使用splice实现高效的代理服务器

很多网络应用场景下， 当原设备与目标设备无法直接建立连接时，这时就需要一台代理服务器进行中转。代理服务器只需要将来自源设备的报文 原封不动的转发给目标设备，而并不需要知道报文的具体内容。在这种情况下，服务器就没必要将客户端的报文读出来。每读取一次都需要从内核态到用户态的一次拷贝，在流量密集型的应用中，这样做显然很消耗服务器性能。而splice就是为了解决这个问题，它实现了不同文件描述符之间的0拷贝操作。关于splice函数的描述如下：

ssize_t splice(int fd_in, loff_t* off_in,int fd_out, loff_t* off_out, size_t len, unsigned int flags);

fd_in参数是带输入的文件描述符

off_in表示从输入数据流的何处开始读取数据，如果fd_in是管道文件描述符，则必须设置为NULL，表示从0开始

fd_out/off_out 表示输出的文件描述符与偏移

len表示要读取的长度

flags 标志，常用的两个标志SPLICE_F_NONBLOCK，表示非阻塞的splice操作，但要依赖于文件描述符本身的阻塞状态；SPLICE_F_MORE表示给内核一个提示，后续splice调用将读取更多数据

使用splice时，fd_in于fd_out必须至少有一个是管道文件描述符

下面给出一个demo，实现两个客户端之间的报文透传

服务器代码：

#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 connfd[] = {};

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 = ; i < number; i++ )
    {
        int sockfd = events[i].data.fd;
        if ( sockfd == listenfd )
        {
            static int conntimes = ;
            struct sockaddr_in client_address;
            socklen_t client_addrlength = sizeof( client_address );
            int fd = accept( listenfd, ( struct sockaddr* )&client_address, &client_addrlength );
            addfd( epollfd, fd, false );
            if(conntimes <= )
            {
                connfd[conntimes] = fd;
            }
            conntimes++;
            printf("new conn, %d\n",fd);
        }
        else if ( events[i].events & EPOLLIN  )
        {
            int pipefd[];
            pipe(pipefd);
            if(sockfd == connfd[])
            {
                splice(connfd[], NULL, pipefd[], NULL, ,SPLICE_F_MORE);
                splice(pipefd[], NULL, connfd[], NULL, ,SPLICE_F_MORE);
            }
            else if(sockfd == connfd[])
            {
                splice(connfd[], NULL, pipefd[], NULL, ,SPLICE_F_MORE);
                splice(pipefd[], NULL, connfd[], NULL, ,SPLICE_F_MORE);
            }
            else
            {
                int recvlen =  read(sockfd,buf,BUFFER_SIZE-);
                buf[recvlen] = ;
                printf("recv buf : %s\n",buf);
            }
            close(pipefd[]);
            close(pipefd[]);
        }
        else
        {
            printf( "something else happened \n" );
        }
    }
}

int main( int argc, char* argv[] )
{
    if( argc <=  )
    {
        printf( "usage: %s ip_address port_number\n", basename( argv[] ) );
        return ;
    }
    const char* ip = argv[];
    int port = atoi( argv[] );

    int ret = ;
    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,  );
    assert( listenfd >=  );

    ret = bind( listenfd, ( struct sockaddr* )&address, sizeof( address ) );
    assert( ret != - );

    ret = listen( listenfd,  );
    assert( ret != - );

    epoll_event events[ MAX_EVENT_NUMBER ];
    int epollfd = epoll_create(  );
    assert( epollfd != - );
    addfd( epollfd, listenfd, true );

    while(  )
    {
        int ret = epoll_wait( epollfd, events, MAX_EVENT_NUMBER, - );
        if ( ret <  )
        {
            printf( "epoll failure\n" );
            break;
        }
        lt( events, ret, epollfd, listenfd );
    }
    close( listenfd );
    return ;
}

客户端代码：

#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 <string.h>
#include <stdlib.h>
#include <poll.h>
#include <fcntl.h>

int connectserver(const char *ip, int port)
{
    int rcv_size = ;
    size_t optlen = sizeof(int);
    struct sockaddr_in server_address;
    bzero( &server_address, sizeof( server_address ) );
    server_address.sin_family = AF_INET;
    inet_pton( AF_INET, ip, &server_address.sin_addr );
    server_address.sin_port = htons( port );
    int sockfd = socket( PF_INET, SOCK_STREAM,  );
    assert( sockfd >=  );
    if ( connect( sockfd, ( struct sockaddr* )&server_address, sizeof( server_address ) ) <  )
    {
        printf( "connection failed\n" );
        close( sockfd );
        return -;
    }
    return sockfd;
}

int main( int argc, char* argv[] )
{
    if( argc <=  )
    {
        printf( "usage: %s ip_address port_number\n",  argv[]  );
        return ;
    }
    const char* ip = argv[];
    int port = atoi( argv[] );
    int pipefd[];
    pipe(pipefd);
    int sockfd = connectserver(ip,port);
    assert(sockfd >= );
    fd_set readsets;
    FD_ZERO(&readsets);
    while()
    {
        FD_SET(STDIN_FILENO,&readsets);
        FD_SET(sockfd, &readsets);
        int ret = select(sockfd+ , &readsets,NULL,NULL,NULL);
        assert(ret >= );
        if(FD_ISSET(STDIN_FILENO, &readsets))
        {
            splice(STDIN_FILENO, NULL, pipefd[], NULL, ,SPLICE_F_MORE);
            splice(pipefd[], NULL, sockfd, NULL, ,SPLICE_F_MORE);
        }
        else if(FD_ISSET(sockfd,&readsets))
        {
            splice(sockfd, NULL, pipefd[], NULL, ,SPLICE_F_MORE);
            splice(pipefd[], NULL, STDOUT_FILENO, NULL, ,SPLICE_F_MORE);
        }
    }
    close(pipefd[]);
    close(pipefd[]);
    close(sockfd);
    return ;
}