Linux学习： TCP粘包问题

TCP协议下：

当发送数据过长过短，或缓冲区大小问题，导致出现了所谓的 TCP“粘包”问题，这是我们的俗称， TCP是流模式，并不是包；

现象解释：

TCP粘包是指发送方发送的若干包数据到接收方接收时粘成一包，从接收缓冲区看，后一包数据的头紧接着前一包数据的尾。
出现粘包现象的原因是多方面的，它既可能由发送方造成，也可能由接收方造成。发送方引起的粘包是由TCP协议本身造成的，TCP为提高传输效率，发送方往往要收集到足够多的数据后才发送一包数据。若连续几次发送的数据都很少，通常TCP会根据优化算法把这些数据合成一包后一次发送出去，这样接收方就收到了粘包数据。接收方引起的粘包是由于接收方用户进程不及时接收数据，从而导致粘包现象。这是因为接收方先把收到的数据放在系统接收缓冲区，用户进程从该缓冲区取数据，若下一包数据到达时前一包数据尚未被用户进程取走，则下一包数据放到系统接收缓冲区时就接到前一包数据之后，而用户进程根据预先设定的缓冲区大小从系统接收缓冲区取数据，这样就一次取到了多包数据。

好了，根据上述的理论我们自己人为制造一起 ”粘包“

server

 1 #include <stdio.h>
 2 #include <stdlib.h>
 3 #include <string.h>
 4 #include <unistd.h>
 5 #include <errno.h>
 6 #include <sys/types.h>
 7 #include <sys/socket.h>
 8 #include <netinet/in.h>
 9 #include <arpa/inet.h>
10 #define ERR_EXIT(m) 
11     do { 
12         perror(m);
13         exit(EXIT_FAILURE);
14     }while(0)
15 
16 void do_service(int sockfd);
17 
18 int main(int argc, const char *argv[])
19 {
20     int listenfd = socket(PF_INET, SOCK_STREAM, 0);
21     if(listenfd == -1)
22         ERR_EXIT("socket");
23 
24     //地址复用
25     int on = 1;
26     if (setsockopt(listenfd, SOL_SOCKET, SO_REUSEADDR, &on, sizeof(on)) < 0)
27         ERR_EXIT("setsockopt");
28 
29     struct sockaddr_in addr;
30     memset(&addr, 0, sizeof addr);
31     addr.sin_family = AF_INET;
32     addr.sin_addr.s_addr = inet_addr("127.0.0.1");
33     addr.sin_port = htons(8976);
34     if(bind(listenfd, (struct sockaddr*)&addr, sizeof addr) == -1)
35         ERR_EXIT("bind");
36 
37     if(listen(listenfd, SOMAXCONN) == -1)
38         ERR_EXIT("listen");
39 
40     int peerfd = accept(listenfd, NULL, NULL);
41     do_service(peerfd);
42 
43     close(peerfd);
44     close(listenfd);
45 
46     return 0;
47 }
48 
49 
50 
51 void do_service(int sockfd)
52 {
53     int cnt = 0;
54     char recvbuf[1024000] = {0};
55     while(1)
56     {
57         int nread = read(sockfd, recvbuf, sizeof recvbuf);
58         if(nread == -1)
59         {
60             if(errno == EINTR)
61                 continue;
62             ERR_EXIT("read");
63         }
64         else if(nread == 0)
65         {
66             printf("close ...
");
67             exit(EXIT_SUCCESS);
68         }
69 
70         printf("count = %d, receive size = %d
", ++cnt, nread);
71         //write(sockfd, recvbuf, strlen(recvbuf));
72         memset(recvbuf, 0, sizeof recvbuf);
73     }
74 }

