Is anybody knows why adding flag SA_RESTART to signal handler will block signal receiving in sysread() at all?

And how to create signal handler which will continue processing signals without interrupting system calls with EINTR?

use POSIX;

$SIG = 10; $SIGNAME = "USR1";
#$SIG = 50; $SIGNAME = "NUM50";

$SIG{$SIGNAME} = sub { $got++ };
sigaction($SIG, undef, my $act=bless({}, "POSIX::SigAction"));
$act->{FLAGS} |= &POSIX::SA_RESTART;
sigaction($SIG, $act);

my $parent_pid = $$;
die "can't fork\n" unless defined(my $chield_pid = fork());
unless ($chield_pid) {
    select(undef,undef,undef, 0.1), kill($SIG, $parent_pid) while 1;
}

my $n = sysread \*STDIN, my $buf, 1;
printf "got=%d n=%s err=%s\n", $got, defined($n) ? $n : "undef", $!;

kill 9, $chield_pid;
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