最高效的进(线)程间通信机制--eventfd发布时间：2022/5/31 13:15:46
我们常用的进程（线程）间通信机制有管道，信号，消息队列，信号量，共享内存，socket等等，其中主要作为进程（线程）间通知/等待的有管道pipe和socketpair。线程还有特别的condition。
今天来看一个liunx较新的系统调用，它是从LINUX 2.6.27版本开始增加的，主要用于进程或者线程间的通信（如通知/等待机制的实现）。
首先来看一下函数原型：
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#include int eventfd(unsigned int initval, int flags); [/ol]
下面是它man手册中的描述，我照着翻译了一遍（我英语四级434，你们要是怀疑下文的话，可以直接去man eventfd ^^）：
eventfd()创建了一个"eventfd object"，能在用户态用做事件wait/notify机制，通过内核取唤醒用户态的事件。这个对象保存了一个内核维护的uint64_t类型的整型counter。这个counter初始值被参数initval指定，一般初值设置为0。
它的标记可以有以下属性：
EFD_CLOECEX，EFD_NONBLOCK，EFD_SEMAPHORE。
在linux直到版本2.6.26，这个flags参数是没用的，必须指定为0。
它返回了一个引用eventfd object的描述符。这个描述符可以支持以下操作：
read：如果计数值counter的值不为0，读取成功，获得到该值。如果counter的值为0，非阻塞模式，会直接返回失败，并把errno的值指纹EINVAL。如果为阻塞模式，一直会阻塞到counter为非0位置。
write：会增加8字节的整数在计数器counter上，如果counter的值达到0xfffffffffffffffe时，就会阻塞。直到counter的值被read。阻塞和非阻塞情况同上面read一样。
close：这个操作不用说了。
重点是支持这个：
poll(2), select(2) (and similar)
The returned file descriptor supports poll(2) (and analogously epoll(7)) and select(2), as follows:
* The file descriptor is readable (the select(2) readfds argument; the poll(2) POLLIN flag) if the counter has a
value greater than 0.
* The file descriptor is writable (the select(2) writefds argument; the poll(2) POLLOUT flag) if it is possible to
write a value of at least "1" without blocking.
* If an overflow of the counter value was detected, then select(2) indicates the file descriptor as being both
readable and writable, and poll(2) returns a POLLERR event. As noted above, write(2) can never overflow the
counter. However an overflow can occur if 2^64 eventfd "signal posts" were performed by the KAIO subsystem (the‐
oretically possible, but practically unlikely). If an overflow has occurred, then read(2) will return that maxi‐
mum uint64_t value (i.e., 0xffffffffffffffff).
The eventfd file descriptor also supports the other file-descriptor multiplexing APIs: pselect(2) and ppoll(2).
它的内核代码实现是这样子的：
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int eventfd_signal(struct eventfd_ctx *ctx, int n) { unsigned long flags; if (n return -EINVAL; spin_lock_irqsave(&ctx->wqh.lock, flags); if (ULLONG_MAX - ctx->count n = (int) (ULLONG_MAX - ctx->count); ctx->count += n; if (waitqueue_active(&ctx->wqh)) wake_up_locked_poll(&ctx->wqh, POLLIN); spin_unlock_irqrestore(&ctx->wqh.lock, flags); return n; } [/ol]
本质就是做了一次唤醒，不用read，也不用write，与eventfd_write的区别是不用阻塞。
说了这么多，我们来看一个例子，理解理解其中的含义：
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#include #include #include #include #include /* Definition of uint64_t */ #define handle_error(msg) \ do { perror(msg); exit(EXIT_FAILURE); } while (0) int main(int argc, char *argv[]) { int efd, j; uint64_t u; ssize_t s; if (argc fprintf(stderr, "Usage: %s ...\n", argv[0]); exit(EXIT_FAILURE); } efd = eventfd(0, 0); if (efd == -1) handle_error("eventfd"); switch (fork()) { case 0: for (j = 1; j printf("Child writing %s to efd\n", argv[j]); u = strtoull(argv[j], NULL, 0); /* strtoull() allows various bases */ s = write(efd, &u, sizeof(uint64_t)); if (s != sizeof(uint64_t)) handle_error("write"); } printf("Child completed write loop\n"); exit(EXIT_SUCCESS); default: sleep(2); printf("Parent about to read\n"); s = read(efd, &u, sizeof(uint64_t)); if (s != sizeof(uint64_t)) handle_error("read"); printf("Parent read %llu (0x%llx) from efd\n", (unsigned long long) u, (unsigned long long) u); exit(EXIT_SUCCESS); case -1: handle_error("fork"); } } [/ol]
输出：
$ ./a.out 1 2 4 7 14
Child writing 1 to efd
Child writing 2 to efd
Child writing 4 to efd
Child writing 7 to efd
Child writing 14 to efd
Child completed write loop
Parent about to read
Parent read 28 (0x1c) from efd
注意：这里用了sleep(2)保证子进程循环写入完毕，得到的值就是综合28。如果不用sleep(2)来保证时序，当子进程写入一个值，父进程会立马从eventfd读出该值。
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