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

On 05/12/2015 08:10 PM, Andrew Cooper wrote:
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@xxxxxxxxxxxxxx>
CC: Ian Campbell <Ian.Campbell@xxxxxxxxxx>
CC: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>
CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  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

Ok, thank you!


   * Array of hypercall buffers.



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