php做网站浏览量,百度关键词广告怎么收费,怎么知道网站是否被百度收录,一个人做网站 没有人写文章怎么办我们常用的进程#xff08;线程#xff09;间通信机制有管道#xff0c;信号#xff0c;消息队列#xff0c;信号量#xff0c;共享内存#xff0c;socket等等#xff0c;其中主要作为进程#xff08;线程#xff09;间通知/等待的有管道pipe和socketpair。线程还有特…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[cpp] view plain copy#include sys/eventfd.h int eventfd(unsigned int initval, int flags); 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_CLOECEXEFD_NONBLOCKEFD_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[cpp] view plain copyint eventfd_signal(struct eventfd_ctx *ctx, int n) { unsigned long flags; if (n 0) return -EINVAL; spin_lock_irqsave(ctx-wqh.lock, flags); if (ULLONG_MAX - ctx-count n) 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; } 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[cpp] view plain copy#include sys/eventfd.h #include unistd.h #include stdlib.h #include stdio.h #include stdint.h /* 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 2) { fprintf(stderr, Usage: %s num...\n, argv[0]); exit(EXIT_FAILURE); } efd eventfd(0, 0); if (efd -1) handle_error(eventfd); switch (fork()) { case 0: for (j 1; j argc; 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); } } 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。
