linux eventfd
eventfd是在linux kernel 2.6.22之后提供的，通过创建一个文件描述符来进行事件通知。可以用于应用程序之间的事件通知，也可用于kernel通知应用程序。
在linux多线程应用开发中，线程之间的通信，经常都是通过消息队列来完成的，当一个线程向队列中写入消息，然后通过pthread_cond_t条件变量来通知那些等待中的其他线程。有了eventfd object，我们就可以通过向eventfd object写入数据，这样就可以通知到那些等待中的线程。最重要的是，eventfd 可以很好的与 select/epoll结合，这样子可以大大提高线程的效率，不至于线程只能 wait unitl contiditon is ready. 在以前没有eventfd时，其实是可以通过管道来解决的，但是管道需要占用两个fd(一个读，一个写)， eventfd object只占用一个fd。
1. 创建一个eventfd object: create a file descriptor for event notification
#include <sys/eventfd.h> int eventfd(unsigned int initval, int flags);
每一个eventfd object，包含有一个无符号的64位整数计数器，并且这个计数器的初始值为initval。
flags的取值有：
1) EFD_NONBLOCK: 设置为非阻塞
2) EFD_CLOEXEC: close on exec
3) EFD_SEMAPHORE: 在2.6.30中提供，如果设置了这个标志，则在读取上会与多线程中的信号量有些类似。
通过返回的文件描述符fd, 我们可以读写此eventfd object，直接通过read, write即可完成读写操作。
2. 读取eventfd
如果eventfd object 的计数器不为0， 则read() 返回 当前的计数值，并将计数器置为0。如果计数器已经为0，此时读取的话，则会被阻塞，直至计数器不为0时，若设置了EFD_NONBLOCK，则会立即返回，并置errno=EAGAIN。
若设置了EFD_SEMAPHORE, 如果eventfd object的计数器不为0，则read()返回1，同时计数器减1.
3. 写eventfd
写eventfd object，会将其计数器加上当前写入值。如果写入后的值超过了64位整数的最大值，则会阻塞，直至被读取，若设置了EFD_NONBLOCK，则立即返回，并置errno=EAGAIN。
4. 当我们不再使用eventfd object时，调用close将其关闭释放。
Note：
在读写eventfd object时，一定要用uint64_t，也就是大小必须是8字节的，否则会返回错误。
example: 采用EFD_NONBLOCK, 写线程每隔1秒写eventfd， main线程通过epoll监听eventfd是否可读，若可读，则读取eventfd的值，并将其打印。
static void *_write_thread(void *arg)
{
int efd = (int)arg;
int i;
int ret;
uint64_t count;
for(i = 0; i < 100; ++i){
count = i + 1;
ret = write(efd, &count, sizeof(count));
if(ret <= 0){
ERR("write eventfd fail");
break;
}
struct timeval tv;
gettimeofday(&tv, NULL);
printf("write %d bytes(%llu) at %lds %ldus\n",
ret, count, tv.tv_sec, tv.tv_usec);
sleep(1);
}
return NULL;
}
int eventfd_test(void)
{
printf("** eventfd test:\n");
int efd = eventfd(0, EFD_NONBLOCK);
assert(efd >= 0);
epoll_fd_t epfd;
assert(epoll_fd_create(&epfd, 1024) == 0);
task_t *write_task = task_create("write_task", _write_thread, (void *)efd);
assert(write_task);
uint64_t count = 0;
assert(epoll_fd_add(&epfd, efd, EPOLLIN, NULL) == 0);
while(1){
int ret = epoll_fd_wait(&epfd, -1);
if(ret <= 0){
ERR("epoll_fd_wait fail");
break;
}
if((ret = read(efd, &count, sizeof(count))) < 0){
ERR("read eventfd fail");
break;
}
struct timeval tv;
gettimeofday(&tv, NULL);
printf("read from efd success, read %d bytes(%llu) at %lds %ldus\n",
ret, count, tv.tv_sec, tv.tv_usec);
}
task_destroy(&write_task, NULL, 1);
epoll_fd_release(&epfd);
close(efd);
printf("** eventfd test success\n");
return 0;
}
程序运行输出：
** eventfd test: read from efd success, read 8 bytes(1) at 1445153954s 567052us write 8 bytes(1) at 1445153954s 567052us read from efd success, read 8 bytes(2) at 1445153955s 567552us write 8 bytes(2) at 1445153955s 567552us read from efd success, read 8 bytes(3) at 1445153956s 568052us write 8 bytes(3) at 1445153956s 568052us read from efd success, read 8 bytes(4) at 1445153957s 568552us write 8 bytes(4) at 1445153957s 568552us read from efd success, read 8 bytes(5) at 1445153958s 569052us write 8 bytes(5) at 1445153958s 569052us read from efd success, read 8 bytes(6) at 1445153959s 569552us write 8 bytes(6) at 1445153959s 569552us read from efd success, read 8 bytes(7) at 1445153960s 570052us write 8 bytes(7) at 1445153960s 570052us read from efd success, read 8 bytes(8) at 1445153961s 570552us write 8 bytes(8) at 1445153961s 570552us read from efd success, read 8 bytes(9) at 1445153962s 571052us write 8 bytes(9) at 1445153962s 571052us read from efd success, read 8 bytes(10) at 1445153963s 571552us write 8 bytes(10) at 1445153963s 571552us read from efd success, read 8 bytes(11) at 1445153964s 572052us write 8 bytes(11) at 1445153964s 572052us read from efd success, read 8 bytes(12) at 1445153965s 572552us write 8 bytes(12) at 1445153965s 572552us read from efd success, read 8 bytes(13) at 1445153966s 573052us write 8 bytes(13) at 1445153966s 573052us .......
