SOCKET网络编程快速上手（二）——细节问题（2）
2.TCP数据包接收问题
对初学者来说，很多都会认为：客户端与服务器最终的打印数据接收或者发送条数都该是一致的，1000条发送打印，1000条接收打印，长度都为1000。但是，事实上并不是这样，发送打印基本不会有什么问题（只是一般情况，如果发生调度或者其他情况，有可能导致差别，因此也要注意封装），接收打印却不是固定的，下面是测试代码：
测试客户端程序：
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#define PORT 1234
#define MAXDATASIZE 1000
int main(int argc, char *argv[])
{
int sockfd, num;
char buf[MAXDATASIZE + 1] = {0};
struct sockaddr_in server;
int iCount = 0;
if (argc != 2)
{
printf("Usage:%s <IP Address>\n", argv[0]);
exit(1);
}
if ((sockfd=socket(AF_INET, SOCK_STREAM, 0)) == -1)
{
printf("socket()error\n");
exit(1);
}
bzero(&server, sizeof(server));
server.sin_family = AF_INET;
server.sin_port = htons(PORT);
server.sin_addr.s_addr = inet_addr(argv[1]);
if (connect(sockfd, (struct sockaddr *)&server, sizeof(server)) == -1)
{
printf("connect()error\n");
exit(1);
}
while (1)
{
memset(buf, 0, sizeof(buf));
if ((num = recv(sockfd, buf, MAXDATASIZE,0)) == -1)
{
printf("recv() error\n");
exit(1);
}
buf[num - 1]='\0';
printf("%dth Recv Length: %d\n", iCount++, num);
}
close(sockfd);
return 0;
}
TCP客户端
1 #include <stdio.h>
2 #include <stdlib.h>
3 #include <unistd.h>
4 #include <string.h>
5 #include <sys/types.h>
6 #include <sys/socket.h>
7 #include <netinet/in.h>
8 #include <netdb.h>
9
10 #define PORT 1234
11 #define MAXDATASIZE 1000
12
13 int main(int argc, char *argv[])
14 {
15 int sockfd, num;
16 char buf[MAXDATASIZE + 1] = {0};
17 struct sockaddr_in server;
18 int iCount = 0;
19
20 if (argc != 2)
21 {
22 printf("Usage:%s <IP Address>\n", argv[0]);
23 exit(1);
24 }
25
26 if ((sockfd=socket(AF_INET, SOCK_STREAM, 0)) == -1)
27 {
28 printf("socket()error\n");
29 exit(1);
30 }
31 bzero(&server, sizeof(server));
32 server.sin_family = AF_INET;
33 server.sin_port = htons(PORT);
34 server.sin_addr.s_addr = inet_addr(argv[1]);
35 if (connect(sockfd, (struct sockaddr *)&server, sizeof(server)) == -1)
36 {
37 printf("connect()error\n");
38 exit(1);
39 }
40
41 while (1)
42 {
43 memset(buf, 0, sizeof(buf));
44 if ((num = recv(sockfd, buf, MAXDATASIZE,0)) == -1)
45 {
46 printf("recv() error\n");
47 exit(1);
48 }
49 buf[num - 1]='\0';
50 printf("%dth Recv Length: %d\n", iCount++, num);
51 }
52
53 close(sockfd);
54
55 return 0;
56 }
测试服务器程序：
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <signal.h>
#define PORT 1234
#define BACKLOG 5
#define MAXDATASIZE 1000
int main()
{
int listenfd, connectfd;
struct sockaddr_in server;
struct sockaddr_in client;
socklen_t addrlen;
char szbuf[MAXDATASIZE] = {0};
int iCount = 0;
int iLength = 0;
if ((listenfd = socket(AF_INET, SOCK_STREAM, 0)) == -1)
{
perror("Creating socket failed.");
exit(1);
}
int opt = SO_REUSEADDR;
setsockopt(listenfd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt));
bzero(&server, sizeof(server));
