on Windows if available.

6.3. Secure RNG support

   Network code very frequently needs a secure, hard-to-predict random number

   generator.  Some operating systems provide a good C implementation of one;

   others do not.  Libevent 2.0 now provides a consistent implementation

   based on the arc4random code originally from OpenBSD.  Libevent (and you)

   can use the evutil_secure_rng_*() functions to access a fairly secure

   random stream of bytes.

6.4. HTTP

   The evhttp uriencoding and uridecoding APIs have updated versions

   that behave more correctly, and can handle strings with internal NULs.

   The evhttp query parsing and URI parsing logic can now detect errors

   more usefully.  Moreover, we include an actual URI parsing function

   (evhttp_uri_parse()) to correctly parse URIs, so as to discourage

   people from rolling their own ad-hoc parsing functions.

   There are now accessor functions for the useful fields of struct http

   and friends; it shouldn't be necessary to access them directly any

   more.

   Libevent now lets you declare support for all specified HTTP methods,

   including OPTIONS, PATCH, and so on.  The default list is unchanged.

   Numerous evhttp bugs also got fixed.

7. Infrastructure improvements

7.1. Better unit test framework

   We now use a unit test framework that Nick wrote called "tinytest".

   The main benefit from Libevent's point of view is that tests which

   might mess with global state can all run each in their own

   subprocess.  This way, when there's a bug that makes one unit test

   crash or mess up global state, it doesn't affect any others.

7.2. Better unit tests

   Despite all the code we've added, our unit tests are much better than

   before.  Right now, iterating over the different backends on various

   platforms, I'm getting between 78% and 81% test coverage, compared

   with less than 45% test coverage in Libevent 1.4.