libevent源码分析：event_assign、event_new

在libevent中，获取event类型对象的方法有两种，event_assign、event_new

1、event_assign()

/**
  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()
  */
EVENT2_EXPORT_SYMBOL
int event_assign(struct event *, struct event_base *, evutil_socket_t, short, event_callback_fn, void *);

实现：

其实event_assign的作用就是把给定的event类型对象的每一个成员赋予一个指定的值。

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);

    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;

    if (events & EV_SIGNAL) {
        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;
        }
        ev->ev_closure = EV_CLOSURE_EVENT_SIGNAL;
    } else {
        if (events & EV_PERSIST) {
            evutil_timerclear(&ev->ev_io_timeout);
            ev->ev_closure = EV_CLOSURE_EVENT_PERSIST;
        } else {
            ev->ev_closure = EV_CLOSURE_EVENT;
        }
    }

    min_heap_elem_init_(ev);

    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;
}

2、event_new()

/**
  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()
 */
EVENT2_EXPORT_SYMBOL
struct event *event_new(struct event_base *, evutil_socket_t, short, event_callback_fn, void *);

实现：

event_new的实现其实是间接的调用的event_assign，首先调用mm_malloc分配一块内存，然后调用event_assign来给event类型的对象各个成员赋值。

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);
}

3、区别

这两种方式的区别就是，event_assign是在栈上分配一个对象，然后给成员赋值；而event_new是在堆上分配一个对象，然后给成员赋值。