可重入函数read（）

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.