compiler: add a sizeof_field() macro

Determining the size of a field is useful when you don't have a struct
variable handy.  Open-coding this is ugly.

This patch adds the sizeof_field() macro, which is similar to
typeof_field().  Existing instances are updated to use the macro.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 20180614164431.29305-1-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/hw/display/xenfb.c b/hw/display/xenfb.c
index f5afcc0358..911291c5c3 100644
--- a/hw/display/xenfb.c
+++ b/hw/display/xenfb.c
@@ -525,8 +525,8 @@ static int xenfb_configure_fb(struct XenFB *xenfb, size_t fb_len_lim,
                               int width, int height, int depth,
                               size_t fb_len, int offset, int row_stride)
 {
-    size_t mfn_sz = sizeof(*((struct xenfb_page *)0)->pd);
-    size_t pd_len = sizeof(((struct xenfb_page *)0)->pd) / mfn_sz;
+    size_t mfn_sz = sizeof_field(struct xenfb_page, pd[0]);
+    size_t pd_len = sizeof_field(struct xenfb_page, pd) / mfn_sz;
     size_t fb_pages = pd_len * XC_PAGE_SIZE / mfn_sz;
     size_t fb_len_max = fb_pages * XC_PAGE_SIZE;
     int max_width, max_height;