[PATCH 01/20] osdep: add 'ignore_value(x)' macro

["Daniel P. Berrangé" <berrange@redhat.com>]

Functions annotated with 'warn_unused_result' cannot be cast
to (void) to silence the compiler. Add the ignore_vaule()
macro which tricks the compiler into thinking the result is
genuinely used, avoiding the need for manual workarounds where
the return value is intentionally ignored.

This clever macro is derived from one originating with the
GNULIB project:

  https://gitweb.git.savannah.gnu.org/gitweb/?p=gnulib.git;a=blob;f=lib/ignore-value.h;hb=HEAD

but simplified since we don't need compat with old GCC and
optimizing on clang is overkill.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
---
 include/qemu/osdep.h | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
index 1ec5b42230b..c1ca2c127b6 100644
--- a/include/qemu/osdep.h
+++ b/include/qemu/osdep.h
@@ -251,6 +251,14 @@ extern "C" {
 #endif
 
 
+/*
+ * Merely casting to (void) is not sufficient for functions
+ * annotated with the "warn_unused_result" attribute
+ */
+#define ignore_value(x) \
+    (__extension__ ({ __typeof__(x) __x = (x); (void) __x; }))
+
+
 /*
  * For mingw, as of v6.0.0, the function implementing the assert macro is
  * not marked as noreturn, so the compiler cannot delete code following an
-- 
2.55.0