accept函数与TCP三次握手

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盼盼编程 2021-06-04 16:00:04 ©著作权

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accept函数和TCP三次握手有没有关系？有什么关系？一段代码让你从此不在疑惑。

服务端：

#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <errno.h>
#include <malloc.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/ioctl.h>
#include <stdarg.h>
#include <fcntl.h>

int main()
{
    int sockSrv = socket(AF_INET, SOCK_STREAM, 0);
    struct sockaddr_in addrSrv;
    addrSrv.sin_family = AF_INET;
    addrSrv.sin_addr.s_addr =htonl(INADDR_ANY);
    addrSrv.sin_port = htons(8888);

    bind(sockSrv, (const struct sockaddr *)&addrSrv, sizeof(struct sockaddr_in));
    listen(sockSrv, 5);

    sockaddr_in addrClient;
    socklen_t len=sizeof(addrClient);

    getchar();
    int sockConn = accept(sockSrv, (struct sockaddr *)&addrClient, &len);
    printf("accept success\n");

    close(sockConn);
    close(sockSrv);

    return 0;
}

客户端：

#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <errno.h>
#include <malloc.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/ioctl.h>
#include <stdarg.h>
#include <fcntl.h>
#include <error.h>

int main()
{
     int ret=0;
     int sockClient = socket(AF_INET, SOCK_STREAM, 0);

     struct sockaddr_in addrSrv;
     addrSrv.sin_addr.s_addr=inet_addr("127.0.0.1");
     addrSrv.sin_family = AF_INET;
     addrSrv.sin_port = htons(8888);
     ret=connect(sockClient, ( const struct sockaddr *)&addrSrv, sizeof(struct sockaddr_in));

     getchar();
     close(sockClient);

     return 0;
}

在服务端代码中，我们在accept函数前面实用getchar()函数，使其不调用accept函数。编译并运行，然后用tcpdump抓包。

[root@localhost mapan]# tcpdump -iany port 8888 -nlps0
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on any, link-type LINUX_SLL (Linux cooked), capture size 65535 bytes
14:55:03.808416 IP 127.0.0.1.39920 > 127.0.0.1.ddi-tcp-1: Flags [S], seq 3756544861, win 65495, options [mss 65495,sackOK,TS val 2960369517 ecr 0,nop,wscale 7], length 0
14:55:03.808434 IP 127.0.0.1.ddi-tcp-1 > 127.0.0.1.39920: Flags [S.], seq 2739095114, ack 3756544862, win 65483, options [mss 65495,sackOK,TS val 2960369517 ecr 2960369517,nop,wscale 7], length 0
14:55:03.808451 IP 127.0.0.1.39920 > 127.0.0.1.ddi-tcp-1: Flags [.], ack 1, win 512, options [nop,nop,TS val 2960369517 ecr 2960369517], length 0

通过抓包的结果可以看出，三次握手已经完成，和accept函数没有啥关系。通过netstat命令也可以看到服务端和客户端都已经是ESTABLISHED