perl中调用recv函数阻塞，设置超时

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eval {

    local $SIG{ALRM} = sub { die "alarm time out" };

    alarm $TIMEOUT;

    $sock->recv($msg, $MAXLEN)      or die "recv: $!";

    alarm 0;

    1;  # return value from eval on normalcy

} or die "recv from $server_host timed out after $TIMEOUT seconds.\n";

#定义ALRM发生信号时的操作，这里发信给客户端并断开链接，子进程退出
$SIG{ALRM} = sub {
print "recv timeout\n";
##syswrite($session,"timeout");
my $bytes=send($session,"timeout",0); 
&log($bytes);
close($session);
exit();
};


alarm( 30 ); #同上，一般放在会超时的操作代码
my $bs=recv($session,$buf,2048,0); 
&log($bs);
alarm( 0 );