一、TCP服务端
1、TCP服务端的默认函数调用顺序
- socket()创建套接字
- bind()分配套接字地址
- listen()等待请求连接状态
- accept()允许连接
- read()/write()数据交换
- close()断开连接
2、进入等待连接请求状态
只有调用了listen函数，客户端才能进入可发出连接请求的状态。即这时客户端才能调用connect()函数（若提前调用将发生错误）。
int listen(int sock, int backlog);
成功返回0，失败返回-1。sock指服务器端套接字，backlog指连接请求队列的长度，若为5，则队列长度为5，表示最多使5个连接请求进入队列。
3、受理客户端连接请求
int accept();
accept函数受理连接请求队列中待处理的客户端连接请求。函数调用成功时，accept函数内部将产生用于数据IO的套接字，并返回其文件描述符。需要强调的是，套接字是自动创建的，并自动与发起连接请求的客户端建立连接。
二、TCP客户端
1、TCP客户端的默认函数调用顺序
- socket()创建套接字
- connect()请求连接
- read/write数据交换
- close()断开连接
2、connect()
客户端调用connect函数后，发生以下情况之一才会返回（完成函数调用）：
- 服务器端接收连接请求。
- 发生断网等异常情况而中断连接请求。
所谓的“接收连接”并不一位置服务器端调用accept函数，其实是服务器端把连接请求信息记录到等待队列。因此，connect函数返回后并不立即进行数据交换。
客户端的IP地址和端口是在调用connect函数时自动分配的。
三、回声服务端/客户端
1、linux
服务端：
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h> #define BUF_SIZE 1024
void error_handling(char * messages); int main(int argc, char *argv[])
{
if(argc != )
{
printf("Usage : %s <port>\n", argv[]);
exit();
}
int serverSock, clientSock;
struct sockaddr_in serverAddr, clientAddr;
socklen_t clientAddrSize; char message[BUF_SIZE];
int strLen; serverSock =socket(PF_INET, SOCK_STREAM, );
if(serverSock == -)
error_handling("socket() error"); memset(&serverAddr, , sizeof(serverAddr));
serverAddr.sin_family = AF_INET;
serverAddr.sin_addr.s_addr = htonl(INADDR_ANY);
serverAddr.sin_port = htons(atoi(argv[])); if(bind(serverSock, (struct sockaddr*) &serverAddr, sizeof(serverAddr)) == -)
error_handling("bind() error");
if(listen(serverSock, ) == -)
error_handling("listen() error"); puts("Server start...");
clientAddrSize = sizeof(clientAddr);
for(int i = ; i < ; i++){
clientSock = accept(serverSock, (struct sockaddr*) &clientAddr, &clientAddrSize);
if(clientSock == -)
error_handling("accept() error");
else
printf("Connected client %d\n", i+);
while((strLen=read(clientSock, message, BUF_SIZE)) != ){
puts("echo");
write(clientSock, message, strLen);
}
printf("Close client %d\n", i+);
close(clientSock);
} close(serverSock);
puts("Server close...");
return ;
} void error_handling(char * messages)
{
puts(messages);
exit();
}
客户端：
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h> #define BUF_SIZE 1024
void error_handling(char * messages); int main(int argc, char *argv[])
{
if(argc != )
{
printf("Usage : %s <IP> <port>\n", argv[]);
exit();
}
int sock;
struct sockaddr_in serv_addr;
char message[BUF_SIZE];
int strLen; sock = socket(PF_INET, SOCK_STREAM, );
if(sock == -)
error_handling("socket() error"); memset(&serv_addr, , sizeof(serv_addr));
serv_addr.sin_family = AF_INET;
serv_addr.sin_addr.s_addr = inet_addr(argv[]);
serv_addr.sin_port = htons(atoi(argv[])); if(connect(sock, (struct sockaddr*) &serv_addr, sizeof(serv_addr)) == -)
error_handling("connect() error");
else
puts("Connected...");
while(){
puts("Input message(Q to quit):");
fgets(message, BUF_SIZE, stdin);
if(!strcmp(message, "q\n") || !strcmp(message, "Q\n"))
break; strLen=strlen(message);
write(sock, message, strlen(message));
int recvLen = ;
while(recvLen < strLen){
int recvCnt = read(sock, &message[recvLen], BUF_SIZE-);
