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:language: C
Creating an event_base
Before you can use any interesting Libevent function, you need to allocate
one or more event_base structures. Each event_base structure holds a set of
events and can poll to determine which events are active.
If an event_base is set up to use locking, it is safe to access it between
multiple threads. Its loop can only be run in a single thread, however. If
you want to have multiple threads polling for IO, you need to have an
event_base for each thread.
TIP: [A future version of Libevent may have support for event_bases that run
events across multiple threads.]
Each event_base has a "method", or a backend that it uses to determine which
events are ready. The recognized methods are:
- select
- poll
- epoll
- kqueue
- devpoll
- evport
- win32
The user can disable specific backends with environment variables. If you
want to turn off the kqueue backend, set the EVENT_NOKQUEUE environment
variable, and so on. If you want to turn off backends from within the
program, see notes on event_config_avoid_method() below.
Setting up a default event_base
The event_base_new() function allocates and returns a new event base with
the default settings. It examines the environment variables and returns
a pointer to a new event_base. If there is an error, it returns NULL.
When choosing among methods, it picks the fastest method that the OS
struct event_base *event_base_new(void);
For most programs, this is all you need.
The event_base_new() function is declared in <event2/event.h>. It first
appeared in Libevent 1.4.3.
Setting up a complicated event_base
If you want more control over what kind of event_base you get, you need to
use an event_config. An event_config is an opaque structure that holds
information about your preferences for an event_base. When you want an
event_base, you pass the event_config to event_base_new_with_config().
struct event_config *event_config_new(void);
struct event_base *event_base_new_with_config(const struct event_config *cfg);
void event_config_free(struct event_config *cfg);
To allocate an event_base with these functions, you call event_config_new()
to allocate a new event_config. Then, you call other functions on the
event_config to tell it about your needs. Finally, you call
event_base_new_with_config() to get a new event_base. When you are done,
you can free the event_config with event_config_free().
int event_config_avoid_method(struct event_config *cfg, const char *method);
enum event_method_feature {
EV_FEATURE_O1 = 0x02,
int event_config_require_features(struct event_config *cfg,
enum event_method_feature feature);
enum event_base_config_flag {
int event_config_set_flag(struct event_config *cfg,
enum event_base_config_flag flag);
Calling event_config_avoid_method tells Libevent to avoid a specific
available backend by name. Calling event_config_require_feature() tells
Libevent not to use any backend that cannot supply all of a set of features.
Calling event_config_set_flag() tells Libevent to set one or more of
the run-time flags below when constructing the event base.
The recognized feature values for event_config_require_features are:
Requires a backend method that supports edge-triggered IO.
Requires a backend method where adding or deleting a single
event, or having a single event become active, is an O(1) operation.
Requires a backend method that can support arbitrary file
descriptor types, and not just sockets.
The recognized option values for event_config_set_flag() are:
Do not allocate locks for the event_base. Setting
this option may save a little time for locking and releasing the
event_base, but will make it unsafe and nonfunctional to access it
from multiple threads.
Do not check the EVENT_* environment
variables when picking which backend method to use. Think hard before
using this flag: it can make it harder for users to debug the interactions
between your program and Libevent.
On Windows only, this flag makes Libevent
enable any necessary IOCP dispatch logic on startup, rather than
Instead of checking the current time every
time the event loop is ready to run timeout callbacks, check it after
every timeout callback. This can use more CPU than you necessarily
intended, so watch out!
Tells Libevent that, if it decides to
use the epoll backend, it is safe to use the faster "changelist"-based
backend. The epoll-changelist backend can avoid needless system calls in
cases where the same fd has its status modified more than once between
calls to the backend's dispatch function, but it also trigger a kernel bug
that causes erroneous results if you give Libevent any fds cloned by
dup() or its variants. This flag has no effect if you use a backend
other than epoll. You can also turn on the epoll-changelist option by
setting the EVENT_EPOLL_USE_CHANGELIST environment variable.
By default, Libevent tries to use the fastest available timing mechanism
that the operating system provides. If there is a slower timing
mechanism that provides more fine-grained timing precision, this
flag tells Libevent to use that timing mechanism instead. If the
operating system provides no such slower-but-more-precise mechanism,
this flag has no effect.
