编写网络应用程序时，可采用非阻塞模式或阻塞模式。 当应用程序需要同时处理多项任务（如服务多个套接字）时，非阻塞模式更具灵活性且不可或缺。但若应用仅执行单一操作（例如从套接字读取数据并写入文件或队列），采用阻塞模式可显著降低程序复杂度。阻塞模式存在一个小问题：若连接出现异常，应用程序可能永远无法察觉，导致程序无限期等待永远不会到达的数据。
这怎么可能发生？想象你的应用程序调用了recv函数，正在等待客户端的数据。由于客户端的网络连接不可靠，其应用程序经历了多次重传。在某个时刻，客户端的TCP协议栈判定连接必须终止，于是通知客户端应用程序并清理套接字。 此时你的应用程序仍在等待数据，而客户端视角下该连接早已关闭。解决此问题的唯一方法是手动终止应用程序并重新启动。终止应用程序后，关联的套接字将进入"TIME_WAIT"状态——除非你设置了REUSEADDR套接字选项，否则该状态将阻止你立即重启应用程序并使其绑定到监听套接字。
不过，您可以指示OpenVOS在recv调用期间为数据等待设置时间限制。结果是，当超时后，该调用将返回-1，且errno将被设置为e$timeout（1081）。 此时应用程序可自主决定后续处理：终止连接、给予客户端重试机会、发送测试数据验证连接状态，或采取任何其他符合应用场景的响应措施。关键在于，对当前状况的诊断与响应机制均由您掌控。
图2中的程序timeout_recv.c展示了实现方法。该程序接受两个参数：监听端口号和超时时间（单位为1/1024秒）。图1展示了执行示例，命令行（粗体 加下划线部分）会被回显——因为我坚信交互式程序应回显所有参数。 请注意我设置了10秒（10240/1024）超时。程序会在调用accept时报告状态，若accept返回超时则会特别标注。虽然前文未提及，但该超时机制同样适用于accept调用，本程序对此进行了演示。建立连接后，程序会在调用recv时报告状态，并记录recv返回超时或字符的具体时间。 所有超时相关消息均已高亮显示。
请注意，设置时间限制的调用是OpenVOS（及VOS）特有的；此方法在其他操作系统下无法运行。
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的示例执行
/* ***************************************************************** 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