Shulou(Shulou.com)06/02 Report--

1. SylixOS epoll introduction

In order to be compatible with Linux's epoll, SylixOS creates a compatible subsystem of epoll and supports some of the functions of epoll. The SylixOS epoll compatible subsystem is simulated by the select subsystem, so it is not as efficient as select.

2. Epoll anomaly analysis

2.1epoll exception scenario

Thread A creates an AF_UNIX anonymous socket to send data; thread B adds the socket to epoll listening and sets the property to be valid once; thread C waits for the epoll event to be generated and reads the data in the socket. As shown in listing 2-1 of the program.

Listing 2-1#include # define READSIZE 7int iEfd = 0poliint iFd [2] = {0}; void * send_data (void * arg) {int iRet = 0 / * * use the socketpair function to create a pair of unnamed, interconnected UNIX domain sockets * and constantly send data at one end * / iRet = socketpair (AF_UNIX, SOCK_STREAM, 0, iFd); if (iRet

< 0) { perror("socketpair"); return NULL; } for (;;) { write(iFd[0], "SylixOS", READSIZE); sleep(1); }}void *test_ctl(void *arg){ struct epoll_event event; int iRet = 0; while (1) { / * * 把套接字加入epoll监听，且设置监听属性为一次有效 */ event.events = 0; event.events = EPOLLIN | EPOLLONESHOT; event.data.fd = iFd[1]; iRet = epoll_ctl(iEfd, EPOLL_CTL_MOD, iFd[1], &event); if (iRet == 0) { printf("test_ctl ctl ok\n"); } sleep(1); }}void *test_wait(void *arg){ struct epoll_event event; int iRet = 0; char cBuf[READSIZE] = {0}; while (1) { / * * 使用epoll等待事件，并读取数据。读取结束后等待下一次事件产生 */ iRet = epoll_wait(iEfd, &event, 1, -1); if (iRet == 1) { printf("test_wait event.data.fd is %d event.events is %x\n", event.data.fd,event.events); read(iFd[1], cBuf, READSIZE); } sleep(1); }}int main(int argc, char **argv){ pthread_t send_tid, wait_tid, ctl_tid; int iRet = 0; struct epoll_event event; /* * 创建一个 epoll 文件描述符 */ iEfd = epoll_create(10); if (0 == iEfd) { perror("epoll create"); return (-1); } iRet = pthread_create(&send_tid, NULL, &send_data, NULL); if (iRet != 0) { fprintf(stderr, "pthread create failed.\n"); return (-1); } sleep(1); / * * 把套接字加入epoll监听，且设置为一次有效 */ event.events = EPOLLIN | EPOLLONESHOT; event.data.fd = iFd[1]; iRet = epoll_ctl(iEfd, EPOLL_CTL_ADD, iFd[1], &event); if (iRet != 0) { perror("epoll_ctl"); return (-1); } iRet = pthread_create(&wait_tid, NULL, &test_wait, NULL); if (iRet != 0) { perror("pthread create"); return (-1); } iRet = pthread_create(&ctl_tid, NULL, &test_ctl, NULL); if (iRet != 0) { perror("pthread create"); return (-1); } pthread_join(send_tid, NULL); pthread_join(wait_tid, NULL); pthread_join(ctl_tid, NULL); return (0);} 该程序运行时，运行结果如图 2-1所示。 图 2-1 运行结果 此时线程test_wait阻塞等待事件产生，根据程序清单 2-1所示，send_data线程一直在写入数据，同时test_ctl线程也在把事件加入epoll中监听。 2.2epoll异常分析 SylixOS epoll兼容子系统是由select子系统模拟出来的，分析epoll_wait源码。epoll_wait代码框架如图 2-2所示。

Figure 2-2 epoll_wait process

According to the epoll_wait implementation process, it can be found that if the event property is set to valid once in epoll_ctl, the event property is left empty after epoll_wait. If epoll_wait is used at this time before the next epoll_ctl operation, the set of file descriptors listening in epoll_wait is empty (because the event property is empty), so select has been listening and no event has been generated.

3. Summary of epoll exception

Set the program in listing 2-1 to ensure that epoll_wait runs after epoll_ctl, and the result is shown in figure 3-1.

Figure 3-1 running result

To use the epoll feature on SylixOS, you need to be careful to ensure that epoll_wait is used after the epoll_ctl is set, so as to ensure that the set of file descriptors that epoll_wait listens to is not empty.

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