带外数据的接收与发送

带外数据比普通数据具有更高的优先级，TCP没有真正的带外数据，而是提供了一个我们要讨论的紧急模式，TCP将数据放置在套机口发送缓冲区的下一个可用位置，并设置这个连接的TCP紧急指针(urgent pointer)为下一个可用位置，TCP紧急指针有一个比用MSG_OOB标志写入的数据多一个字节的序列号。由于接收端的带外缓冲只有1B所以发送端发送的多字节带外数据只有最后1B被当做带外数据，若接收端TCP连接设置了SO_OOBINLINE选项则带外数据和普通数据一样存放于TCP接收缓冲区中。带外数据的发送与接收可以调用含有MSG_OOB标志的send和recv。sockatmark(int sockfd)可以判断sockfd下一个数据是否含有带外数据，若有就可以调用含有MSG_OOB标志的recv接收带外数据。接收带外数据的方式是带有MSG_OOB的recv接收，那么检测带外数据到来的方法：1 sockatmark、2 由于带外数据对于select返回是异常事件所以可以由select返回判断带外数据到来、3 通过SIG_URG信号处理函数

发送带外数据的客户端程序：

#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[] )
{
    if( argc <= 2 )
    {
        printf( "usage: %s ip_address port_number
", basename( argv[0] ) );
        return 1;
    }
    const char* ip = argv[1];
    int port = atoi( argv[2] );

    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
" );
    }
    else
    {
        printf( "send oob data out
" );
        const char* oob_data = "abc";
        const char* normal_data = "123";
        send( sockfd, normal_data, strlen( normal_data ), 0 );
        send( sockfd, oob_data, strlen( oob_data ), MSG_OOB );//MSG_OOB标志的send发送带外数据，注意带外数据，接收端的带外缓存只有1B所以只有c是真正的带外数据，同理带MSG_OOB标志的recv能接收带外数据
        send( sockfd, normal_data, strlen( normal_data ), 0 );//发送普通数据
    }

    close( sockfd );
    return 0;
}

带MSG_OOB标志的recv

#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <assert.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>

#define BUF_SIZE 1024

int main( int argc, char* argv[] )
{
    if( argc <= 2 )
    {
        printf( "usage: %s ip_address port_number
", basename( argv[0] ) );
        return 1;
    }
    const char* ip = argv[1];
    int port = atoi( argv[2] );

    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 sock = socket( PF_INET, SOCK_STREAM, 0 );
    assert( sock >= 0 );

    int ret = bind( sock, ( struct sockaddr* )&address, sizeof( address ) );
    assert( ret != -1 );

    ret = listen( sock, 5 );
    assert( ret != -1 );

    struct sockaddr_in client;
    socklen_t client_addrlength = sizeof( client );
    int connfd = accept( sock, ( struct sockaddr* )&client, &client_addrlength );
    if ( connfd < 0 )
    {
        printf( "errno is: %d
", errno );
    }
    else
    {
        char buffer[ BUF_SIZE ];

        memset( buffer, '', BUF_SIZE );
        ret = recv( connfd, buffer, BUF_SIZE-1, 0 );
        printf( "got %d bytes of normal data '%s'
", ret, buffer );

        memset( buffer, '', BUF_SIZE );
        ret = recv( connfd, buffer, BUF_SIZE-1, MSG_OOB );//带MSG_OOB标志的recv函数能接收带外数据
        printf( "got %d bytes of oob data '%s'
", ret, buffer );

        memset( buffer, '', BUF_SIZE );
        ret = recv( connfd, buffer, BUF_SIZE-1, 0 );
        printf( "got %d bytes of normal data '%s'
", ret, buffer );

        close( connfd );
    }

    close( sock );
    return 0;
}

通过select返回异常事件来检测带外数据到来

#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[] )
{
	if( argc <= 2 )
	{
		printf( "usage: %s ip_address port_number
", basename( argv[0] ) );
		return 1;
	}
	const char* ip = argv[1];
	int port = atoi( argv[2] );
	printf( "ip is %s and port is %d
", 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 connfd = accept( listenfd, ( struct sockaddr* )&client_address, &client_addrlength );
	if ( connfd < 0 )
	{
		printf( "errno is: %d
", errno );
		close( listenfd );
	}

	char remote_addr[INET_ADDRSTRLEN];
	//printf( "connected with ip: %s and port: %d
", inet_ntop( AF_INET, &client_address.sin_addr, remote_addr, INET_ADDRSTRLEN ), ntohs( client_address.sin_port ) );

	char buf[1024];
        fd_set read_fds;//可读事件集合
        fd_set exception_fds;//异常事件集合(含有带外数据的事件是异常事件)

