Changing bufferevent callbacks when data is pending.

switching.c

#include <signal.h>
#include <stdio.h>

#include <event2/event.h>
#include <event2/bufferevent.h>

struct event_base *base;

void signal_cb(evutil_socket_t signal, short action, void *arg)
{
    printf("Sorry\n");
    event_base_loopexit(base, NULL);
}

void read2(struct bufferevent *bev, void *arg)
{
    printf("Success!\n");
    event_base_loopexit(base, NULL);
}

void switch_cb(evutil_socket_t fd, short action, void *arg)
{
    struct bufferevent *bev = arg;

    printf("Step 2\n");
    bufferevent_setcb(bev, read2, NULL, NULL, NULL);
    //bufferevent_setwatermark(bev, EV_READ, 0, 16384);
    //bufferevent_disable(bev, EV_READ);
    //bufferevent_enable(bev, EV_READ);

    // works with BEV_FLUSH for _pair but some implementations ignore flush
    //bufferevent_flush(bev, EV_READ, BEV_NORMAL);
}

void read1(struct bufferevent *bev, void *arg)
{
    struct event *event = arg;

    printf("Step 1\n");
    event_active(event, 0, 0);
}

int main()
{
    struct bufferevent *pair[2];
    struct event *event;

    base = event_base_new();
    bufferevent_pair_new(base, 0, pair);
    event = event_new(base, -1, 0, switch_cb, pair[1]);

    bufferevent_setcb(pair[1], read1, NULL, NULL, event);
    bufferevent_enable(pair[1], EV_READ);
    bufferevent_write(pair[0], " ", 1);

    event = event_new(base, SIGINT, EV_SIGNAL, signal_cb, NULL);
    event_add(event, NULL);

    event_base_dispatch(base);
    printf("Goodbye!\n");

    return 0;
}