线程间通信之eventfd发布时间：2022/5/31 13:13:38
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
2 #include
3 #include
4 #include
5 #include
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
例二、eventfd可以被epoll监控， 一旦有状态变化，可以触发通知
1 #include
2 #include
3 #include
4 #include
5 #include
6 #include
7 #include
8 #include
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.out
97 Read 1 (0x1) from iEvtfd
98 Read 1 (0x1) from iEvtfd
99 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
#include
#include
#include
#include
#include
#include
#include
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 iEvtfd
Read 1 (0x1) from iEvtfd
Read 1 (0x1) from iEvtfd
Read 1 (0x1) from iEvtfd
Read 1 (0x1) from iEvtfd
Read 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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