一、event_new
主要用來創建事件結構體，根據監聽事件類型，文件描述符，以及回調函數，回調函數參數等創建，可以看成是事件的初始化過程，主要是設定事件的初始狀態，此時事件結構體剛剛創建出來還沒有添加到event_base的激活或者等待列表中，是孤立存在的，需要調用event_add函數將此事件添加到event_base中。
/**
Allocate and asssign a new event structure, ready to be added.
The function event_new() returns a new event that can be used in
future calls to event_add() and event_del(). The fd and events
arguments determine which conditions will trigger the event; the
callback and callback_arg arguments tell Libevent what to do when the
event becomes active.
If events contains one of EV_READ, EV_WRITE, or EV_READ|EV_WRITE, then
fd is a file descriptor or socket that should get monitored for
readiness to read, readiness to write, or readiness for either operation
(respectively). If events contains EV_SIGNAL, then fd is a signal
number to wait for. If events contains none of those flags, then the
event can be triggered only by a timeout or by manual activation with
event_active(): In this case, fd must be -1.
The EV_PERSIST flag can also be passed in the events argument: it makes
event_add() persistent until event_del() is called.
The EV_ET flag is compatible with EV_READ and EV_WRITE, and supported
only by certain backends. It tells Libevent to use edge-triggered
events.
The EV_TIMEOUT flag has no effect here.
It is okay to have multiple events all listening on the same fds; but
they must either all be edge-triggered, or all not be edge triggerd.
When the event becomes active, the event loop will run the provided
callbuck function, with three arguments. The first will be the provided
fd value. The second will be a bitfield of the events that triggered:
EV_READ, EV_WRITE, or EV_SIGNAL. Here the EV_TIMEOUT flag indicates
that a timeout occurred, and EV_ET indicates that an edge-triggered
event occurred. The third event will be the callback_arg pointer that
you provide.
@param base the event base to which the event should be attached.
@param fd the file descriptor or signal to be monitored, or -1.
@param events desired events to monitor: bitfield of EV_READ, EV_WRITE,
EV_SIGNAL, EV_PERSIST, EV_ET.
@param callback callback function to be invoked when the event occurs
@param callback_arg an argument to be passed to the callback function
@return a newly allocated struct event that must later be freed with
event_free().
@see event_free(), event_add(), event_del(), event_assign()
*/
// 分配新的event結構，並準備好添加。函數event_new返回新的事件，可以用在
// event_add和event_del函數中。fd和events參數決定了什麼條件可以觸發這個事件；
// 回調函數和回調函數參數告訴libevent當event激活時需要做什麼。
// 如果events包括EV_READ,EV_WRITE,EV_READ|EV_WRITE之一，那麼fd是文件描述符或者socket，
// 這兩個都是可以在讀、寫條件下被監控的。如果events包含EV_SIGNAL，那麼fd是等待的信號值。
// 如果events不包含上述任何一種，那麼事件是定時事件或者通過event_active進行人工觸發：如果是
// 這種情況，fd必需是－1。
// EV_PERSIST可以通過events參數傳遞：它使得event_add永久生效，直到event_del調用；
// EV_ET與EV_READ以及EV_WRITE是兼容的，並且只能被部分後臺方法支持。它告訴libevent使用邊沿觸發事件；
// EV_TIMEOUT此處沒有影響；
// 在同一個fd上監聽多個events是合法的；但是它們必需都是邊沿觸發，或者都不是邊沿觸發。
// 當事件激活時，event loop將運行提供的回調函數，就使用這三個參數：第一個就是提供的fd；
// 第二個是觸發的events，EV_READ，EV_WRITE，EV_SIGNAL，EV_TIMEOUT表示超時事件發生，EV_ET
// 表示邊沿觸發事件發生；第三個參數是提供的callback_arg指針。
// base：需要綁定的base
// fd：需要監聽的文件描述符或者信號，或者爲－1，如果爲－1則是定時事件
// events：事件類型，信號事件和IO事件不能同時存在
// callback：回調函數，信號發生或者IO事件或者定時事件發生時
// callback_arg：傳遞給回調函數的參數，一般是base
// 返回值：是指事件結構體，必須由event_free釋放
// 相關查看event_free，event_add，event_del，event_assign
struct event *
event_new(struct event_base *base, evutil_socket_t fd, short events, void (*cb)(evutil_socket_t, short, void *), void *arg)
{
struct event *ev;
ev = mm_malloc(sizeof(struct event));
if (ev == NULL)
return (NULL);
if (event_assign(ev, base, fd, events, cb, arg) < 0) {
mm_free(ev);
return (NULL);
}
return (ev);
}二、event_assign
準備將新創建的event添加到event_base中，注意，這裏的添加到event_base中只是將事件和event_base關聯起來，並不是加入到event_base的激活隊列或者等待隊列中。
/**
Prepare a new, already-allocated event structure to be added.
The function event_assign() prepares the event structure ev to be used
in future calls to event_add() and event_del(). Unlike event_new(), it
doesn't allocate memory itself: it requires that you have already
allocated a struct event, probably on the heap. Doing this will
typically make your code depend on the size of the event structure, and
thereby create incompatibility with future versions of Libevent.