The above functions that manipulate an event_config all return 0 on success,
-1 on failure.
NOTE: It is easy to set up an event_config that requires a backend that your
OS does not provide. For example, as of Libevent 2.0.1-alpha, there is no
O(1) backend for Windows, and no backend on Linux that provides both
EV_FEATURE_FDS and EV_FEATURE_O1. If you have made a configuration that
Libevent can't satisfy, event_base_new_with_config() will return NULL.
int event_config_set_num_cpus_hint(struct event_config *cfg, int cpus)
This function is currently only useful with Windows when using IOCP, though
it may become useful for other platforms in the future. Calling it tells the
event_config that the event_base it generates should try to make good use of
a given number of CPUs when multithreading. Note that this is only a hint:
the event base may wind up using more or fewer CPUs than you select.
int event_config_set_max_dispatch_interval(struct event_config *cfg,
const struct timeval *max_interval, int max_callbacks,
int min_priority);
This function prevents priority inversion by limiting how many low-priority
event callbacks can be invoked before checking for more high-priority events.
If max_interval is non-null, the event loop checks the time after each
callback, and re-scans for high-priority events if max_interval has passed.
If max_callbacks is nonnegative, the event loop also checks for more events
after max_callbacks callbacks have been invoked. These rules apply to any
event of min_priority or higher.
//BUILD: FUNCTIONBODY INC:event2/event.h
.Example: Preferring edge-triggered backends
struct event_config *cfg;
struct event_base *base;
int i;
/* My program wants to use edge-triggered events if at all possible. So
I'll try to get a base twice: Once insisting on edge-triggered IO, and
once not. */
for (i=0; i<2; ++i) {
cfg = event_config_new();
/* I don't like select. */
event_config_avoid_method(cfg, "select");
if (i == 0)
event_config_require_features(cfg, EV_FEATURE_ET);
base = event_base_new_with_config(cfg);
if (base)
/* If we get here, event_base_new_with_config() returned NULL. If
this is the first time around the loop, we'll try again without
setting EV_FEATURE_ET. If this is the second time around the
loop, we'll give up. */
//BUILD: FUNCTIONBODY INC:event2/event.h
.Example: Avoiding priority-inversion
struct event_config *cfg;
struct event_base *base;
cfg = event_config_new();
if (!cfg)
/* Handle error */;
/* I'm going to have events running at two priorities. I expect that
some of my priority-1 events are going to have pretty slow callbacks,
so I don't want more than 100 msec to elapse (or 5 callbacks) before
checking for priority-0 events. */
struct timeval msec_100 = { 0, 100*1000 };
event_config_set_max_dispatch_interval(cfg, &msec_100, 5, 1);
base = event_base_new_with_config(cfg);
if (!base)
/* Handle error */;
event_base_priority_init(base, 2);
These functions and types are declared in <event2/event.h>.
The EVENT_BASE_FLAG_IGNORE_ENV flag first appeared in Libevent 2.0.2-alpha.
The EVENT_BASE_FLAG_PRECISE_TIMER flag first appeared in Libevent
2.1.2-alpha. The event_config_set_num_cpus_hint() function was new in Libevent
2.0.7-rc, and event_config_set_max_dispatch_interval() was new in 2.1.1-alpha.
Everything else in this section first appeared in Libevent 2.0.1-alpha.
Examining an event_base's backend method
Sometimes you want to see which features are actually available in an
event_base, or which method it's using.
const char **event_get_supported_methods(void);
The event_get_supported_methods() function returns a pointer to an array of
the names of the methods supported in this version of Libevent. The
last element in the array is NULL.
//BUILD: FUNCTIONBODY INC:event2/event.h
int i;
const char **methods = event_get_supported_methods();
printf("Starting Libevent %s. Available methods are:\n",
for (i=0; methods[i] != NULL; ++i) {
printf(" %s\n", methods[i]);
NOTE: This function returns a list of the methods that Libevent was compiled
to support. It is possible that your operating system will not in fact
support them all when Libevent tries to run. For example, you could be on a
version of OSX where kqueue is too buggy to use.
const char *event_base_get_method(const struct event_base *base);
enum event_method_feature event_base_get_features(const struct event_base *base);
The event_base_get_method() call returns the name of the actual method in use
by an event_base. The event_base_get_features() call returns a bitmask of
the features that it supports.
