Re: select for UNIX sockets?

[Jesse Pollard <jesse@cats-chateau.net>]

On Monday 02 June 2003 19:08, Krzysztof Halasa wrote:
> Hi,
>
> Should something like this work correctly?
>
> while(1) {
>         FD_ZERO(&set);
>         FD_SET(fd, &set);
>         select(FD_SETSIZE, NULL, &set, NULL, NULL); <<<<<<< for writing
>
>         if (FD_ISSET(fd, &set))
>                 sendto(fd, &datagram, 1, 0, ...);
> }
>
> fd is a normal local datagram socket. It looks select() returns with
> "fd ready for write" and sendto() then blocks as the queue is full.
>
> I don't know if it's expected behaviour or just a not yet known bug.
> Of course, I have a more complete test program if needed.
>
> 2.4.21rc6, haven't tried any other version.
>
> strace shows:
>
> select(1024, NULL, [3], NULL, NULL)     = 1 (out [3])
> sendto(3, "\0", 1, 0, {sa_family=AF_UNIX, path="/tmp/tempUn"}, 13 <<<
> blocks

Could. There may be room for the buffer, but unless it is set to nonblock, 
you may have a stream open to another host that may not accept the data (busy,
network congestion...) With the required acks, the return may (should?) be
delayed until the ack arrives.