The easiest way to avoid this problem is just to use event_new() and
event_free() instead.
A slightly harder way to future-proof your code is to use
event_get_struct_event_size() to determine the required size of an event
at runtime.
Note that it is NOT safe to call this function on an event that is
active or pending. Doing so WILL corrupt internal data structures in
Libevent, and lead to strange, hard-to-diagnose bugs. You _can_ use
event_assign to change an existing event, but only if it is not active
or pending!
The arguments for this function, and the behavior of the events that it
makes, are as for event_new().
@param ev an event struct to be modified
@param base the event base to which ev should be attached.
@param fd the file descriptor to be monitored
@param events desired events to monitor; can be EV_READ and/or EV_WRITE
@param callback callback function to be invoked when the event occurs
@param callback_arg an argument to be passed to the callback function
@return 0 if success, or -1 on invalid arguments.
@see event_new(), event_add(), event_del(), event_base_once(),
event_get_struct_event_size()
*/
// 準備新的已經分配的事件結構體以添加到event_base中；
// 函數event_assign準備事件結構體ev，以備將來event_add和event_del調用。
// 不像event_new，他沒有自己分配內存：他需要你已經分配了一個事件結構體，應該是在堆上分配的；
// 這樣做一般會讓你的代碼依賴於事件結構體的尺寸，並且會造成與未來版本不兼容。
// 避免自己分配出現問題的最簡單的辦法是使用event_new和event_free來申請和釋放。
// 讓代碼未來兼容的稍微困難的方式是使用event_get_struct_event_size來確定運行時事件的尺寸。
// 注意，如果事件處於激活或者未決狀態，調用這個函數是不安全的。這樣會破壞libevent內部數據結構，
// 導致一些奇怪的，難以調查的問題。你只能使用event_assign來改變一個存在的事件，但是事件狀態不能
// 是激活或者未決的。
// ev：需要修正的事件結構體
// base： 將ev添加到的event_base
// fd： 需要監控的文件描述符
// events： 需要監控的事件類型；可以是EV_READ或者EV_WRITE
// callback：當事件發生時的回調函數
// callback_arg: 傳遞給回調函數的參數，一般是event_base句柄
// 如果成功則返回0，如果失敗則返回－1
// 相關查看 event_new,event_add,event_del,event_base_once,event_get_struct_event_size
int
event_assign(struct event *ev, struct event_base *base, evutil_socket_t fd, short events, void (*callback)(evutil_socket_t, short, void *), void *arg)
{
if (!base)
base = current_base;
if (arg == &event_self_cbarg_ptr_)
arg = ev;
event_debug_assert_not_added_(ev);
// 事件結構體初始設置
ev->ev_base = base;
ev->ev_callback = callback;
ev->ev_arg = arg;
ev->ev_fd = fd;
ev->ev_events = events;
ev->ev_res = 0;
ev->ev_flags = EVLIST_INIT;
ev->ev_ncalls = 0;
ev->ev_pncalls = NULL;
// 如果是信號事件
if (events & EV_SIGNAL) {
// 信號事件與IO事件不能同時存在
if ((events & (EV_READ|EV_WRITE|EV_CLOSED)) != 0) {
event_warnx("%s: EV_SIGNAL is not compatible with "
"EV_READ, EV_WRITE or EV_CLOSED", __func__);
return -1;
}
// 設置事件關閉時執行回調函數的類型：evcb_callback
ev->ev_closure = EV_CLOSURE_EVENT_SIGNAL;
} else {
// 如果是其它類型的事件：IO事件、定時事件
// 如果事件是永久事件，即每次調用之後不會移除出事件列表
// 清空IO超時控制，並設置事件關閉時回調函數類型：evcb_callback
if (events & EV_PERSIST) {
evutil_timerclear(&ev->ev_io_timeout);
ev->ev_closure = EV_CLOSURE_EVENT_PERSIST;
} else {
// 設置事件回調函數類型：evcb_callback
ev->ev_closure = EV_CLOSURE_EVENT;
}
}
// 事件超時控制最小堆初始化
min_heap_elem_init_(ev);
// 默認情況下，事件優先級設置爲中間優先級
if (base != NULL) {
/* by default, we put new events into the middle priority */
ev->ev_pri = base->nactivequeues / 2;
}
event_debug_note_setup_(ev);
return 0;
}三、event_free
釋放由event_new申請的事件，可以看出源碼中是調用事件刪除函數event_del將事件從event_base中刪除，然後調用內部空間釋放函數mm_free釋放事件結構體的空間。
/**
Deallocate a struct event * returned by event_new().
If the event is pending or active, first make it non-pending and
non-active.
*/
// 釋放event_new創建的事件結構體；如果事件正處於未決或者激活狀態，首先需要將事件
// 變爲非未決或者非活躍狀態
void
event_free(struct event *ev)
{
/* This is disabled, so that events which have been finalized be a
* valid target for event_free(). That's */
// event_debug_assert_is_setup_(ev);
/* make sure that this event won't be coming back to haunt us. */
// 調用實際執行函數
event_del(ev);
event_debug_note_teardown_(ev);
mm_free(ev);
}