Re: LuaSocket - how can a tcp client detect a closed connection?

["Mark Edgar" <medgar123@...>]

On 3/5/07, gary ng <garyng2000@yahoo.com> wrote:
> However, given the generic and widespread usage of
> fork()/exec(), even 2 file descriptors may not be
> enough. As they are in general passed to childs which
> unless explicitly closed in the forked child, would
> have the same effect as it is now. So a shutdown() is
> still needed which effectively means, close this
> pipe/socket regardless of how many fd are dup().

Neglecting to explicitly (or implicity via FD_CLOEXEC) close unused
descriptors is a bug in the program, not a flaw in the design of
POSIX.

shutdown() is not legal for use with pipes.  It is specific to sockets.

In the hypothetical design of dual-fd sockets (let's call these
twockets), a close() on the writing twocket is equivalent to
shutdown(sock, SHUT_RD) on the socket, and a close() on the reading
twocket is equivalent to shutdown(sock, SHUT_WR) on the socket.

> I was in this situation trying to create a loopback
> pipe in lua like pipethrough("gzip -dc"). Initially, I
> juse use pipe(2) create the pipe pair then dup
> stdin/stdout and fork()/exec(). However, even I close
> the write end of the parent, I still don't get the
> result because it was passed to the fork() child. I
> have to explicitly close them before the exec().

Right.  You need to ensure that each unused end is closed before
calling exec, or use FD_CLOEXEC instead to accomplish this.

BTW, the "ex" API can do this for you in "pure" Lua.  See the popen2()
example at http://lua-users.org/wiki/ExtensionProposal

    -Mark