libevent源码分析：event_assign、event_new

 1 /**
 2   Prepare a new, already-allocated event structure to be added.
 3 
 4   The function event_assign() prepares the event structure ev to be used
 5   in future calls to event_add() and event_del().  Unlike event_new(), it
 6   doesn't allocate memory itself: it requires that you have already
 7   allocated a struct event, probably on the heap.  Doing this will
 8   typically make your code depend on the size of the event structure, and
 9   thereby create incompatibility with future versions of Libevent.
10 
11   The easiest way to avoid this problem is just to use event_new() and
12   event_free() instead.
13 
14   A slightly harder way to future-proof your code is to use
15   event_get_struct_event_size() to determine the required size of an event
16   at runtime.
17 
18   Note that it is NOT safe to call this function on an event that is
19   active or pending.  Doing so WILL corrupt internal data structures in
20   Libevent, and lead to strange, hard-to-diagnose bugs.  You _can_ use
21   event_assign to change an existing event, but only if it is not active
22   or pending!
23 
24   The arguments for this function, and the behavior of the events that it
25   makes, are as for event_new().
26 
27   @param ev an event struct to be modified
28   @param base the event base to which ev should be attached.
29   @param fd the file descriptor to be monitored
30   @param events desired events to monitor; can be EV_READ and/or EV_WRITE
31   @param callback callback function to be invoked when the event occurs
32   @param callback_arg an argument to be passed to the callback function
33 
34   @return 0 if success, or -1 on invalid arguments.
35 
36   @see event_new(), event_add(), event_del(), event_base_once(),
37     event_get_struct_event_size()
38   */
39 EVENT2_EXPORT_SYMBOL
40 int event_assign(struct event *, struct event_base *, evutil_socket_t, short, event_callback_fn, void *);

 1 int
 2 event_assign(struct event *ev, struct event_base *base, evutil_socket_t fd, short events, void (*callback)(evutil_socket_t, short, void *), void *arg)
 3 {
 4     if (!base)
 5         base = current_base;
 6     if (arg == &event_self_cbarg_ptr_)
 7         arg = ev;
 8 
 9     event_debug_assert_not_added_(ev);
10 
11     ev->ev_base = base;
12 
13     ev->ev_callback = callback;
14     ev->ev_arg = arg;
15     ev->ev_fd = fd;
16     ev->ev_events = events;
17     ev->ev_res = 0;
18     ev->ev_flags = EVLIST_INIT;
19     ev->ev_ncalls = 0;
20     ev->ev_pncalls = NULL;
21 
22     if (events & EV_SIGNAL) {
23         if ((events & (EV_READ|EV_WRITE|EV_CLOSED)) != 0) {
24             event_warnx("%s: EV_SIGNAL is not compatible with "
25                 "EV_READ, EV_WRITE or EV_CLOSED", __func__);
26             return -1;
27         }
28         ev->ev_closure = EV_CLOSURE_EVENT_SIGNAL;
29     } else {
30         if (events & EV_PERSIST) {
31             evutil_timerclear(&ev->ev_io_timeout);
32             ev->ev_closure = EV_CLOSURE_EVENT_PERSIST;
33         } else {
34             ev->ev_closure = EV_CLOSURE_EVENT;
35         }
36     }
37 
38     min_heap_elem_init_(ev);
39 
40     if (base != NULL) {
41         /* by default, we put new events into the middle priority */
42         ev->ev_pri = base->nactivequeues / 2;
43     }
44 
45     event_debug_note_setup_(ev);
46 
47     return 0;
48 }

 1 /**
 2   Allocate and asssign a new event structure, ready to be added.
 3 
 4   The function event_new() returns a new event that can be used in
 5   future calls to event_add() and event_del().  The fd and events
 6   arguments determine which conditions will trigger the event; the
 7   callback and callback_arg arguments tell Libevent what to do when the
 8   event becomes active.
 9 
10   If events contains one of EV_READ, EV_WRITE, or EV_READ|EV_WRITE, then
11   fd is a file descriptor or socket that should get monitored for
12   readiness to read, readiness to write, or readiness for either operation
13   (respectively).  If events contains EV_SIGNAL, then fd is a signal
14   number to wait for.  If events contains none of those flags, then the
15   event can be triggered only by a timeout or by manual activation with
16   event_active(): In this case, fd must be -1.
17 
18   The EV_PERSIST flag can also be passed in the events argument: it makes
19   event_add() persistent until event_del() is called.
20 
21   The EV_ET flag is compatible with EV_READ and EV_WRITE, and supported
22   only by certain backends.  It tells Libevent to use edge-triggered
23   events.
24 
25   The EV_TIMEOUT flag has no effect here.
26 
27   It is okay to have multiple events all listening on the same fds; but
28   they must either all be edge-triggered, or all not be edge triggerd.
29 
30   When the event becomes active, the event loop will run the provided
31   callbuck function, with three arguments.  The first will be the provided
32   fd value.  The second will be a bitfield of the events that triggered:
33   EV_READ, EV_WRITE, or EV_SIGNAL.  Here the EV_TIMEOUT flag indicates
34   that a timeout occurred, and EV_ET indicates that an edge-triggered
35   event occurred.  The third event will be the callback_arg pointer that
36   you provide.
37 
38   @param base the event base to which the event should be attached.
39   @param fd the file descriptor or signal to be monitored, or -1.
40   @param events desired events to monitor: bitfield of EV_READ, EV_WRITE,
41       EV_SIGNAL, EV_PERSIST, EV_ET.
42   @param callback callback function to be invoked when the event occurs
43   @param callback_arg an argument to be passed to the callback function
44 
45   @return a newly allocated struct event that must later be freed with
46     event_free().
47   @see event_free(), event_add(), event_del(), event_assign()
48  */
49 EVENT2_EXPORT_SYMBOL
50 struct event *event_new(struct event_base *, evutil_socket_t, short, event_callback_fn, void *);

 1 struct event *
 2 event_new(struct event_base *base, evutil_socket_t fd, short events, void (*cb)(evutil_socket_t, short, void *), void *arg)
 3 {
 4     struct event *ev;
 5     ev = mm_malloc(sizeof(struct event));
 6     if (ev == NULL)
 7         return (NULL);
 8     if (event_assign(ev, base, fd, events, cb, arg) < 0) {
 9         mm_free(ev);
10         return (NULL);
11     }
12 
13     return (ev);
14 }