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我们先来看看:
xxxxxx$ netstat -ano | head Active Internet connections (servers and established)Proto Recv-Q Send-Q Local Address Foreign Address State Timertcp 0 0 127.0.0.1:42222 0.0.0.0:* LISTEN off (0.00/0/0)tcp 0 0 10.100.70.140:48369 0.0.0.0:* LISTEN off (0.00/0/0)tcp 0 0 10.100.70.140:13942 0.0.0.0:* LISTEN off (0.00/0/0)tcp 0 0 10.100.70.140:10586 0.0.0.0:* LISTEN off (0.00/0/0)tcp 0 0 10.100.70.140:63227 0.0.0.0:* LISTEN off (0.00/0/0)tcp 0 0 0.0.0.0:8765 0.0.0.0:* LISTEN off (0.00/0/0)tcp 0 0 10.100.70.140:20126 0.0.0.0:* LISTEN off (0.00/0/0)tcp 0 0 10.100.70.140:23456 0.0.0.0:* LISTEN off (0.00/0/0)
第二列表内核recv缓冲区中的字节数(接收缓冲区), 第三列表示内核send缓冲区中的字节数(发送缓冲区)。 所以, 对于一个tcp连接的两端而言, 有四个内核缓冲区。
来看程序, 服务端:
#include#include #include #include #include #include #include #include #include #include #include #include #include #include #include int main(){ int sockSrv = socket(AF_INET, SOCK_STREAM, 0); struct sockaddr_in addrSrv; addrSrv.sin_family = AF_INET; addrSrv.sin_addr.s_addr = INADDR_ANY; addrSrv.sin_port = htons(8765); bind(sockSrv, (const struct sockaddr *)&addrSrv, sizeof(struct sockaddr_in)); listen(sockSrv, 5); struct sockaddr_in addrClient; int len = sizeof(struct sockaddr_in); int sockConn = accept(sockSrv, (struct sockaddr *)&addrClient, (socklen_t*)&len); while(1) { getchar(); char szRecvBuf[1001] = {0}; int iRet = recv(sockConn, szRecvBuf, sizeof(szRecvBuf) - 1, 0); printf("iRet is %d\n", iRet); } getchar(); close(sockConn); close(sockSrv); return 0;}
客户端:
#include#include #include #include #include #include #include #include #include #include #include #include #include #include #include #include int main(){ int sockClient = socket(AF_INET, SOCK_STREAM, 0); struct sockaddr_in addrSrv; addrSrv.sin_addr.s_addr = inet_addr("10.100.70.140"); addrSrv.sin_family = AF_INET; addrSrv.sin_port = htons(8765); connect(sockClient, ( const struct sockaddr *)&addrSrv, sizeof(struct sockaddr_in)); #define N 2000 char szSendBuf[N] = {0}; for(unsigned int i = 0; i < N; i++) //×Ö·ûÊý×é×îºóÒ»¸ö×Ö·û²»ÒªÇóÊÇ¡®\0¡¯ { szSendBuf[i] = 'a'; } int total = 0; while(1) { int iRet = send(sockClient, szSendBuf, sizeof(szSendBuf) , 0); total += iRet; printf("iRet is %d, total send is %d\n", iRet, total); getchar(); } close(sockClient); return 0;}
我们先开启服务端, 再看起客户端, 此时客户端给服务端发送了2000字节, 但服务端没有去取出这2000字节, 我们来看看服务端的情况:
xxxxxx$ netstat -ano | grep 8765tcp 0 0 0.0.0.0:8765 0.0.0.0:* LISTEN off (0.00/0/0)tcp 2000 0 10.100.70.140:8765 10.100.70.139:43634 ESTABLISHED off (0.00/0/0)
再看看客户端, 客户端都发送完了, 没有字节积压, 所以发送缓冲区中么有字节, 如下:
xxxxxx$ netstat -ano | grep 8765tcp 0 0 10.100.70.139:43634 10.100.70.140:8765 ESTABLISHED off (0.00/0/0)
此时, 如果我们在服务端用recv函数取出1000字节, 会怎样呢? 显然, 内核缓冲区中还剩100字节, 如下:
xxxxxx$ netstat -ano | grep 8765tcp 0 0 0.0.0.0:8765 0.0.0.0:* LISTEN off (0.00/0/0)tcp 1000 0 10.100.70.140:8765 10.100.70.139:43634 ESTABLISHED off (0.00/0/0)
如何才能在客户端上看到内核缓冲区中的数据呢? 很简单, 让客户端一只发发发, 服务端的内核缓冲区数据塞满后, 自然开始在客户端的内核缓冲区积压了。 有兴趣的朋友可以试试, 这对理解tcp很有帮助。
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