server.sin_family = AF_INET;
server.sin_port = htons(PORT);
server.sin_addr.s_addr = htonl(INADDR_ANY);
if (bind(listenfd, (struct sockaddr *)&server, sizeof(server)) == -1)
{
perror("Bind()error.");
exit(1);
}
if (listen(listenfd, BACKLOG) == -1)
{
perror("listen()error\n");
exit(1);
}
addrlen = sizeof(client);
if ((connectfd = accept(listenfd, (struct sockaddr*)&client, &addrlen)) == -1)
{
perror("accept()error\n");
exit(1);
}
printf("You got a connection from cient's ip is %s, prot is %d\n", inet_ntoa(client.sin_addr), htons(client.sin_port));
memset(szbuf, 'a', sizeof(szbuf));
while (iCount < 1000)
{
iLength = send(connectfd, szbuf, sizeof(szbuf), 0);
printf("%dth Server Send Length %d\n", iCount++, iLength);
}
printf("send over!\n");
sleep(10);
close(connectfd);
close(listenfd);
return 0;
}
TCP服务器程序
1 #include <stdio.h>
2 #include <stdlib.h>
3 #include <string.h>
4 #include <unistd.h>
5 #include <sys/types.h>
6 #include <sys/socket.h>
7 #include <netinet/in.h>
8 #include <arpa/inet.h>
9 #include <signal.h>
10
11 #define PORT 1234
12 #define BACKLOG 5
13 #define MAXDATASIZE 1000
14
15 int main()
16 {
17 int listenfd, connectfd;
18 struct sockaddr_in server;
19 struct sockaddr_in client;
20 socklen_t addrlen;
21 char szbuf[MAXDATASIZE] = {0};
22 int iCount = 0;
23 int iLength = 0;
24
25 if ((listenfd = socket(AF_INET, SOCK_STREAM, 0)) == -1)
26 {
27 perror("Creating socket failed.");
28 exit(1);
29 }
30
31 int opt = SO_REUSEADDR;
32 setsockopt(listenfd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt));
33
34 bzero(&server, sizeof(server));
35 server.sin_family = AF_INET;
36 server.sin_port = htons(PORT);
37 server.sin_addr.s_addr = htonl(INADDR_ANY);
38 if (bind(listenfd, (struct sockaddr *)&server, sizeof(server)) == -1)
39 {
40 perror("Bind()error.");
41 exit(1);
42 }
43 if (listen(listenfd, BACKLOG) == -1)
44 {
45 perror("listen()error\n");
46 exit(1);
47 }
48
49 addrlen = sizeof(client);
50 if ((connectfd = accept(listenfd, (struct sockaddr*)&client, &addrlen)) == -1)
51 {
52 perror("accept()error\n");
53 exit(1);
54 }
55 printf("You got a connection from cient's ip is %s, prot is %d\n", inet_ntoa(client.sin_addr), htons(client.sin_port));
56
57 memset(szbuf, 'a', sizeof(szbuf));
58 while (iCount < 1000)
59 {
60 iLength = send(connectfd, szbuf, sizeof(szbuf), 0);
61 printf("%dth Server Send Length %d\n", iCount++, iLength);
62 }
63
64 printf("send over!\n");
65 sleep(10);
66
67 close(connectfd);
68 close(listenfd);
69
70 return 0;
71 }
客户端接收打印片段如下：
936th Recv Length: 1000
937th Recv Length: 1000
938th Recv Length: 1000
939th Recv Length: 1000
940th Recv Length: 1000
941th Recv Length: 1000
942th Recv Length: 384
943th Recv Length: 616
944th Recv Length: 1000
945th Recv Length: 1000
