[PATCH v17.1 01/14] helpers: Move static_assert definition to shared

[Daniel Stone]

Collect the fallback definitions of static_assert() from desktop-shell
and the test shell, and move them to helpers.h. This allows code
throughout the tree to use static_assert() for build-time assertions,
where it is supported by the compiler.

As GCC goes out of its way to only add static_assert() when C11 has been
explicitly requested - which we don't do - make sure to use the more
widely available _Static_assert() if that is provided.

This will be used in future patches to ensure two array lengths don't go
out of sync.

Signed-off-by: Daniel Stone <daniels at collabora.com>
---
 desktop-shell/shell.c             |  4 ----
 shared/helpers.h                  | 33 +++++++++++++++++++++++++++++++
 tests/weston-test-desktop-shell.c |  4 ----
 3 files changed, 33 insertions(+), 8 deletions(-)

On Mon, 9 Jul 2018 at 14:54, Pekka Paalanen <ppaalanen at gmail.com> wrote:
> I'm fairly paranoid of copying GPL code. The implementation sure is
> trivial, but what exactly did you take from the kernel?
> The documentation bit looks already copyrightable.

They probably have better documentation. :) I wrote that documentation
myself, so it's just the macro body which was taken from the kernel. My
understanding is that this isn't copyrightable.

> Did you know of the existing uses of static_assert in Weston?
> Oh, but there are no uses left, all we have left are the fallback empty
> definitions... two of them. >_<

But that's a good point. How about the following patch instead, which
uses static_assert() where possible? It turns out static_assert() was
actually mostly not defined, but I tried this one with:
static_assert(1 != 0, "must not trigger");
static_assert(1 == 0, "must trigger");

... and that worked.


diff --git a/desktop-shell/shell.c b/desktop-shell/shell.c
index 8b7a23ade..ea3c45354 100644
--- a/desktop-shell/shell.c
+++ b/desktop-shell/shell.c
@@ -47,10 +47,6 @@
 #define DEFAULT_NUM_WORKSPACES 1
 #define DEFAULT_WORKSPACE_CHANGE_ANIMATION_LENGTH 200
 
-#ifndef static_assert
-#define static_assert(cond, msg)
-#endif
-
 struct focus_state {
 	struct desktop_shell *shell;
 	struct weston_seat *seat;
diff --git a/shared/helpers.h b/shared/helpers.h
index 46f745d1b..9e1de947e 100644
--- a/shared/helpers.h
+++ b/shared/helpers.h
@@ -100,6 +100,39 @@ extern "C" {
 	(type *)( (char *)__mptr - offsetof(type,member) );})
 #endif
 
+/**
+ * Build-time static assertion support
+ *
+ * A build-time equivalent to assert(), will generate a compilation error
+ * if the supplied condition does not evaluate true.
+ *
+ * The following example demonstrates use of static_assert to ensure that
+ * arrays which are supposed to mirror each other have a consistent
+ * size.
+ *
+ * This is only a fallback definition; support must be provided by the
+ * compiler itself.
+ *
+ * @code
+ * int small[4];
+ * long expanded[4];
+ *
+ * static_assert(ARRAY_LENGTH(small) != ARRAY_LENGTH(expanded));
+ * for (i = 0; i < ARRAY_LENGTH(small); i++)
+ *     expanded[i] = small[4];
+ * @endcode
+ *
+ * @param condition Expression to check for truth
+ * @param msg Message to print on failure
+ */
+#ifndef static_assert
+# ifdef _Static_assert
+#  define static_assert(cond, msg) _Static_assert(cond, msg)
+# else
+#  define static_assert(cond, msg)
+# endif
+#endif
+
 #ifdef  __cplusplus
 }
 #endif
diff --git a/tests/weston-test-desktop-shell.c b/tests/weston-test-desktop-shell.c
index de8442512..c780316d9 100644
--- a/tests/weston-test-desktop-shell.c
+++ b/tests/weston-test-desktop-shell.c
@@ -41,10 +41,6 @@
 #include "shared/helpers.h"
 #include "libweston-desktop/libweston-desktop.h"
 
-#ifndef static_assert
-#define static_assert(cond, msg)
-#endif
-
 struct desktest_shell {
 	struct wl_listener compositor_destroy_listener;
 	struct weston_desktop *desktop;
-- 
2.17.1