Question

如何判断SOCKET已经断开

Answer 1

Socket在连接以后，如果客户端非正常退出，比如网线掉了、超时退出等，服务器端的socket仍然有缓存，此时可以从该socket描述符中读出一些数据，短时间内甚至可以写，因此判断TCP socket是否断开及其不易，网上有很多方法，都不能精确判断。本人用select()和recv写了一个，在缓冲区被读完以后判断非常管用。这里，recv时使用MSG_PEEK参数，数据不会从缓冲区中清除掉，因此不会影响正常的读写。
/* check whether socket is connected. Actually, it is not accurate if the client
* crashs and the server don't know about it because the socket buffer is ready.
* So the return value '0' doesn't mean the socket is really valid. Of course,
* '-1' shows the socket is invalid*/int checksock(int s){fd_set fds;
char buf[2];
int nbread;
FD_ZERO(&fds);
FD_SET(s,&fds);
if ( select()(s+1, &fds, (fd_set *)0, (fd_set *)0, NULL) == -1 ) {
//log(LOG_ERR,"select()(): %s\n",strerror(errno)) ;return -1;}if (!FD_ISSET(s,&fds)) {
//log(LOG_ERR,"select() returns OK but FD_ISSET not\n") ;return -1;}/* read one byte from socket */