设置connect超时

TCP一般默认的connect超时时间为75s，相对较长，可以自定义超时时间，使得超时时间短于75s

typedef sighandler_t  Sigfunc
static void connect_alarm(int);
int connect_timeout(int sockfd, const struct sockaddr *addr,socklen_t addrlen,int sec)
{
 Sigfunc *func;
 int n ;
 func = signal(SIGALRM,connect_alarm);

 if(alarm(sec) != 0)
  {
   printf("the alarm  alreadly exsited");
  }
  
 if((n = connect(sockfd,addr,addrlen)) < 0)
  {
   close(sockfd);
   printf("connect failed");
   if(errno == EINTR)
    errno = ETIMEDOUT;
  }
 alarm(0);  //cancel alarm
 func = signal(SIGALRM,func);
 return n;
}

static void connect_alarm(int signo)
{
 return ;  //just interrupt connect()
}