注意， server端的接收缓冲区应该足够大，否则无法接收 “黏在一块的数据包”

client端

 1 #include <stdio.h>
 2 #include <stdlib.h>
 3 #include <string.h>
 4 #include <unistd.h>
 5 #include <errno.h>
 6 #include <sys/types.h>
 7 #include <sys/socket.h>
 8 #include <netinet/in.h>
 9 #include <arpa/inet.h>
10 #define ERR_EXIT(m) 
11     do { 
12         perror(m);
13         exit(EXIT_FAILURE);
14     }while(0)
15 
16 void do_service(int sockfd);
17 void nano_sleep(double val);
18 
19 int main(int argc, const char *argv[])
20 {
21     int peerfd = socket(PF_INET, SOCK_STREAM, 0);
22     if(peerfd == -1)
23         ERR_EXIT("socket");
24 
25     struct sockaddr_in addr;
26     memset(&addr, 0, sizeof addr);
27     addr.sin_family = AF_INET;
28     addr.sin_addr.s_addr = inet_addr("127.0.0.1"); //localhost
29     addr.sin_port = htons(8976);
30     socklen_t len = sizeof addr;
31     if(connect(peerfd, (struct sockaddr*)&addr, len) == -1)
32         ERR_EXIT("Connect");
33 
34     do_service(peerfd);
35 
36 
37     return 0;
38 }
39 
40 
41 
42 void do_service(int sockfd)
43 {
44     //const int kSize = 1024;
45     #define SIZE 1024
46     char sendbuf[SIZE + 1] = {0};
47     int i;
48     for(i = 0; i < SIZE; ++i)
49         sendbuf[i] = 'a';
50 
51     int cnt = 0; //次数
52     while(1)
53     {
54         int i;
55         for(i = 0; i < 10; ++i)
56         {
57             write(sockfd, sendbuf, SIZE);
58             printf("count = %d, write %d bytes
", ++cnt, SIZE);
59         }
60         nano_sleep(4);
61 
62         memset(sendbuf, 0, sizeof sendbuf);
63     }
64 }
65 
66 void nano_sleep(double val)
67 {
68     struct timespec tv;
69     tv.tv_sec = val; //取整
70     tv.tv_nsec = (val - tv.tv_sec) * 1000 * 1000 * 1000;
71 
72     int ret;
73     do
74     {
75         ret = nanosleep(&tv, &tv);
76     }while(ret == -1 && errno == EINTR);
77 }

客户端应该短时间发送大量的数据，使server端处理接收时造成粘包；

可以看到我们连续发送了 10次长度为1024 的全是a的字符串；看下server端打印如何

count = 1, receive size = 1024
count = 2, receive size = 1024
count = 3, receive size = 1024
count = 4, receive size = 1024
count = 5, receive size = 1024
count = 6, receive size = 5120
count = 7, receive size = 10240
count = 8, receive size = 10240
count = 9, receive size = 10240

可以看到，当第6次读取时便出现了粘包；数据出现了相连的问题；

而我们的客户端是均匀的每次发送1024字节的数据

count = 1, write 1024 bytes
count = 2, write 1024 bytes
count = 3, write 1024 bytes
count = 4, write 1024 bytes
count = 5, write 1024 bytes
count = 6, write 1024 bytes
count = 7, write 1024 bytes
count = 8, write 1024 bytes
count = 9, write 1024 bytes
count = 10, write 1024 bytes
count = 11, write 1024 bytes
count = 12, write 1024 bytes
count = 13, write 1024 bytes
count = 14, write 1024 bytes
count = 15, write 1024 bytes
count = 16, write 1024 bytes
count = 17, write 1024 bytes
count = 18, write 1024 bytes
count = 19, write 1024 bytes
count = 20, write 1024 bytes
count = 21, write 1024 bytes
count = 22, write 1024 bytes
count = 23, write 1024 bytes
count = 24, write 1024 bytes
count = 25, write 1024 bytes
count = 26, write 1024 bytes
count = 27, write 1024 bytes
count = 28, write 1024 bytes
count = 29, write 1024 bytes
count = 30, write 1024 bytes
count = 31, write 1024 bytes
count = 32, write 1024 bytes
count = 33, write 1024 bytes
count = 34, write 1024 bytes
count = 35, write 1024 bytes
count = 36, write 1024 bytes
count = 37, write 1024 bytes
count = 38, write 1024 bytes
count = 39, write 1024 bytes
count = 40, write 1024 bytes