if(recvCnt == -)
error_handling("read() error");
recvLen+=recvCnt;
}
message[recvLen]=;
printf("Message from server: %s\n", message);
}
close(sock);
return ;
} void error_handling(char * messages)
{
puts(messages);
exit();
}
2、windows
服务端：
#include <stdio.h>
#include <stdlib.h>
#include <WinSock2.h> #define BUF_SIZE 1024
void ErrorHandling(char *message); int main(int argc, char *argv[])
{
if (argc != ) {
printf("Usage : %s <port>\n", argv[]);
exit();
} WSADATA wsaData;
SOCKET serverSock, clientSock;
SOCKADDR_IN serverAddr, clientAddr;
int szClientAddr;
char message[BUF_SIZE];
int slen; if (WSAStartup(MAKEWORD(, ), &wsaData) != )
ErrorHandling("WSAStartup() error"); serverSock = socket(PF_INET, SOCK_STREAM, );
if (serverSock == INVALID_SOCKET)
ErrorHandling("socket() error");
memset(&serverAddr, , sizeof(serverAddr));
serverAddr.sin_family = AF_INET;
serverAddr.sin_addr.s_addr = htonl(INADDR_ANY);
serverAddr.sin_port = htons(atoi(argv[])); if (bind(serverSock, (SOCKADDR*)&serverAddr, sizeof(serverAddr)) == SOCKET_ERROR)
ErrorHandling("bind() error"); if (listen(serverSock, ) == SOCKET_ERROR)
ErrorHandling("listen() error");
for (int i = ; i < ; ++i) {
szClientAddr = sizeof(clientAddr);
clientSock = accept(serverSock, (SOCKADDR*)&clientAddr, &szClientAddr);
if (clientSock == INVALID_SOCKET)
ErrorHandling("accept() error");
else
printf("Connected client %d\n", i);
while ((slen =recv(clientSock, message, BUF_SIZE, )) != )
{
send(clientSock, message, slen, );
}
closesocket(clientSock);
printf("Closed client %d\n", i);
}
closesocket(serverSock);
WSACleanup();
return ;
} void ErrorHandling(char *message) {
fputs(message, stderr);
fputc('\n', stderr);
}
客户端：
#pragma warning(disable:4996)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <winsock2.h> #define BUF_SIZE 1024
void ErrorHandling(char *message); int main(int argc, char* argv[]) {
if (argc != ) {
printf("Usage : %s <IP> <port>\n", argv[]);
exit();
} WSADATA wsaData;
SOCKET sock;
SOCKADDR_IN serverAddr;
char message[BUF_SIZE];
int sLen, recvLen; if (WSAStartup(MAKEWORD(, ), &wsaData) != )
ErrorHandling("WSAStartup() error"); sock = socket(PF_INET, SOCK_STREAM, );
if (sock == INVALID_SOCKET)
ErrorHandling("socket() error"); memset(&serverAddr, , sizeof(serverAddr));
serverAddr.sin_family = AF_INET;
serverAddr.sin_addr.s_addr = inet_addr(argv[]);
serverAddr.sin_port = htons(atoi(argv[])); if (connect(sock, (SOCKADDR*)&serverAddr, sizeof(serverAddr)) == SOCKET_ERROR)
ErrorHandling("connect() error");
else
printf("Connected.......\n");
while ()
{
fputs("Input message(Q to quit):", stdout);
fgets(message, BUF_SIZE, stdin);
if(!strcmp(message, "q\n") || !strcmp(message, "Q\n"))
break;
sLen = send(sock, message, strlen(message), );
recvLen = ;
while (recvLen < sLen) {
int cnt = recv(sock, message, BUF_SIZE - , );
if (cnt == -)
ErrorHandling("ercv() error");
recvLen += cnt;
}
message[recvLen] = ;
printf("Message from server: %s\n", message);
}
closesocket(sock);
WSACleanup();
return ;
} void ErrorHandling(char *message) {
fputs(message, stderr);
fputc('\n', stderr);
}