path: root/whatsnew-2.0.txt

What's New In Libevent 2.0 so far:

0. 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 subversion trunk as of 27 Dec 2007.

  NOTE 1: If any features or fixes get backported from trunk to 1.4,
  they should get moved from here into whatsnew-14.txt, since they
  will no longer be differences between 1.4 and this version.

2. New and Improved APIs

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

  All existing code that worked with should 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.

2.1. New header layout for improved 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
  functions you want to start using instead of the old ones. 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 of 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, and are
  willing to rebuild their software whenever they want to link it against a
  new version of libevent.

  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.

  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-compatibile 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 use 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" 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().


   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 stack 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().

2.X. 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);

2.4. 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.

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.X. kqueue event ordering consistency

2.X. 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.X. evthread_* functions for thread-safe structures.

  Libevent structures can now be built with locking support.  You can
  enable this on a per-event-base level by writing functions to implement
  mutexes and thread IDs, and passing them to evthread_set_locking_callback
  and evthread_set_id_callback.  This makes it safe to add, remove,
  and activate events on an event base from a different thread.

  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.)

  If you want threading support and you're using Windows, you can just
  call evthread_use_windows_threads().

2.X. bufferevent_setfd/cb

2.X. Bufferevent IO filters (????)

2.X. Edge-triggered events on some backends.

2.X. Multiple callbacks per evbuffer

3. Big bugfixes

3.X. Win32 bufferevents work

4. Big performance improvements

4.X. Faster windows backend(s)

4.X. Faster evbuffer implementation

4.X. Generic notify support

5. Extras improvements

5.X. DNS: IPv6 nameservers

5.X. DNS: 0x20 hack support

5.X. DNS: Better security.


6. Removed/Deprecated code and features