Re: [PATCH v2] tools/libxc: Implement writev_exact() in the same style as write_exact()

[David Vrabel <david.vrabel@citrix.com>]

On 07/07/14 11:18, Andrew Cooper wrote:
> This implementation of writev_exact() will cope with an iovcnt greater than
> IOV_MAX because glibc will actually let this work anyway, and it is very
> useful not to have to work about this in the caller of writev_exact().  The
> caller is still required to ensure that the sum of iov_len's doesn't overflow
> a ssize_t.
> 
> Promote the MAX() and MIN() macro definitions from xg_save_restore.h to
> xc_private.h
[...]
> --- a/tools/libxc/xc_private.c
> +++ b/tools/libxc/xc_private.c
> @@ -854,6 +854,37 @@ int write_exact(int fd, const void *data, size_t size)
>      return 0;
>  }
>  
> +int writev_exact(int fd, const struct iovec *iov, int iovcnt)
> +{
[...]
> +        /* writev() guarentees atomicity of individual iov[] elements.  Sanity
> +         * check that the returned len did lie on an iov[] element boundary. */
> +        assert(len == 0);

There's nothing in the writev(2) man page that says this.  I think you
need to handle partial writes of an entry.

> --- a/tools/libxc/xc_private.h
> +++ b/tools/libxc/xc_private.h
[...]
> +#ifndef MAX
> +#define MAX(_a, _b) ((_a) >= (_b) ? (_a) : (_b))
> +#endif
> +#ifndef MIN
> +#define MIN(_a, _b) ((_a) <= (_b) ? (_a) : (_b))
> +#endif

Do we really want to use these unsafe macros in more places?

David