显然不是我们发送数据时造成的问题，而是TCP本身的缺陷。

下面有两种解决“粘包的问题” 我们来介绍一下

1. 每当我们发送数据时，先行将4个字节的将要发送的数据的长度信息发送过去

同理，通过约定，接收方也先行接收长度信息，按照长度信息来接收后面的字节流；这样可以防止数据粘包的问题；

server端

 1 #include <stdio.h>
 2 #include <stdlib.h>
 3 #include <string.h>
 4 #include <unistd.h>
 5 #include <errno.h>
 6 #include <sys/types.h>
 7 #include <sys/socket.h>
 8 #include <netinet/in.h>
 9 #include <arpa/inet.h>
10 #include "sysutil.h"
11 #define ERR_EXIT(m) 
12     do { 
13         perror(m);
14         exit(EXIT_FAILURE);
15     }while(0)
16 
17 void do_service(int sockfd);
18 
19 int main(int argc, const char *argv[])
20 {
21     int listenfd = socket(PF_INET, SOCK_STREAM, 0);
22     if(listenfd == -1)
23         ERR_EXIT("socket");
24 
25     //地址复用
26     int on = 1;
27     if (setsockopt(listenfd, SOL_SOCKET, SO_REUSEADDR, &on, sizeof(on)) < 0)
28         ERR_EXIT("setsockopt");
29 
30     struct sockaddr_in addr;
31     memset(&addr, 0, sizeof addr);
32     addr.sin_family = AF_INET;
33     addr.sin_addr.s_addr = inet_addr("127.0.0.1");
34     addr.sin_port = htons(8976);
35     if(bind(listenfd, (struct sockaddr*)&addr, sizeof addr) == -1)
36         ERR_EXIT("bind");
37 
38     if(listen(listenfd, SOMAXCONN) == -1)
39         ERR_EXIT("listen");
40 
41     int peerfd = accept(listenfd, NULL, NULL);
42     do_service(peerfd);
43 
44     close(peerfd);
45     close(listenfd);
46 
47     return 0;
48 }
49 
50 
51 
52 void do_service(int sockfd)
53 {
54     int cnt = 0;
55     char recvbuf[1024000] = {0};
56     while(1)
57     {
58         //先接收报文长度
59         int32_t len = recv_int32(sockfd);
60         //接收len长度的报文
61         int nread = readn(sockfd, recvbuf, len);
62         if(nread == -1)
63             ERR_EXIT("readn");
64         else if(nread == 0 || nread < len)
65         {
66             printf("client close ....
");
67             exit(EXIT_FAILURE);
68         }
69 
70         printf("count = %d, receive size = %d
", ++cnt, nread);
71         //write(sockfd, recvbuf, strlen(recvbuf));
72         memset(recvbuf, 0, sizeof recvbuf);
73     }
74 }

client端

 1 #include <stdio.h>
 2 #include <stdlib.h>
 3 #include <string.h>
 4 #include <unistd.h>
 5 #include <errno.h>
 6 #include <sys/types.h>
 7 #include <sys/socket.h>
 8 #include <netinet/in.h>
 9 #include <arpa/inet.h>
10 #include "sysutil.h"
11 #define ERR_EXIT(m) 
12     do { 
13         perror(m);
14         exit(EXIT_FAILURE);
15     }while(0)
16 
17 void do_service(int sockfd);
18 
19 int main(int argc, const char *argv[])
20 {
21     int listenfd = socket(PF_INET, SOCK_STREAM, 0);
22     if(listenfd == -1)
23         ERR_EXIT("socket");
24 
25     //地址复用
26     int on = 1;
27     if (setsockopt(listenfd, SOL_SOCKET, SO_REUSEADDR, &on, sizeof(on)) < 0)
28         ERR_EXIT("setsockopt");
29 
30     struct sockaddr_in addr;
31     memset(&addr, 0, sizeof addr);
32     addr.sin_family = AF_INET;
33     addr.sin_addr.s_addr = inet_addr("127.0.0.1");
34     addr.sin_port = htons(8976);
35     if(bind(listenfd, (struct sockaddr*)&addr, sizeof addr) == -1)
36         ERR_EXIT("bind");
37 
38     if(listen(listenfd, SOMAXCONN) == -1)
39         ERR_EXIT("listen");
40 
41     int peerfd = accept(listenfd, NULL, NULL);
42     do_service(peerfd);
43 
44     close(peerfd);
45     close(listenfd);
46 
47     return 0;
48 }
49 
50 
51 
52 void do_service(int sockfd)
53 {
54     int cnt = 0;
55     char recvbuf[1024000] = {0};
56     while(1)
57     {
58         //先接收报文长度
59         int32_t len = recv_int32(sockfd);
60         //接收len长度的报文
61         int nread = readn(sockfd, recvbuf, len);
62         if(nread == -1)
63             ERR_EXIT("readn");
64         else if(nread == 0 || nread < len)
65         {
66             printf("client close ....
");
67             exit(EXIT_FAILURE);
68         }
69 
70         printf("count = %d, receive size = %d
", ++cnt, nread);
71         //write(sockfd, recvbuf, strlen(recvbuf));
72         memset(recvbuf, 0, sizeof recvbuf);
73     }
74 }