946th Recv Length: 1000
947th Recv Length: 1000
948th Recv Length: 1000
949th Recv Length: 1000
950th Recv Length: 1000
951th Recv Length: 1000
952th Recv Length: 1000
953th Recv Length: 1000
954th Recv Length: 1000
955th Recv Length: 1000
956th Recv Length: 1000
957th Recv Length: 1000
958th Recv Length: 1000
959th Recv Length: 1000
960th Recv Length: 1000
961th Recv Length: 1000
962th Recv Length: 384
963th Recv Length: 616
964th Recv Length: 1000
965th Recv Length: 1000
966th Recv Length: 1000
967th Recv Length: 1000
968th Recv Length: 1000
969th Recv Length: 1000
970th Recv Length: 1000
971th Recv Length: 1000
972th Recv Length: 1000
973th Recv Length: 1000
974th Recv Length: 1000
975th Recv Length: 1000
976th Recv Length: 1000
977th Recv Length: 1000
978th Recv Length: 1000
979th Recv Length: 1000
980th Recv Length: 1000
981th Recv Length: 1000
982th Recv Length: 384
983th Recv Length: 616
984th Recv Length: 1000
985th Recv Length: 1000
986th Recv Length: 1000
987th Recv Length: 1000
988th Recv Length: 1000
989th Recv Length: 1000
990th Recv Length: 1000
991th Recv Length: 1000
992th Recv Length: 1000
993th Recv Length: 1000
994th Recv Length: 1000
995th Recv Length: 1000
996th Recv Length: 1000
997th Recv Length: 1000
998th Recv Length: 1000
999th Recv Length: 1000
1000th Recv Length: 1000
1001th Recv Length: 1000
1002th Recv Length: 384
1003th Recv Length: 616
1004th Recv Length: 1000
1005th Recv Length: 1000
1006th Recv Length: 1000
1007th Recv Length: 1000
1008th Recv Length: 1000
1009th Recv Length: 1000
1010th Recv Length: 1000
1011th Recv Length: 1000
1012th Recv Length: 1000
1013th Recv Length: 1000
1014th Recv Length: 1000
1015th Recv Length: 1000
1016th Recv Length: 1000
1017th Recv Length: 1000
1018th Recv Length: 1000
1019th Recv Length: 1000
1020th Recv Length: 1000
1021th Recv Length: 1000
1022th Recv Length: 384
1023th Recv Length: 616
1024th Recv Length: 1000
1025th Recv Length: 1000
1026th Recv Length: 1000
1027th Recv Length: 1000
1028th Recv Length: 1000
1029th Recv Length: 1000
1030th Recv Length: 1000
1031th Recv Length: 1000
1032th Recv Length: 1000
1033th Recv Length: 1000
1034th Recv Length: 1000
1035th Recv Length: 1000
1036th Recv Length: 1000
1037th Recv Length: 1000
1038th Recv Length: 1000
1039th Recv Length: 1000
1040th Recv Length: 1000
1041th Recv Length: 1000
1042th Recv Length: 384
1043th Recv Length: 616
1044th Recv Length: 1000
1045th Recv Length: 1000
1046th Recv Length: 1000
1047th Recv Length: 1000
1048th Recv Length: 1000
1049th Recv Length: 1000
1050th Recv Length: 1000
客户端接收打印片段
1 936th Recv Length: 1000
2 937th Recv Length: 1000
3 938th Recv Length: 1000
4 939th Recv Length: 1000
5 940th Recv Length: 1000
6 941th Recv Length: 1000
7 942th Recv Length: 384
