--- /dev/null
+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.
+
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