我修改了Beej教程中的一个例子,试图找出它.我想在for循环中做的是每次迭代我等待4秒.如果有可用的读取,我会打印“按下了一个键”,如果超时,则会打印“超时”.然后我会清除该设置并重复该过程9次.但似乎一旦设置了文件描述符0,即使在调用FD_ZERO()和/或FD_CLR()之后它也永远不会被设置.换句话说,在循环的第一次迭代中按下一个键之后,将为剩余的迭代设置文件描述符,并且不再进行等待.所以一定有我遗失的东西,但我不知道是什么.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>

#define SERVERPORT 4950

int main(int argc,char *argv[]) {
    struct sockaddr_in their_addr; // connector's address information
    struct hostent *he;
    int numbytes;
    int broadcast = 1;

    if ((he=gethostbyname(argv[1])) == NULL) {  // get the host info
        perror("gethostbyname");
        exit(1);
    }

    // this call is what allows broadcast packets to be sent:
    if (setsockopt(sockfd,SOL_SOCKET,SO_BROADCAST,&broadcast,sizeof broadcast) == -1) {
        perror("setsockopt (SO_BROADCAST)");
        exit(1);
    }
    their_addr.sin_family = AF_INET;     // host byte order
    their_addr.sin_port = htons(SERVERPORT); // short,network byte order
    their_addr.sin_addr = *((struct in_addr *)he->h_addr);
    memset(their_addr.sin_zero,'\0',sizeof their_addr.sin_zero);

    struct timeval tv;
    fd_set broadcastfds;

    int i;
    for(i=0; i < 10; i++) {
        tv.tv_sec = 4;
        tv.tv_usec = 500000;

        FD_ZERO(&broadcastfds);
        FD_CLR(0,&broadcastfds);
        FD_SET(0,&broadcastfds);
        if(select(0+1,&broadcastfds,NULL,&tv) == -1) perror("select");

        if (FD_ISSET(0,&broadcastfds)) printf("A key was pressed!\n");
        else printf("Timed out.\n");
        fflush(stdout); 
    }   
    close(sockfd);
    return 0;
}