UDP connect

在UDP客户端和服务端的例子中，客户端代码如下：

#include <stdio.h>
#include <strings.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <stdlib.h>
#include <arpa/inet.h>
#include <errno.h>
void test( int sd, struct sockaddr_in *addr, int len)  
{  
    char buf[4];  
    int ret;
    /*
    ret = connect(sd, (struct sockaddr *)addr, len);  
    printf("connect ret = %d\n",ret);
    if(ret<0)
        perror("connect");
    */
    ret = sendto(sd, buf, 4, 0, (struct sockaddr *)addr, len);  
    if(ret < 0)
        perror("first sendto");
    /*
    ret = sendto(sd, buf, 4, 0, (struct sockaddr *)addr, len);  
    if(ret < 0)
        perror("second sendto");
    */
   ret = recvfrom(sd, buf, 4, 0, (struct sockaddr *)addr,&len);  
    if(ret < 0)
        perror("recvfrom ");
}  

int main(int argc, char **argv)  
{  
    int sd;  
    struct sockaddr_in addr;  
    bzero(&addr, sizeof(struct sockaddr_in));  
    addr.sin_family = AF_INET;  
    addr.sin_port = htons(12345);  
    inet_pton(AF_INET, "127.0.0.1", &addr.sin_addr);  
    sd = socket(AF_INET, SOCK_DGRAM, 0);  
    test(sd, &addr, sizeof(addr));  
    return 0;  
}

1. 调用connect sendto后会收到icmp消息，之后进行的任何sendto or recvfrom 均会显示connect refused
2. 不掉用conncet, sendto 后会收到icmp消息，但是之后recvfrom会阻塞，sendto会继续产生icmp