在编写网络应用时，你可以使用非阻塞模式或阻塞模式。非阻塞模式更加灵活，当应用程序需要做多件事情时，比如服务于多个套接字时，就需要使用非阻塞模式。但是如果应用程序只做一件事，例如从一个套接字读取数据并将数据写入一个文件或队列，那么使用阻塞模式可以大大降低应用程序的复杂性。阻断模式有一个小问题，如果连接出了问题，应用程序可能永远不会知道；它将永远等待永远不会到达的数据。
怎么会发生这种情况呢？想象一下，你的应用程序已经调用了recv，正在等待来自客户端的数据。客户端的网络连接是不可靠的，客户端应用程序正在经历多次重传。在某些时候，客户端的TCP协议栈决定必须终止连接；它通知客户端应用程序并清理套接字。你的应用程序被留在一个连接上等待数据，而这个连接对客户端来说已经被关闭了。清除问题的唯一方法是手动终止应用程序并重新启动它。终止应用程序会使连接的套接字处于"TIME_WAIT"状态，除非你设置了REUSEADDR套接字选项，否则你将无法立即重启应用程序并让它绑定到监听套接字。
但是你可以告诉OpenVOS在recv调用中设置一个等待数据的时间限制。结果是，在时间限制到期后，调用将返回一个-1，errno将被设置为e$timeout (1081)。然后你的应用程序可以决定如何继续。它可以终止连接，再给客户机一次机会，给客户机发送一些东西来测试连接，或者其他任何适合应用程序的响应。关键是，对情况的诊断和响应都在你的控制之下。
图2中的程序timeout_recv.c就是一个例子。它需要两个参数，一个是要监听的端口号，一个是超时时间，单位是1/1024秒。图1是一个执行示例，命令行（粗体和下划线）是有回声的，因为我坚信所有的参数都应该被交互式程序回声。注意我设置了10秒（10240 / 1024）的超时。程序会在调用accept时报告，如果accept返回超时。虽然我在前一段没有提到，但超时也会在accept调用时起作用，程序演示了这一点。在建立连接后，它报告何时调用recv以及recv返回超时或字符的时间。我已经高亮了所有的超时信息。
请注意，设置时间限制的调用是OpenVOS（和VOS）特有的，这种方法在其他操作系统下是行不通的。
timeout_recv 5647 10240command line was: timeout_recv 5647 10240 timeout_recv: 12:14:27 calling accepttimeout_recv: 12:14:37 accept returned a timeouttimeout_recv: 12:14:37 calling accepttimeout_recv: 12:14:47 accept returned a timeouttimeout_recv: 12:14:47 calling accepttimeout_recv: 12:14:48 calling recvtimeout_recv: 12:14:58 recv returned a timeouttimeout_recv: 12:14:58 calling recvtimeout_recv: 12:15:00 recv return 1 characterstimeout_recv: 12:15:00 calling recvtimeout_recv: 12:15:10 recv returned a timeouttimeout_recv: 12:15:10 calling recvtimeout_recv: 12:15:16 recv return 1 characterstimeout_recv: 12:15:16 calling recvtimeout_recv: 12:15:23 recv return 1 characterstimeout_recv: 12:15:23 calling recvtimeout_recv: 12:15:30 recv return 1 characterstimeout_recv: 12:15:30 calling recvtimeout_recv: 12:15:38 recv return 1 characterstimeout_recv: 12:15:38 calling recvtimeout_recv: 12:15:46 recv return 1 characterstimeout_recv: 12:15:46 calling recvtimeout_recv: 12:15:56 recv returned a timeouttimeout_recv: 12:15:56 calling recvtimeout_recv: client terminated connection at 12:16:00ready 12:16:00 |
图1 - timeout_recv的执行示例
/* ***************************************************************** timeout_recv written by NSDavids Stratus CAC 10-04-23 version 1.0 initial release This is a demonstration of timing out blocking accept and recv callsand returning control to the program. ***************************************************************** */ #include <sys/socket.h>#include <arpa/inet.h>#include <stdio.h>#include <stdlib.h>#include <string.h>#include <time.h> #define bzero(s, len) memset((char *)(s), 0, len) int errno; void s$set_io_time_limit(short int *, long int *, short int *);int s$c_get_portid_from_fildes(int file_des); /* this routine returns the current time as a string */ char * GetCurrentTime (char * szT){time_t tTime;struct tm *tmLT; tTime = time ((time_t *) 0);tmLT = localtime (&tTime); sprintf (szT, "%02ld:%02ld:%02ld", tmLT -> tm_hour, tmLT -> tm_min, tmLT -> tm_sec); return (szT);} /* this routine sets the time out value on the port associated with the socket. These calls are VOS specific, this program is not portable to other environments. i.e. Windows or Linux */ int SetTimeOut (int iSocket, int iTimeLimit, char * szType){long lPortID;short sPortID;long lTimeLimit;short sError; if ((lPortID = s$c_get_portid_from_fildes(iSocket)) == -1) { printf ("timeout_recv: Error getting port ID of %s socketn", szType); exit (-1); } sPortID = (short)lPortID; if (iTimeLimit > 0) { lTimeLimit = iTimeLimit; s$set_io_time_limit (&sPortID, &lTimeLimit, &sError); if (sError != 0) { printf ("timeout_recv: Error %d setting time out of %s socketn", szType, sError); exit (sError); } }return (0);} main (argc, argv)int argc;char *argv []; { int iCliLen; struct sockaddr_in sockCliAddr, sockServAddr; short sPortNo; int iBytes; int iTimeLimit; char szT [32]; int iListenSock, iAcceptedSock; #define BUFFERLEN 100 char szBuffer [BUFFERLEN]; /* process the arguments */ if (argc == 3) { sPortNo = atoi (argv [1]); iTimeLimit = atoi (argv [2]); printf ("command line was: timeout_recv %d %dnn", sPortNo, iTimeLimit); } else { printf ("nUsage: timeout_recv <port_number> <time_limit in 1/1024s of a sec)n"); exit (-1); } /* set up a listening socket */ if ((iListenSock = socket (AF_INET, SOCK_STREAM, 0)) < 0) printf ("timeout_recv: Error %d can't open stream socket", errno); bzero ( (char *) &sockServAddr, sizeof (sockServAddr)); sockServAddr.sin_family = AF_INET; sockServAddr.sin_addr.s_addr = htonl (INADDR_ANY); sockServAddr.sin_port = htons (sPortNo); if (bind (iListenSock, (struct sockaddr *) &sockServAddr, sizeof (sockServAddr)) < 0) { printf ("timeout_recv: Error %d can't bind local addressn", errno); exit (errno); } listen (iListenSock, 5); SetTimeOut (iListenSock, iTimeLimit, "Listening"); /* In most cases I expect that an application will just block waiting for a connection. However, I wanted to show that you can also timeout the call to accept */ iAcceptedSock = 0; while (iAcceptedSock < 1) { printf ("timeout_recv: %s calling acceptn", GetCurrentTime (szT)); iAcceptedSock = accept (iListenSock, (struct sockaddr *) &sockCliAddr, &iCliLen); if (iAcceptedSock < 0) { if (errno == 1081) printf ("timeout_recv: %s accept returned a timeoutn", GetCurrentTime (szT)); else { printf ("timeout_recv: %s accept returned the unexpected error %dn", GetCurrentTime (szT), errno); exit (errno); } iAcceptedSock = 0; } } /* set the timeout on the newly accepted socket */ SetTimeOut (iAcceptedSock, iTimeLimit, "Accepted"); /* main loop, call recv and process the return value. If the return value is -1 then it is an error. An errno of 1081 (e$timeout) is expected, report it and continue. Any other error is unexpected and fatal. If the return value is 0 it means the client terminated the connection, report it and exit. If the return value is > 0 it indicates the number of characters received, report it and continue the loop. */ while (1) { printf ("timeout_recv: %s calling recvn", GetCurrentTime (szT)); iBytes = recv (iAcceptedSock, szBuffer, BUFFERLEN, 0); if (iBytes == -1) { if (errno == 1081) printf ("timeout_recv: %s recv returned a timeoutn", GetCurrentTime (szT)); else { printf ("timeout_recv: %s recv returned the unexpected error %dn", GetCurrentTime (szT), errno); exit (errno); } } else if (iBytes == 0) { printf ("timeout_recv: client terminated connection at %sn", GetCurrentTime (szT)); exit (0); } else printf ("timeout_recv: %s recv return %d charactersn", GetCurrentTime (szT), iBytes); }} |
图2 - timeout_recv.c
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