線程間通信之eventfd

man手冊中的解釋：
eventfd()創建了一個“eventfd對象”， 通過它能夠實現用戶態程序間(我覺得這里主要指線程而非進程)的等待/通知機制，以及內核態向用戶態通知的機制（未考證）。
此對象包含了一個被內核所維護的計數（uint64_t）, 初始值由initval來決定。

int eventfd(unsigned int initval, int flags);創建一個eventfd文件描述符
int eventfd_read(int fd, eventfd_t *value); 向eventfd中寫入一個值
int eventfd_write(int fd, eventfd_t value); 從eventfd中讀出一個值

例一、子線程多次寫入多個值，主線程一次讀出所有值的和

 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 運行結果
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 如果有寫入操作，但是并沒有導致初始值變化，則主線程會一直掛在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監控， 一旦有狀態變化，可以觸發通知

  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 運行結果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監控的eventfd，如果在子線程中被多次寫入，在主線程中是怎么讀的？

#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;
}運行結果：
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

例一中并沒有epoll做監控，
因此在read前，如果eventfd被寫多次，在read的時候也是一次全部讀出。

?

注：eventfd中的SEMAPHORE標志用法

* 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.

通過測試發現。如果eventfd在創建的時候傳入EFD_SEMAPHORE 標志，則會按上面man手冊中提到的那樣，每次在eventfd_read的時候只減一，并不是把值一次性全部讀出。見下例 ：