Re: [Qemu-trivial] [PATCH] iov_send_recv(): Handle zero bytes case even if OS does not

[Michael Tokarev <mjt@tls.msk.ru>]

On 12.08.2012 01:24, Peter Maydell wrote:
> POSIX allows sendmsg() and recvmsg() to fail EMSGSIZE if passed a zero
> msg.msg_iovlen (in particular the MacOS X implementation will do this).

> Handle the case where iov_send_recv() is passed a zero byte count
> explicitly, to avoid accidentally depending on the OS to treat zero
> msg_iovlen as a no-op.

> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
> This is what was causing 'make check' to fail on MacOS X.
> The other option was to declare that a zero bytecount was illegal, I guess.

I don't think sending/receiving zero bytes is a good idea in the
first place.  Which test were failed on MacOS?  Was it failing at
test-iov "random I/O"?

I thought I ensured that the test does not call any i/o function
with zero "count" argument.  Might be I was wrong, and in that
case THAT place should be fixed instead.

Can you provide a bit more details please?

The whole thing is actually interesting: this is indeed a system-
dependent corner case which should be handled in the code to make
the routine consistent.  But how to fix this is an open question
I think.  Your approach seems to be best, but we as well may
print a warning there...

Thank you!

/mjt

>  iov.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/iov.c b/iov.c
> index b333061..60705c7 100644
> --- a/iov.c
> +++ b/iov.c
> @@ -146,6 +146,13 @@ ssize_t iov_send_recv(int sockfd, struct iovec *iov, unsigned iov_cnt,
>  {
>      ssize_t ret;
>      unsigned si, ei;            /* start and end indexes */
> +    if (bytes == 0) {
> +        /* Catch the do-nothing case early, as otherwise we will pass an
> +         * empty iovec to sendmsg/recvmsg(), and not all implementations
> +         * accept this.
> +         */
> +        return 0;
> +    }
>  
>      /* Find the start position, skipping `offset' bytes:
>       * first, skip all full-sized vector elements, */