Re: [PATCH v5] tools/libxc: Implement writev_exact() in the same style as write_exact()

[Ian Campbell <ian.campbell@citrix.com>]

On Thu, 2015-02-19 at 11:45 +0000, Andrew Cooper wrote:

> +    while ( iov_idx < iovcnt )
> +    {
> +        /* Skip over iov[] entries with 0 length. */
> +        while ( iov[iov_idx].iov_len == 0 )
> +            if ( ++iov_idx == iovcnt )
> +                goto out;

Is this required for some reason or just an optimisation?

> +
> +        len = writev(fd, &iov[iov_idx], min(iovcnt - iov_idx, IOV_MAX));
> +        saved_errno = errno;
> +
> +        if ( (len == -1) && (errno == EINTR) )
> +            continue;
> +        if ( len <= 0 )
> +        {
> +            rc = -1;
> +            goto out;
> +        }
> +
> +        /* Check iov[] to see whether we had a partial or complete write. */
> +        while ( len > 0 && (iov_idx < iovcnt) )
> +        {
> +            if ( len >= iov[iov_idx].iov_len )
> +                len -= iov[iov_idx++].iov_len;
> +            else
> +            {
> +                /* Partial write of iov[iov_idx]. Copy iov so we can adjust
> +                 * element iov_idx and resubmit the rest. */

I suppose we can't / don't want to just declare that the input is
non-const and potentially corrupted?

> +                if ( !local_iov )
> +                {
> +                    local_iov = malloc(iovcnt * sizeof(*iov));
> +                    if ( !local_iov )
> +                    {
> +                        saved_errno = ENOMEM;
> +                        goto out;

What is rc at this point? I think it is 0, but I think you want it to be
-1?

It might be better to drop the inialiser of rc and set it in the one or
two places which would then need it (the goto out in the skip-0-length
loop and just before the out label AFAICT).