0x01 缘由
最近又在复习一些基础知识，现在看看epoll的相关知识点。
0x02 介绍
epoll是Linux内核为处理大批量文件描述符而作了改进的poll，是Linux下多路复用IO接口select/poll的增强版本，它能显著提高程序在大量并发连接中只有少量活跃的情况下的系统CPU利用率。另一点原因就是获取事件的时候，它无须遍历整个被侦听的描述符集，只要遍历那些被内核IO事件异步唤醒而加入Ready队列的描述符集合就行了。epoll除了提供select/poll那种IO事件的水平触发（Level Triggered）外，还提供了边缘触发（Edge Triggered），这就使得用户空间程序有可能缓存IO状态，减少epoll_wait/epoll_pwait的调用，提高应用程序效率。（摘自百度百科）
两种模式（简单理解）：
LT（水平触发）: 对于采用LT工作模式的文件描述符，当epoll_wait检测到其上有事件发生并将此事件通知应用程序后，应用程序可以不立即处理该事件，当应用程序下一次调用epoll_wait时，epoll_waite还会再次向应用程序通告此事件，直到该时间被处理。
ET（边缘触发）:对于采用ET工作模式的文件描述符，当epoll_wait检测到其上有事件发生并将此事件通知应用程序后，应用程序必须立即处理该事件，因后续的epoll_wait调用将不再向应用程序通知这件事。
0x03 示例和测试
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <assert.h>
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <fcntl.h>
#include <stdlib.h>
#include <sys/epoll.h>
#include <pthread.h>
#define MAX_EVENT_NUMBER 1024
#define BUFFER_SIZE 10
int setnonblocking( int fd )
{
int old_option = fcntl( fd, F_GETFL );
int new_option = old_option | O_NONBLOCK;
fcntl( fd, F_SETFL, new_option );
return old_option;
}
void addfd( int epollfd, int fd, bool enable_et )
{
epoll_event event;
event.data.fd = fd;
event.events = EPOLLIN;
if( enable_et )
{
event.events |= EPOLLET;
}
epoll_ctl( epollfd, EPOLL_CTL_ADD, fd, &event );
setnonblocking( fd );
}
void lt( epoll_event* events, int number, int epollfd, int listenfd )
{
char buf[ BUFFER_SIZE ];
for ( int i = 0; i < number; i++ )
{
int sockfd = events[i].data.fd;
if ( sockfd == listenfd )
{
struct sockaddr_in client_address;
socklen_t client_addrlength = sizeof( client_address );
int connfd = accept( listenfd, ( struct sockaddr* )&client_address, &client_addrlength );
addfd( epollfd, connfd, false );
}
else if ( events[i].events & EPOLLIN )
{
printf( "event trigger once\n" );
memset( buf, '\0', BUFFER_SIZE );
int ret = recv( sockfd, buf, BUFFER_SIZE-1, 0 );
if( ret <= 0 )
{
close( sockfd );
continue;
}
printf( "get %d bytes of content: %s\n", ret, buf );
}
else
{
printf( "something else happened \n" );
}
}
}
void et( epoll_event* events, int number, int epollfd, int listenfd )
{
char buf[ BUFFER_SIZE ];
for ( int i = 0; i < number; i++ )
{
int sockfd = events[i].data.fd;
if ( sockfd == listenfd )
{
struct sockaddr_in client_address;
socklen_t client_addrlength = sizeof( client_address );
int connfd = accept( listenfd, ( struct sockaddr* )&client_address, &client_addrlength );
addfd( epollfd, connfd, true );
}
else if ( events[i].events & EPOLLIN )
{
printf( "event trigger once\n" );
while( 1 )
{
memset( buf, '\0', BUFFER_SIZE );
int ret = recv( sockfd, buf, BUFFER_SIZE-1, 0 );
if( ret < 0 )
{
if( ( errno == EAGAIN ) || ( errno == EWOULDBLOCK ) )
{
printf( "read later\n" );
break;
}
close( sockfd );
break;
}
else if( ret == 0 )
{
close( sockfd );
}
else
{
printf( "get %d bytes of content: %s\n", ret, buf );
}
}
}
else
{
printf( "something else happened \n" );
}
}
}
int main( int argc, char* argv[] )
{
if( argc <= 2 )
{
printf( "usage: %s ip_address port_number\n", basename( argv[0] ) );
return 1;
}
const char* ip = argv[1];
int port = atoi( argv[2] );
int ret = 0;
struct sockaddr_in address;
bzero( &address, sizeof( address ) );
address.sin_family = AF_INET;
inet_pton( AF_INET, ip, &address.sin_addr );
address.sin_port = htons( port );
int listenfd = socket( PF_INET, SOCK_STREAM, 0 );
assert( listenfd >= 0 );
ret = bind( listenfd, ( struct sockaddr* )&address, sizeof( address ) );
assert( ret != -1 );
ret = listen( listenfd, 5 );
assert( ret != -1 );
epoll_event events[ MAX_EVENT_NUMBER ];
int epollfd = epoll_create( 5 );
assert( epollfd != -1 );
addfd( epollfd, listenfd, true );
while( 1 )
{
int ret = epoll_wait( epollfd, events, MAX_EVENT_NUMBER, -1 );
if ( ret < 0 )
{
printf( "epoll failure\n" );
break;
}
lt( events, ret, epollfd, listenfd );
//et( events, ret, epollfd, listenfd );
}
close( listenfd );
return 0;
}
编译两个程序：
gcc -o lt main.cpp
gcc -o et main.cpp
运行：
lt模式：
et/20180108163925729?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvcGFuZ3llbWVuZw==/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast" align="middle" alt="" />et/20180108163943195?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvcGFuZ3llbWVuZw==/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast" align="middle" alt="" />
两次事件触发。buffer中的数据做两次触发读出。
et模式：
et/20180108164020474?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvcGFuZ3llbWVuZw==/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast" align="middle" alt="" />
et/20180108164042034?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvcGFuZ3llbWVuZw==/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast" align="middle" alt="" />
仅触发了一次事件，将buffer中的数据全部读出。
