why recvfrom() still block when a SIGALRM has been captured?

I want to use alarm() to set the timeout of recvfrom. But found that when use signal() to register a handler for SIGALRM, and a SIGALRM has been captured and then invoked the signal handler. But after returned from handler, the recvfrom() still blocks while there is no data coming and there is no EINTR error. Why? Does the signal() set the SA_RESTART flag automatically? Here is the code:

signal(SIGALRM, sig_handler);
while(1)
{
    alarm(5);
    n = recvfrom(sock, buf, BUF_MAX, 0, (struct sockaddr*)&addr, &len);
    if(n < 0)
    {
        if(errno == EINTR)
        {
            printf("recvfrom timeoutn");
            continue;
        }
        else
        {
            printf("recvfrom errorn");
        }
    }
    else
    {
         printf("data: %sn", buf);
         alarm(0);
    }
}

void sig_handler(int signo)
{
    return;
}