Socket 入门+ linux协议栈

giant1st

From: http://topic.csdn.net/u/20110818/11/2ba543bf-a34a-417f-87d7-3567122664a7.html?seed=2089953752&r=75291466#r_75291466

tcp的server socket流程是socket()创建套接字，bind()绑定，listen()监听，accept()接收然后才是send()，recv()什么 的，最后关闭。client是 socket()，connect()，recv()，send()最后close()。

注意一：
Linux服务器 1024 以下的端口号必须以超级管理员身份使用，或者授权。这个和Windows就不一样。
也就是说，楼主如果使用默认的端口号7，则运行你的这个程序应先切换到root身份，然后再这个权限下运行你写的那个程序。
以普通用户身份，系统不会把1024下的端口号给你用的。

//-------
建议在做Linux下网络编程的的时候，实验端口最好在10000以上，因为如果你的Linux装的服务软件多的话，可能会遇到端口号冲突的问题。别让这小问题绊住你。

端口号取值范围 0 - 65535(即2的16次方)


编译 gcc -o TCPEchoClient TCPEchoClient.c DieWithError.c
对于编译好的 程序 ./TCPEchoClient 127.0.0.1 "echo this"  
如果程序运行结果正常 ：Recived ：echo this 
但是显示 connect refuse 
TCPEchoClient.c
C/C++ code

1. #include <stdio.h> /* for printf() and fprintf() */
   
2. #include <sys/socket.h> /* for socket(), connect(), send(), and recv() */
   
3. #include <arpa/inet.h> /* for sockaddr_in and inet_addr() */
   
4. #include <stdlib.h> /* for atoi() and exit() */
   
5. #include <string.h> /* for memset() */
   
6. #include <unistd.h> /* for close() */
   
7. 
   
8. #define RCVBUFSIZE 32 /* Size of receive buffer */
   
9. 
   
10. void DieWithError(char *errorMessage); /* Error handling function */
    
11. 
    
12. int main(int argc, char *argv[])
    
13. {
    
14.     int sock; /* Socket descriptor */
    
15.     struct sockaddr_in echoServAddr; /* Echo server address */
    
16.     unsigned short echoServPort; /* Echo server port */
    
17.     char *servIP; /* Server IP address (dotted quad) */
    
18.     char *echoString; /* String to send to echo server */
    
19.     char echoBuffer[RCVBUFSIZE]; /* Buffer for echo string */
    
20.     unsigned int echoStringLen; /* Length of string to echo */
    
21.     int bytesRcvd, totalBytesRcvd; /* Bytes read in single recv()
    
22.                                         and total bytes read */
    
23. 
    
24.     if ((argc < 3) || (argc > 4)) /* Test for correct number of arguments */
    
25.     {
    
26.        fprintf(stderr, "Usage: %s <Server IP> <Echo Word> [<Echo Port>]\n",
    
27.                argv[0]);
    
28.        exit(1);
    
29.     }
    
30. 
    
31.     servIP = argv[1]; /* First arg: server IP address (dotted quad) */
    
32.     echoString = argv[2]; /* Second arg: string to echo */
    
33. 
    
34.     if (argc == 4)
    
35.         echoServPort = atoi(argv[3]); /* Use given port, if any */
    
36.     else
    
37.         echoServPort = 7; /* 7 is the well-known port for the echo service */
    
38. 
    
39.     /* Create a reliable, stream socket using TCP */
    
40.     if ((sock = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0)
    
41.         DieWithError("socket() failed");
    
42. 
    
43.     /* Construct the server address structure */
    
44.     memset(&echoServAddr, 0, sizeof(echoServAddr)); /* Zero out structure */
    
45.     echoServAddr.sin_family = AF_INET; /* Internet address family */
    
46.     echoServAddr.sin_addr.s_addr = inet_addr(servIP); /* Server IP address */
    
47.     echoServAddr.sin_port = htons(echoServPort); /* Server port */
    
48. 
    
49.     /* Establish the connection to the echo server */
    
50.     if (connect(sock, (struct sockaddr *) &echoServAddr, sizeof(echoServAddr)) < 0)
    
51.         DieWithError("connect() failed");
    
52. 
    
53.     echoStringLen = strlen(echoString); /* Determine input length */
    
54. 
    
55.     /* Send the string to the server */
    
56.     if (send(sock, echoString, echoStringLen, 0) != echoStringLen)
    
57.         DieWithError("send() sent a different number of bytes than expected");
    
58. 
    
59.     /* Receive the same string back from the server */
    
60.     totalBytesRcvd = 0;
    
61.     printf("Received: "); /* Setup to print the echoed string */
    
62.     while (totalBytesRcvd < echoStringLen)
    
63.     {
    
64.         /* Receive up to the buffer size (minus 1 to leave space for
    
65.            a null terminator) bytes from the sender */
    
66.         if ((bytesRcvd = recv(sock, echoBuffer, RCVBUFSIZE - 1, 0)) <= 0)
    
67.             DieWithError("recv() failed or connection closed prematurely");
    
68.         totalBytesRcvd += bytesRcvd; /* Keep tally of total bytes */
    
69.         echoBuffer[bytesRcvd] = '\0'; /* Terminate the */
    
70.         printf("%s", echoBuffer); /* Print the echo buffer */
    
71.     }
    
72. 
    
73.     printf("\n"); /* Print a final linefeed */
    
74. 
    
75.     close(sock);
    
76.     exit(0);
    
77. }

DieWithError.c
C/C++ code

1. #include <stdio.h> /* for perror() */
   
2. #include <stdlib.h> /* for exit() */
   
3. 
   
4. void DieWithUserMessage(const char *msg,const char *detail)
   
5. {
   
6.     fputs(msg,stderr);
   
7.     fputs(":",stderr);
   
8.     fputs("detail",stderr);
   
9.     fputs("\n",stderr);
   
10.     exit(1);
    
11. }
    
12. void DieWithError(char *errorMessage)
    
13. {
    
14.     perror(errorMessage);
    
15.     exit(1);
    
16. }

Linux协议栈：
sys_socketcall

Linux 中的 socket 结构是 struct sock，这个结构是在 linux/include/net/sock.h 中定义的。