8 943th Recv Length: 616
9 944th Recv Length: 1000
10 945th Recv Length: 1000
11 946th Recv Length: 1000
12 947th Recv Length: 1000
13 948th Recv Length: 1000
14 949th Recv Length: 1000
15 950th Recv Length: 1000
16 951th Recv Length: 1000
17 952th Recv Length: 1000
18 953th Recv Length: 1000
19 954th Recv Length: 1000
20 955th Recv Length: 1000
21 956th Recv Length: 1000
22 957th Recv Length: 1000
23 958th Recv Length: 1000
24 959th Recv Length: 1000
25 960th Recv Length: 1000
26 961th Recv Length: 1000
27 962th Recv Length: 384
28 963th Recv Length: 616
29 964th Recv Length: 1000
30 965th Recv Length: 1000
31 966th Recv Length: 1000
32 967th Recv Length: 1000
33 968th Recv Length: 1000
34 969th Recv Length: 1000
35 970th Recv Length: 1000
36 971th Recv Length: 1000
37 972th Recv Length: 1000
38 973th Recv Length: 1000
39 974th Recv Length: 1000
40 975th Recv Length: 1000
41 976th Recv Length: 1000
42 977th Recv Length: 1000
43 978th Recv Length: 1000
44 979th Recv Length: 1000
45 980th Recv Length: 1000
46 981th Recv Length: 1000
47 982th Recv Length: 384
48 983th Recv Length: 616
49 984th Recv Length: 1000
50 985th Recv Length: 1000
51 986th Recv Length: 1000
52 987th Recv Length: 1000
53 988th Recv Length: 1000
54 989th Recv Length: 1000
55 990th Recv Length: 1000
56 991th Recv Length: 1000
57 992th Recv Length: 1000
58 993th Recv Length: 1000
59 994th Recv Length: 1000
60 995th Recv Length: 1000
61 996th Recv Length: 1000
62 997th Recv Length: 1000
63 998th Recv Length: 1000
64 999th Recv Length: 1000
65 1000th Recv Length: 1000
66 1001th Recv Length: 1000
67 1002th Recv Length: 384
68 1003th Recv Length: 616
69 1004th Recv Length: 1000
70 1005th Recv Length: 1000
71 1006th Recv Length: 1000
72 1007th Recv Length: 1000
73 1008th Recv Length: 1000
74 1009th Recv Length: 1000
75 1010th Recv Length: 1000
76 1011th Recv Length: 1000
77 1012th Recv Length: 1000
78 1013th Recv Length: 1000
79 1014th Recv Length: 1000
80 1015th Recv Length: 1000
81 1016th Recv Length: 1000
82 1017th Recv Length: 1000
83 1018th Recv Length: 1000
84 1019th Recv Length: 1000
85 1020th Recv Length: 1000
86 1021th Recv Length: 1000
87 1022th Recv Length: 384
88 1023th Recv Length: 616
89 1024th Recv Length: 1000
90 1025th Recv Length: 1000
91 1026th Recv Length: 1000
92 1027th Recv Length: 1000
93 1028th Recv Length: 1000
94 1029th Recv Length: 1000
95 1030th Recv Length: 1000
96 1031th Recv Length: 1000
97 1032th Recv Length: 1000
98 1033th Recv Length: 1000
99 1034th Recv Length: 1000
100 1035th Recv Length: 1000
101 1036th Recv Length: 1000
102 1037th Recv Length: 1000
103 1038th Recv Length: 1000
104 1039th Recv Length: 1000
105 1040th Recv Length: 1000
106 1041th Recv Length: 1000
107 1042th Recv Length: 384