//BUILD: FUNCTIONBODY INC:event2/event.h
struct event_base *base;
enum event_method_feature f;
base = event_base_new();
if (!base) {
puts("Couldn't get an event_base!");
} else {
printf("Using Libevent with backend method %s.",
f = event_base_get_features(base);
if ((f & EV_FEATURE_ET))
printf(" Edge-triggered events are supported.");
if ((f & EV_FEATURE_O1))
printf(" O(1) event notification is supported.");
if ((f & EV_FEATURE_FDS))
printf(" All FD types are supported.");
These functions are defined in <event2/event.h>. The event_base_get_method()
call was first available in Libevent 1.4.3. The others first appeared in
Libevent 2.0.1-alpha.
Deallocating an event_base
When you are finished with an event_base, you can deallocate it with
void event_base_free(struct event_base *base);
Note that this function does not deallocate any of the events that are
currently associated with the event_base, or close any of their sockets, or
free any of their pointers.
The event_base_free() function is defined in <event2/event.h>. It was first
implemented in Libevent 1.2.
Setting priorities on an event_base
Libevent supports setting multiple priorities on an event. By default,
though, an event_base supports only a single priority level. You can set the
number of priorities on an event_base by calling event_base_priority_init().
int event_base_priority_init(struct event_base *base, int n_priorities);
This function returns 0 on success and -1 on failure. The 'base' argument is
the event_base to modify, and n_priorities is the number of priorities to
support. It must be at least 1. The available priorities for new events
will be numbered from 0 (most important) to n_priorities-1 (least important).
There is a constant, EVENT_MAX_PRIORITIES, that sets the upper bound on the
value of n_priorities. It is an error to call this function with a higher
value for n_priorities.
NOTE: You *must* call this function before any events become active. It is
best to call it immediately after creating the event_base.
To find the number of priorities currently supported by a base, you can
call event_base_getnpriorities().
int event_base_get_npriorities(struct event_base *base);
The return value is equal to the number of priorities configured in the
base. So if event_base_get_npriorities() returns 3, then allowable priority
values are 0, 1, and 2.
For an example, see the documentation for event_priority_set below.
By default, all new events associated with this base will be initialized
with priority equal to n_priorities / 2.
The event_base_priority_init function is defined in <event2/event.h>. It has
been available since Libevent 1.0. The event_base_get_npriorities() function
was new in Libevent 2.1.1-alpha.
Reinitializing an event_base after fork()
Not all event backends persist cleanly after a call to fork(). Thus, if your
program uses fork() or a related system call in order to start a new process,
and you want to continue using an event_base after you have forked, you may
need to reinitialize it.
int event_reinit(struct event_base *base);
The function returns 0 on success, -1 on failure.
//BUILD: FUNCTIONBODY INC:event2/event.h INC:../example_stubs/Ref2.h INC:unistd.h
struct event_base *base = event_base_new();
/* ... add some events to the event_base ... */
if (fork()) {
/* In parent */
continue_running_parent(base); /*...*/
} else {
/* In child */
continue_running_child(base); /*...*/
The event_reinit() function is defined in <event2/event.h>. It was first
available in Libevent 1.4.3-alpha.
Obsolete event_base functions
Older versions of Libevent relied pretty heavily on the idea of a
"current" event_base. The "current" event_base was a global setting
shared across all threads. If you forgot to specify which event_base
you wanted, you got the current one. Since event_bases weren't
threadsafe, this could get pretty error-prone.
Instead of event_base_new(), there was:
struct event_base *event_init(void);
This function worked like event_base_new(), and set the current base
to the allocated base. There was no other way to change the
current base.
Some of the event_base functions in this section had variants that
operated on the current base. These functions behaved as the current
functions, except that they took no base argument.
| Current function | Obsolete current-base version
| event_base_priority_init() | event_priority_init()
| event_base_get_method() | event_get_method()