如果eventfd_create()时设置了EFD_SEMAPHORE时，程序输出结果如下：
** eventfd test: read from efd success, read 8 bytes(1) at 1445154897s 318192us write 8 bytes(1) at 1445154897s 322194us write 8 bytes(2) at 1445154898s 322694us read from efd success, read 8 bytes(1) at 1445154898s 322694us read from efd success, read 8 bytes(1) at 1445154898s 322694us read from efd success, read 8 bytes(1) at 1445154899s 327196us read from efd success, read 8 bytes(1) at 1445154899s 327196us read from efd success, read 8 bytes(1) at 1445154899s 327196us write 8 bytes(3) at 1445154899s 339202us write 8 bytes(4) at 1445154900s 339702us read from efd success, read 8 bytes(1) at 1445154900s 339702us read from efd success, read 8 bytes(1) at 1445154900s 339702us read from efd success, read 8 bytes(1) at 1445154900s 339702us read from efd success, read 8 bytes(1) at 1445154900s 339702us write 8 bytes(5) at 1445154901s 340202us read from efd success, read 8 bytes(1) at 1445154901s 340202us read from efd success, read 8 bytes(1) at 1445154901s 340202us read from efd success, read 8 bytes(1) at 1445154901s 340202us read from efd success, read 8 bytes(1) at 1445154901s 340202us read from efd success, read 8 bytes(1) at 1445154901s 340202us write 8 bytes(6) at 1445154902s 340702us read from efd success, read 8 bytes(1) at 1445154902s 340702us read from efd success, read 8 bytes(1) at 1445154902s 340702us read from efd success, read 8 bytes(1) at 1445154902s 340702us read from efd success, read 8 bytes(1) at 1445154902s 340702us read from efd success, read 8 bytes(1) at 1445154902s 340702us read from efd success, read 8 bytes(1) at 1445154902s 340702us write 8 bytes(7) at 1445154903s 341202us read from efd success, read 8 bytes(1) at 1445154903s 341202us read from efd success, read 8 bytes(1) at 1445154903s 341202us read from efd success, read 8 bytes(1) at 1445154903s 341202us read from efd success, read 8 bytes(1) at 1445154903s 341202us read from efd success, read 8 bytes(1) at 1445154903s 341202us read from efd success, read 8 bytes(1) at 1445154903s 341202us read from efd success, read 8 bytes(1) at 1445154903s 341202us
因为写线程每次写入的数值都是递加的，但是此时main线程每次读取的值是1，所以会出现写1次数值，然后多次读取的情况。
在学习eventfd的时候，发现linux还提供了signalfd, timerfd。
signalfd: 把信号抽象为一个文件描述符，当有信号发生时，可以对其read, 可以与epoll/select结合，进行监听；
timerfd: 把定时器抽象为一个文件描述符，定时器到期时，可以对其read，所以也是可以通过epoll、select进行监听。
在下面的参考文档中都有详细的说明。
参考：
http://man7.org/linux/man-pages/man2/eventfd.2.html
http://man7.org/linux/man-pages/man2/signalfd.2.html
http://man7.org/linux/man-pages/man2/timerfd_create.2.html