TCP 用select 监听带外数据(MSG_OOB)

        select 监听带外数据时,OOB数据会出现在exceptionSet中，但是如果设置了SO_OOBINLINE,OOB数据会出现在readSet中。

        OOB消息不会因为流量控制而影响它的传输，但是带外数据会。如果在select中用exceptionSet去监听OOB_data，只有等到带外数据到了，才会触发select exceptionSet
返回。这样实际上丢失带了OOB即时传送的这一个很重要的特点。测试代码如下：

发送端：

#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>

int main(int argc, char **argv)

{

    const char* ip = "192.168.1.99";

    int port = 65531 ;

    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, 0 );

    assert( sockfd >= 0 );

    if ( connect( sockfd, ( struct sockaddr* )&server_address, sizeof( server_address ) ) < 0 )

    {

        printf( "connection failed\n" );

    }

    int size=16384;

    setsockopt(sockfd,SOL_SOCKET,SO_SNDBUF,&size,sizeof(size));

    char buff[16384];

    memset(buff,1,16384);

    while(1)

    {

        int  n  =send(sockfd, buff,16384,0);

        printf("wrote %d bytes of normal data\n",n);

        sleep(1);

        send(sockfd, "a", 1, MSG_OOB);

        printf("wrote 1 byte of OOB data\n");

        sleep(1);

        n  =send(sockfd, buff,1024,0);

        return 0;

    }

    send(sockfd, "123", 3,0);

    printf("wrote 3 bytes of normal data\n");

    sleep(1);

    send(sockfd, "4", 1, MSG_OOB);

    printf("wrote 1 byte of OOB data\n");

    sleep(1);

    send(sockfd, "56", 2,0);

    printf("wrote 2 bytes of normal data\n");

    sleep(1);

    send(sockfd, "7", 1, MSG_OOB);

    printf("wrote 1 byte of OOB data\n");

    sleep(1);

    send(sockfd, "89", 2,0);

    printf("wrote 2 bytes of normal data\n");

    sleep(1);

    exit(0);

}

接收端：

#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>

int main(int argc, char * * argv)

{

    int n, justreadoob = 0; /* 声明了一个叫做justreadoob的变量来指示我们是否刚读过带外数据，这个标志决定是否select异常条件 */

    char buff[100]={0};

    fd_set rset, xset;

    const char* ip = "192.168.1.99";

    int port = 65531;

    printf( "ip is %s and port is %d\n", ip, port );

    int ret = 0;

    struct sockaddr_in address;//服务端地址

    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 );

    struct sockaddr_in client_address;//客户端地址

    socklen_t client_addrlength = sizeof( client_address );

    int size=4096;

    setsockopt(listenfd,SOL_SOCKET,SO_RCVBUF,&size,sizeof(size));

    int connfd = accept( listenfd, ( struct sockaddr* )&client_address, &client_addrlength );

    if ( connfd < 0 )

    {

        printf( "errno is: %d\n", errno );

        close( listenfd );

    }

    FD_ZERO(&rset);

    FD_ZERO(&xset);

    for( ; ; )

    {

        FD_SET(connfd, &rset);

        if(justreadoob == 0)

            FD_SET(connfd, &xset);

        select(connfd+1, &rset, NULL, &xset, NULL);

        memset(buff,0,100);

        if(FD_ISSET(connfd, &xset))

        {

            n = recv(connfd, buff, sizeof(buff)-1, MSG_OOB);

            buff[n] = 0; /* null terminate */

            printf("read %d OOB bytes: %s\n", n, buff);

            justreadoob = 1; /* 当我们设置justreadoob标志时，我们还必须清除这个描述字在异常集合中的那一位 */

            FD_CLR(connfd, &xset);

        }

        if(FD_ISSET(connfd, &rset))

        {

            n = recv(connfd, buff, sizeof(buff)-1,0) ;

            if( n  == 0 )

            {

                printf("received EOF\n");

                exit(0);

            }

            buff[n] = 0;  /* null terminate */

            printf("read %d bytes: %s \n", n, buff);

            justreadoob = 0;

        }

    }

}

上例中把接收端的接收缓存放小，保证发送的数据一定会寒满接收缓存，然后发一次OOB，测试的效果是接收端只有在收到OOB数据后才会触发Read操作，每次都能正确的读出OOB_Data，而不是返回一个EWOULDBLOCK。

所以如果用OOB写HeartBeat还是抛弃Select用直接信号或是直接recv的方式比较合理