线程间通信之eventfd
man手册中的解释：
eventfd()创建了一个“eventfd对象”， 通过它能够实现用户态程序间(我觉得这里主要指线程而非进程)的等待/通知机制，以及内核态向用户态通知的机制（未考证）。
此对象包含了一个被内核所维护的计数（uint64_t）, 初始值由initval来决定。
int eventfd(unsigned int initval, int flags);创建一个eventfd文件描述符
int eventfd_read(int fd, eventfd_t *value); 向eventfd中写入一个值
int eventfd_write(int fd, eventfd_t value); 从eventfd中读出一个值
例一、子线程多次写入多个值，主线程一次读出所有值的和
1 #include <sys/eventfd.h>2 #include <unistd.h>3 #include <stdlib.h>4 #include <stdio.h>5 #include <stdint.h> 6 7 int main(int argc, char**argv[])8 {9 int efd, j;
10 uint64_t u;
11 ssize_t s;
12
13 if (argc < 2)
14 {
15 printf("number of argc is wrong!\n");
16 return 0;
17 }
18
19 efd = eventfd(0,0);
20 if (-1 == efd)
21 {
22 printf("failed to create eventfd\n");
23 }
24
25 switch(fork())
26 {
27 case 0:
28 {
29 for(j=1; j<argc;j++)
30 {
31 printf("child writing %s to efd\n", argv[j]);
32 u = strtoull(argv[j], NULL, 0);
33 s = write(efd, &u, sizeof(uint64_t));
34 if (s!=sizeof(uint64_t))
35 {
36 printf("write efd failed\n");
37 }
38 }
39 printf("Child completed write loop\n");
40 exit(0);
41 }
42 default:
43 sleep(2);
44 printf("Parents about to read\n");
45 s = read(efd, &u, sizeof(uint64_t));
46 if (s != sizeof(uint64_t))
47 {
48 printf("read efd failed\n");
49 }
50 printf("Parents first read %llu (0x%llx) from efd\n", u, u);
51 exit(0);
52 case -1:
53 {
54 printf("fork error\n");
55 }
56 }
57
58 return 0;
59 }
60
61 运行结果
62 kane@kanelinux:/mnt/hgfs/kanelinuxshare/eventfd$ ./a.out 1 2 3 4
63 child writing 1 to efd
64 child writing 2 to efd
65 child writing 3 to efd
66 child writing 4 to efd
67 Child completed write loop
68 Parents about to read
69 Parents first read 10 (0xa) from efd
70
71 如果有写入操作，但是并没有导致初始值变化，则主线程会一直挂在read操作上
72 kane@kanelinux:/mnt/hgfs/kanelinuxshare/eventfd$ ./a.out 0 0 0 0
73 child writing 0 to efd
74 child writing 0 to efd
75 child writing 0 to efd
76 child writing 0 to efd
77 Child completed write loop
78 Parents about to read
79 ^C
例二、eventfd可以被epoll监控， 一旦有状态变化，可以触发通知
1 #include <sys/eventfd.h>2 #include <unistd.h>3 #include <stdlib.h>4 #include <stdio.h>5 #include <stdint.h> 6 #include <sys/epoll.h> 7 #include <string.h> 8 #include <pthread.h> 9 10 int g_iEvtfd = -1;11 12 void *eventfd_child_Task(void *pArg)13 {14 uint64_t uiWrite = 1;15 16 while(1)17 {18 sleep(2);19 if (0 != eventfd_write(g_iEvtfd, uiWrite))20 {21 printf("child write iEvtfd failed\n");22 } 23 }24 25 return;26 }27 28 int main(int argc, char**argv[])29 {30 int iEvtfd, j;31 uint64_t uiWrite = 1;32 uint64_t uiRead;33 ssize_t s;34 int iEpfd;35 struct epoll_event stEvent;36 int iRet = 0;37 struct epoll_event stEpEvent;38 pthread_t stWthread;39 40 iEpfd = epoll_create(1);41 if (-1 == iEpfd)42 {43 printf("Create epoll failed.