Re: [PATCH] poll.2, select.2: fix erroneous description of "available for write".

["Michael Kerrisk (man-pages)" <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

Hi Rusty,

On 08/19/2014 03:57 PM, Rusty Russell wrote:
> POSIX says: "POLLOUT Normal data may be written without blocking.".
> This is "may" is misleading, see the POSIX write page:
> 
> Write requests to a pipe or FIFO shall be handled in the same way as
> a regular file with the following exceptions: ... If the O_NONBLOCK
> flag is clear, a write request may cause the thread to block, but on
> normal completion it shall return nbyte.
> 
> ... When attempting to write to a file descriptor (other than a pipe
> or FIFO) that supports non-blocking writes and cannot accept the data
> immediately:
> 
> If the O_NONBLOCK flag is clear, write() shall block the calling
> thread until the data can be accepted.
> 
> If the O_NONBLOCK flag is set, write() shall not block the thread. If
> some data can be written without blocking the thread, write() shall
> write what it can and return the number of bytes written. Otherwise,
> it shall return -1 and set errno to [EAGAIN].> 
> The net result is that write() of more than 1 byte on a socket, pipe or FIFO
> which is "ready" may block: write() (unlike read!) will attempt to write
> the entire buffer and only return a short write under exceptional
> circumstances.
> 
> Indeed, this is the behaviour we see in Linux:
> 
> https://github.com/rustyrussell/ccan/commit/897626152d12d7fd13a8feb36989eb5c8c1f3485
> https://plus.google.com/103188246877163594460/posts/BkTGTMHDFgZ
> 
> Signed-off-by: Rusty Russell <rusty-8n+1lVoiYb80n/F98K4Iww@public.gmane.org>

Thanks for the detailed log message. This all makes sense and
I've applied the patch. One minor correction your point
above. On pipes/FIFOs at least, select() and poll()
return ready only if at least PIPE_BUF bytes of space
are available. (On some other implementations that I've
tested, they do however return true even if there is just
one byte of space available.

Cheers,

Michael


> diff --git a/man2/poll.2 b/man2/poll.2
> index 53aec82..9167472 100644
> --- a/man2/poll.2
> +++ b/man2/poll.2
> @@ -167,7 +167,10 @@ There is urgent data to read (e.g., out-of-band data on TCP socket;
>  pseudoterminal master in packet mode has seen state change in slave).
>  .TP
>  .B POLLOUT
> -Writing now will not block.
> +Writing is now possible, though a write larger that the available space
> +in a socket or pipe will still block (unless
> +.B O_NONBLOCK
> +is set).
>  .TP
>  .BR POLLRDHUP " (since Linux 2.6.17)"
>  Stream socket peer closed connection,
> diff --git a/man2/select.2 b/man2/select.2
> index ac70b85..ea35986 100644
> --- a/man2/select.2
> +++ b/man2/select.2
> @@ -86,9 +86,10 @@ allow a program to monitor multiple file descriptors,
>  waiting until one or more of the file descriptors become "ready"
>  for some class of I/O operation (e.g., input possible).
>  A file descriptor is considered ready if it is possible to
> -perform the corresponding I/O operation (e.g.,
> -.BR read (2))
> -without blocking.
> +perform a corresponding I/O operation (e.g.,
> +.BR read (2)
> +without blocking, or a sufficiently small
> +.BR write (2)).
>  .PP
>  The operation of
>  .BR select ()
> @@ -131,8 +132,8 @@ available for reading (more precisely, to see if a read will not
>  block; in particular, a file descriptor is also ready on end-of-file),
>  those in
>  .I writefds
> -will be watched to see if a write will not block, and
> -those in
> +will be watched to see if space is available for write (though a large
> +write may still block), and those in
>  .I exceptfds
>  will be watched for exceptions.
>  On exit, the sets are modified in place
> 


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
--
To unsubscribe from this list: send the line "unsubscribe linux-man" in
the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html