Question

I have found a server by select(), which I want to receive from some clients.

But I find that the server will get blocked in read() by gdb.

So I thought of solving it by adding a SIGALRM, but
when a timeout occurs, it’s still blocked in read().

This happens because, system calls are automatically restarted, the read()
is not interrupted when the SIGALRM signal handler returns.

Is this interpretation correct?

Answer 1

The usual solution to this problem is to use SOCK_NONBLOCK to socket(2) or O_NONBLOCK to fcntl(2)‘s F_SETFL command. Once the socket is marked non-blocking, it’ll never block when you try to read from it, and you won’t need to try to straddle the divide between blocking or non-blocking. Are you sure select(2) set the filedescriptor? The select(2) manpage does describe one reason why you see what you’re seeing, but it doesn’t seem likely:

Under Linux, select() may report a socket file descriptor as
“ready for reading”, while nevertheless a subsequent read
blocks. This could for example happen when data has arrived
but upon examination has wrong checksum and is discarded.
There may be other circumstances in which a file descriptor is
spuriously reported as ready. Thus it may be safer to use
O_NONBLOCK on sockets that should not block.

If you really just want to prevent the automatic restart, look into SA_RESTART in sigaction(2) to prevent restartable system calls from restarting.