Re: [PATCH v4 06/14] tools/libxc: add a check in xc_hypercall_buffer_free_pages macro

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 12/05/15 12:25, Yang Hongyang wrote:
> When we use a DECLARE_HYPERCALL_BUFFER_SHADOW define a user
> pointer '_name' and a shadow xc_hypercall_buffer_t.
> then call xc_hypercall_buffer_free_pages(_xch, _name, _nr),
> the complier will report '_name' unused error, it's because
> xc_hypercall_buffer_free_pages() is a MACRO and '_name'
> transparently converted to the hypercall buffer. it confused
> the caller because xc_hypercall_buffer_free_pages() do look
> like a function and take '_name' as an arg.
> Add an if check to let the compiler think we are actually
> using the argument '_name'.
>
> Signed-off-by: Yang Hongyang <yanghy@cn.fujitsu.com>
> CC: Ian Campbell <Ian.Campbell@citrix.com>
> CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
> CC: Wei Liu <wei.liu2@citrix.com>
> CC: Andrew Cooper <andrew.cooper3@citrix.com>
> ---
>  tools/libxc/include/xenctrl.h | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/tools/libxc/include/xenctrl.h b/tools/libxc/include/xenctrl.h
> index 0804257..35fb3ac 100644
> --- a/tools/libxc/include/xenctrl.h
> +++ b/tools/libxc/include/xenctrl.h
> @@ -353,7 +353,9 @@ void xc__hypercall_buffer_free(xc_interface *xch, xc_hypercall_buffer_t *b);
>  void *xc__hypercall_buffer_alloc_pages(xc_interface *xch, xc_hypercall_buffer_t *b, int nr_pages);
>  #define xc_hypercall_buffer_alloc_pages(_xch, _name, _nr) xc__hypercall_buffer_alloc_pages(_xch, HYPERCALL_BUFFER(_name), _nr)
>  void xc__hypercall_buffer_free_pages(xc_interface *xch, xc_hypercall_buffer_t *b, int nr_pages);
> -#define xc_hypercall_buffer_free_pages(_xch, _name, _nr) xc__hypercall_buffer_free_pages(_xch, HYPERCALL_BUFFER(_name), _nr)
> +#define xc_hypercall_buffer_free_pages(_xch, _name, _nr)        \
> +    if ( _name )                                                \
> +        xc__hypercall_buffer_free_pages(_xch, HYPERCALL_BUFFER(_name), _nr)

This needs to be wrapped in a standard "do { ... } while (0)" for macros

~Andrew

>  
>  /*
>   * Array of hypercall buffers.