这种方式的关键是在收发送数据前的 send_int32 和 recv_int32 用于发收 4字节长度的数据长度信息

相当于发送方先告诉收方，我要发送多长的信息，你按照这个长度收，这样每条信息之间就会条理清晰不至于“粘包”

两个函数代码如下 (原理相当简答，不过是一个包装过的writenn 和readn)

 1 void send_int32(int sockfd, int32_t val)
 2 {
 3     //先转化为网络字节序
 4     int32_t tmp = htonl(val);
 5     if(writen(sockfd, &tmp, sizeof(int32_t)) != sizeof(int32_t))
 6         ERR_EXIT("send_int32");
 7 }
 8 
 9 int32_t recv_int32(int sockfd)
10 {
11     int32_t tmp;
12     if(readn(sockfd, &tmp, sizeof(int32_t)) != sizeof(int32_t))
13         ERR_EXIT("recv_int32");
14     return ntohl(tmp); //转化为主机字节序
15 }

2. 另外一种防止粘包的处理方式更加简答，通过以当做每条信息之间的标志；

处理方式在逻辑上更加明了，事实上各大网络公司也是通过这种方式处理粘包问题的

下面只用修改几行代码即可

把 server端和 client 端中的 do_service逻辑稍加修改即可

client 每次发送的数据缓冲区末尾加一个做标示

void do_service(int sockfd)
{
    //const int kSize = 1024;
    #define SIZE 1024
    char sendbuf[SIZE + 1] = {0};
    int i;
    for(i = 0; i < SIZE-1; ++i)
        sendbuf[i] = 'a';
    sendbuf[SIZE - 1] = '
';
    // aaaaaa....aaaaa


    int cnt = 0; //次数
    while(1)
    {
        int i;
        for(i = 0; i < 10; ++i)
        {
            //write(sockfd, sendbuf, SIZE);
            //我们每次发送的报文均以
作为结尾
            if(writen(sockfd, sendbuf, SIZE) != SIZE)
                ERR_EXIT("writen");
            
            printf("count = %d, write %d bytes
", ++cnt, SIZE);
        }
        nano_sleep(4);

        //memset(sendbuf, 0, sizeof sendbuf);
    }
}

server用 readline即可因为readline 遇到便返回了。

 1 void do_service(int sockfd)
 2 {
 3     int cnt = 0;
 4     char recvbuf[1024000] = {0};
 5     while(1)
 6     {
 7         int nread = readline(sockfd, recvbuf, sizeof recvbuf);
 8         if(nread == -1)
 9             ERR_EXIT("readn");
10         else if(nread == 0)
11         {
12             printf("client close ....
");
13             exit(EXIT_FAILURE);
14         }
15 
16         printf("count = %d, receive size = %d
", ++cnt, nread);
17         //write(sockfd, recvbuf, strlen(recvbuf));
18         memset(recvbuf, 0, sizeof recvbuf);
19     }
20 }

以上代码均通过测试无误，暂时解决了粘包问题，上述代码依然存在缺陷， readline的效率问题。