\n");44 return 0;45 }46 47 iEvtfd = eventfd(0,0);48 if (-1 == iEvtfd)49 {50 printf("failed to create eventfd\n");51 return 0;52 }53 54 g_iEvtfd = iEvtfd;55 56 memset(&stEvent, 0, sizeof(struct epoll_event));57 stEvent.events = (unsigned long) EPOLLIN;58 stEvent.data.fd = iEvtfd;59 iRet = epoll_ctl(iEpfd, EPOLL_CTL_ADD, g_iEvtfd, &stEvent);60 if (0 != iRet)61 {62 printf("failed to add iEvtfd to epoll\n");63 close(g_iEvtfd);64 close(iEpfd);65 return 0;66 }67 68 iRet = pthread_create(&stWthread, NULL, eventfd_child_Task, NULL);69 if (0 != iRet)70 {71 close(g_iEvtfd);72 close(iEpfd);73 return;74 }75 76 for(;;)77 {78 iRet = epoll_wait(iEpfd, &stEpEvent, 1, -1);79 if (iRet > 0)80 {81 s = eventfd_read(iEvtfd, &uiRead);82 if (s != 0)83 {84 printf("read iEvtfd failed\n");85 break;86 }87 printf("Read %llu (0x%llx) from iEvtfd\n", uiRead, uiRead);88 }89 }90 91 close(g_iEvtfd);92 close(iEpfd);93 return 0;94 }95 运行结果96 kane@kanelinux:/mnt/hgfs/kanelinuxshare/eventfd$ ./a.out97 Read 1 (0x1) from iEvtfd98 Read 1 (0x1) from iEvtfd99 Read 1 (0x1) from iEvtfd
100 Read 1 (0x1) from iEvtfd
101 Read 1 (0x1) from iEvtfd
102 Read 1 (0x1) from iEvtfd
103 Read 1 (0x1) from iEvtfd
104 Read 1 (0x1) from iEvtfd
105 ^C
例三、被epoll监控的eventfd，如果在子线程中被多次写入，在主线程中是怎么读的？
#include <sys/eventfd.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <stdint.h>
#include <sys/epoll.h>
#include <string.h>
#include <pthread.h> int g_iEvtfd = -1;void *eventfd_child_Task(void *pArg)
{uint64_t uiWrite = 1;while(1){sleep(2);eventfd_write(g_iEvtfd, uiWrite);eventfd_write(g_iEvtfd, uiWrite);}return;
}int main(int argc, char**argv[])
{int iEvtfd, j;uint64_t uiWrite = 1;uint64_t uiRead;ssize_t s;int iEpfd;struct epoll_event stEvent;int iRet = 0;struct epoll_event stEpEvent;pthread_t stWthread;iEpfd = epoll_create(1);if (-1 == iEpfd){printf("Create epoll failed.\n");return 0;}iEvtfd = eventfd(0,0);if (-1 == iEvtfd){printf("failed to create eventfd\n");return 0;}g_iEvtfd = iEvtfd;memset(&stEvent, 0, sizeof(struct epoll_event));stEvent.events = (unsigned long) EPOLLIN;stEvent.data.fd = iEvtfd;iRet = epoll_ctl(iEpfd, EPOLL_CTL_ADD, g_iEvtfd, &stEvent);if (0 != iRet){printf("failed to add iEvtfd to epoll\n");close(g_iEvtfd);close(iEpfd);return 0;}iRet = pthread_create(&stWthread, NULL, eventfd_child_Task, NULL);if (0 != iRet){close(g_iEvtfd);close(iEpfd);return;}for(;;){iRet = epoll_wait(iEpfd, &stEpEvent, 1, -1);if (iRet > 0){s = eventfd_read(iEvtfd, &uiRead);if (s != 0){printf("read iEvtfd failed\n");break;}printf("Read %llu (0x%llx) from iEvtfd\n", uiRead, uiRead);}}close(g_iEvtfd);close(iEpfd);return 0;
}运行结果：
kane@kanelinux:/mnt/hgfs/kanelinuxshare/eventfd$ ./a.out
Read 1 (0x1) from iEvtfd
Read 1 (0x1) from iEvtfdRead 1 (0x1) from iEvtfd
Read 1 (0x1) from iEvtfdRead 1 (0x1) from iEvtfd
Read 1 (0x1) from iEvtfdRead 1 (0x1) from iEvtfd
Read 1 (0x1) from iEvtfd^C
例一中并没有epoll做监控，
因此在read前，如果eventfd被写多次，在read的时候也是一次全部读出。
注：eventfd中的SEMAPHORE标志用法
* If EFD_SEMAPHORE was not specified and the eventfd counter
has a nonzero value, then a read(2) returns 8 bytes contain‐
ing that value, and the counter's value is reset to zero.
* If EFD_SEMAPHORE was specified and the eventfd counter has a
nonzero value, then a read(2) returns 8 bytes containing the
value 1, and the counter's value is decremented by 1.
通过测试发现。如果eventfd在创建的时候传入EFD_SEMAPHORE 标志，则会按上面man手册中提到的那样，每次在eventfd_read的时候只减一，并不是把值一次性全部读出。见下例 ：
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