[PATCH] libs/gnttab: introduce BUILD_BUG_ON

[Wei Liu <wei.liu2@citrix.com>]

The implementation is taken from libxc.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
---
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Paulina Szubarczyk <paulinaszubarczyk@gmail.com>

I could have put it in a header file accessible to all libraries under
libs but this construct is only relevant to xengnttab library at the
moment so it's put under gnttab/private.h. It can be easily moved to
a common place when other libraries under libs require it.

This patch is necessary to unblock Paulina on her gnttab copy work.
---
 tools/libs/gnttab/private.h | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/tools/libs/gnttab/private.h b/tools/libs/gnttab/private.h
index d286c86..da487f2 100644
--- a/tools/libs/gnttab/private.h
+++ b/tools/libs/gnttab/private.h
@@ -4,6 +4,13 @@
 #include <xentoollog.h>
 #include <xengnttab.h>
 
+/* Force a compilation error if condition is true */
+#if __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 6)
+#define BUILD_BUG_ON(p) ({ _Static_assert(!(p), "!(" #p ")"); })
+#else
+#define BUILD_BUG_ON(p) ((void)sizeof(struct { int:-!!(p); }))
+#endif
+
 struct xengntdev_handle {
     xentoollog_logger *logger, *logger_tofree;
     int fd;
-- 
2.1.4


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