108 1043th Recv Length: 616
109 1044th Recv Length: 1000
110 1045th Recv Length: 1000
111 1046th Recv Length: 1000
112 1047th Recv Length: 1000
113 1048th Recv Length: 1000
114 1049th Recv Length: 1000
115 1050th Recv Length: 1000
服务器发送打印片段整理时发现丢失了，大家可以自己试试，没有问题。
不难发现，服务器发送正常，客户端在接收时却和我们想的很不一样，但发送和接收的总数据量是一致的，就是说数据没有丢失。如果编程者认为TCP情况下发送和接收的数据长度都一致的，那就极有可能在代码中体现出这一思想，最终出现问题。
其实，这就是所谓的“粘包”现象，Stevens很明确地已经指出了这一点，他说，“UDP是长度固定的、无连接的不可靠报文传输；TCP是有序、可靠、双向的面向连接字节流”。他没说TCP是长度固定的，有没有？当然我更倾向于这样的理解，UDP是面向报文的，报文在传输时是不能被分割的（只是从应用层来看）；TCP是面向字节流的，接收多少数据完全取决于发送和接收的速度了，有多少数据recv就返回多少，数据长度并不和send保持一致，也没这个必要。
那么这个问题怎么解决呢？其实，我们只要将recv封装一层就可以了，那就是我们熟悉的readn函数（该函数不是系统调用），代码如下：
int readn(int connfd, void *vptr, int n)
{
int nleft;
int nread;
char *ptr;
struct timeval select_timeout;
fd_set rset;
ptr = vptr;
nleft = n;
while (nleft > 0)
{
FD_ZERO(&rset);
FD_SET(connfd, &rset);
select_timeout.tv_sec = 5;
select_timeout.tv_usec = 0;
if (select(connfd+1, &rset, NULL, NULL, &select_timeout) <= 0)
{
return -1;
}
if ((nread = recv(connfd, ptr, nleft, 0)) < 0)
{
if(errno == EINTR)
{
nread = 0;
}
else
{
return -1;
}
}
else if (nread == 0)
{
break;
}
nleft -= nread;
ptr += nread;
}
return(n - nleft);
}
readn
1 int readn(int connfd, void *vptr, int n)
2 {
3 int nleft;
4 int nread;
5 char *ptr;
6 struct timeval select_timeout;
7 fd_set rset;
8
9 ptr = vptr;
10 nleft = n;
11
12 while (nleft > 0)
13 {
14 FD_ZERO(&rset);
15 FD_SET(connfd, &rset);
16 select_timeout.tv_sec = 5;
17 select_timeout.tv_usec = 0;
18 if (select(connfd+1, &rset, NULL, NULL, &select_timeout) <= 0)
19 {
20 return -1;
21 }
22 if ((nread = recv(connfd, ptr, nleft, 0)) < 0)
23 {
24 if(errno == EINTR)
25 {
26 nread = 0;
27 }
28 else
29 {
30 return -1;
31 }
32 }
33 else if (nread == 0)
34 {
35 break;
36 }
37 nleft -= nread;
38 ptr += nread;
39 }
40 return(n - nleft);
41 }
相应的也有writen函数
int writen(int connfd, void *vptr, size_t n)
{
int nleft, nwritten;
char *ptr;
ptr = vptr;
nleft = n;
while(nleft>0)
{
if((nwritten = send(connfd, ptr, nleft, 0)) == ERROR)
{
if(errnoGet() == EINTR)
{
//PRT_ERR(("EINTR\n"));
nwritten = 0;
}
else
{
//PRT_ERR(("Send() error, 0x%x\n", errnoGet()));
return ERROR;
}
}
nleft -= nwritten;
ptr += nwritten;
}
return(n);
}
writen
1 int writen(int connfd, void *vptr, size_t n)
2 {
3 int nleft, nwritten;
4 char *ptr;
5
6 ptr = vptr;
7 nleft = n;
8
9 while(nleft>0)
10 {
11 if((nwritten = send(connfd, ptr, nleft, 0)) == ERROR)
12 {
13 if(errnoGet() == EINTR)
14 {
15 //PRT_ERR(("EINTR\n"));
16 nwritten = 0;
17 }
18 else
19 {
20 //PRT_ERR(("Send() error, 0x%x\n", errnoGet()));
21 return ERROR;
22 }
23 }
24 nleft -= nwritten;
25 ptr += nwritten;
26 }
27
28 return(n);
29 }
函数中为什么对EINTR进行处理后面再说，也是必不可少的。
在处理TCP发送和接收部分时，可以说必须要使用上述封装，否则等到造成数据不完整或者不一致后再去找问题，可能就麻烦了。这个是必不可少滴。