        FD_ZERO( &read_fds );//事件集合清零
        FD_ZERO( &exception_fds );

        int nReuseAddr = 1;
	setsockopt( connfd, SOL_SOCKET, SO_OOBINLINE, &nReuseAddr, sizeof( nReuseAddr ) );//
	while( 1 )
	{
		memset( buf, '', sizeof( buf ) );
		FD_SET( connfd, &read_fds );//将文件描述符connfd和可读事件集合绑定,由于select内核在线修改事件集合以通知应用程序就绪事件，所以下次select调用前都需要重新绑定
		FD_SET( connfd, &exception_fds );

        	ret = select( connfd + 1, &read_fds, NULL, &exception_fds, NULL );//select无限期等待事件就绪
		//printf( "select one
" );
        	if ( ret < 0 )
        	{
                	printf( "selection failure
" );
                	break;
        	}

        	if ( FD_ISSET( connfd, &read_fds ) )//处理可读事件，这里表示客户端发送普通数据到来
		{
        		ret = recv( connfd, buf, sizeof( buf )-1, 0 );
			if( ret <= 0 )
			{
				break;
			}
			printf( "get %d bytes of normal data: %s
", ret, buf );
		}
		else if( FD_ISSET( connfd, &exception_fds ) )//处理异常事件，这里表示客户端发送带外数据到来
        	{
        		ret = recv( connfd, buf, sizeof( buf )-1, MSG_OOB );//MSG_OOB标志
			if( ret <= 0 )
			{
				break;
			}
			printf( "get %d bytes of oob data: %s
", ret, buf );
        	}

	}

	close( connfd );
	close( listenfd );
	return 0;
}

采用SIGURG信号检测带外数据的到来

#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <assert.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <signal.h>
#include <fcntl.h>

#define BUF_SIZE 1024//缓冲区大小

static int connfd;

void sig_urg( int sig )//信号处理函数(SIGURG信号处理带外数据)
{
    int save_errno = errno;
    
    char buffer[ BUF_SIZE ];
    memset( buffer, '', BUF_SIZE );
    int ret = recv( connfd, buffer, BUF_SIZE-1, MSG_OOB );//接收带外数据
    printf( "got %d bytes of oob data '%s'
", ret, buffer );//输出带外数据

    errno = save_errno;
}

void addsig( int sig, void ( *sig_handler )( int ) )//信号安装
{
    struct sigaction sa;//sigaction结构体
    memset( &sa, '', sizeof( sa ) );
    sa.sa_handler = sig_handler;
    sa.sa_flags |= SA_RESTART;//被信号中断的系统调用自动重启
    sigfillset( &sa.sa_mask );//设置全部信号为进程信号掩码
    assert( sigaction( sig, &sa, NULL ) != -1 );//信号处理
}

int main( int argc, char* argv[] )
{
    if( argc <= 2 )
    {
        printf( "usage: %s ip_address port_number
", basename( argv[0] ) );
        return 1;
    }
    const char* ip = argv[1];
    int port = atoi( argv[2] );

    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 sock = socket( PF_INET, SOCK_STREAM, 0 );
    assert( sock >= 0 );

    int ret = bind( sock, ( struct sockaddr* )&address, sizeof( address ) );
    assert( ret != -1 );

    ret = listen( sock, 5 );
    assert( ret != -1 );

    struct sockaddr_in client;//客户端地址
    socklen_t client_addrlength = sizeof( client );
    connfd = accept( sock, ( struct sockaddr* )&client, &client_addrlength );//建立客户端连接
    if ( connfd < 0 )
    {
        printf( "errno is: %d
", errno );
    }
    else
    {
        addsig( SIGURG, sig_urg );//添加SIGURG信号
        fcntl( connfd, F_SETOWN, getpid() );//SIGURG的前提条件是进程必须持有文件描述符connfd

        char buffer[ BUF_SIZE ];
        while( 1 )
        {
            memset( buffer, '', BUF_SIZE );
            ret = recv( connfd, buffer, BUF_SIZE-1, 0 );//接收客户端发送来的数据，若有带外数据到来则激活SIGURG信号处理函数
            if( ret <= 0 )
            {
                break;
            }
            printf( "got %d bytes of normal data '%s'
", ret, buffer );
        }

        close( connfd );
    }

    close( sock );
    return 0;
}