Subj : Re: close while read-ing
To   : comp.programming.threads
From : David Schwartz
Date : Tue May 24 2005 08:15 pm


"Lyonya" <leonews@optonline.com> wrote in message 
news:XuQke.870$So7.823@fe10.lga...

> what might happen if one thread is blocked on a read() from a socket and 
> another thread tries to close() it?

    You must design your code so that can never, ever happen. There is no 
way the thread that is trying to close the socket can no for sure that the 
thread is blocked in read (as opposed to being *about* to call read). So you 
cannot write code that does what you suggest without invoking undefined (and 
extremely dangerous) behavior. For a TCP socket, you can unblock and 
terminate the connection by calling "shutdown". But you must not release a 
shared resource while another thread is or might be using it.

    If thread A does this:
A1) close(s);
    and Thread B does this:
B1) read(s);

    Your example assumes that A1 comes *after* B1. But since there's no 
synchronization (and no way to put any in, try it), you can also have A1 
come *before* B1. Now imagine this, there's some other thread somewhere in 
the system that allocates a socket. Like this:

C1) j=socket();
C2) bind(j);

    Now, what happens if you get A1, C1, C2, B1? You now have thread B 
reading on the wrong socket entirely, and thread C and its network 
connection will get completely screwed up.

    You will find that any design that allows what you suggest will also 
allow what I warn about. It is literally impossible to prevent it (without 
doing crazy things like suspending threads, finding out where they are, and 
so on).

    DS

.