Using select on FIFO's

[Leslie Mikesell]

In article <BLARSEN.91May30090732 at spider.uio.no> Bjorn.Larsen at usit.uio.no writes:
>Yesterday, I wanted to use select(2) to wait on a number of FIFO's
>that were opened for reading. It turned out to be harder than I
>expected.
[...]
>So I came up with the following 'solution':
>
>	int fd, wfd;
>
>/* Open the FIFO for nonblocking read */
>	fd = open("/tmp/FIFO", O_RDONLY|O_NONBLOCK);
>
>/* Open the FIFO for write (won't hang, since it is open for read) */
>	wfd = open("/tmp/FIFO", O_WRONLY);
>
>/* Close the read descriptor */
>	close(fd);
>
>/* Open the FIFO for blocking read */
>	fd = open("/tmp/FIFO", O_RDONLY);
>
>After these acrobatics, the file descriptor 'fd' can be used with
>select().

I don't have select(), but I've always just opened the FIFO with O_RDWR
on the reader side which (a) doesn't block during the open, and (b)
does cause the read()s to continue to block for data instead of returning
EOF after a writer has closed.  This allows the writers to come and
go as necessary.  If you can arrange for every action the daemon is
supposed to perform to be triggered by a write to the FIFO, you can
just let the read block.  Of course if you want to see EOF when the
writer exits, you can't do it this way.  I would expect select() to
wake up when the last writer closes, so you would control seeing this
by whether or not the reading process also opens for writing.  However,
if the reading process does not open for writing, after the FIFO has
been opened and closed by a writer it will return EOF immediately
instead of blocking whenever no writer has it open.  Select() shouldn't
block in this case either.

Les Mikesell
  les at chinet.chi.il.us