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authorThomas Haller <thaller@redhat.com>2020-10-13 09:34:04 +0200
committerThomas Haller <thaller@redhat.com>2020-10-21 10:38:25 +0200
commit143130066be214280a3c6d4abc4cfe97c10a5f6f (patch)
tree144d1ce99ee406bd73c0e58d2a0cd4723e143265
parentb6d6a16b2a1c5e4593ba5ff22a2e11f23dca93c9 (diff)
c-rbtree: fix struct alignment of CRBTree on m68k architectures
On m68k, 32bit integer are aligned to only 2 bytes. This breaks assumptions and a static assertion of c-rbtree. Explicitly require that the first field is aligned to at least 4 bytes. This fixes the build and ensures that all valid pointers to a CRBTree have the lowest two bits unset (so they can be used for storing 2 additional flags). Use a union instead of aligning __parent_and_flags itself. That is because alignas() cannot lower the natural alignment, so if we would want to align __parent_and_flags, we could only do alignas(sizeof(unsigned long) > 4 ? sizeof(unsigned long) : 4) That would not be correct if "long" is 8 bytes long but had a natural alignment of only 4. The union allows us to specify an alignment of at least 4, but otherwise follow natural alignment. https://github.com/c-util/c-rbtree/commit/10d973a9e6718516a6839c8b87a3e630c5391d53
-rw-r--r--shared/c-rbtree/src/c-rbtree.h6
1 files changed, 5 insertions, 1 deletions
diff --git a/shared/c-rbtree/src/c-rbtree.h b/shared/c-rbtree/src/c-rbtree.h
index a9bbce52f..d1941185f 100644
--- a/shared/c-rbtree/src/c-rbtree.h
+++ b/shared/c-rbtree/src/c-rbtree.h
@@ -27,6 +27,7 @@ extern "C" {
#endif
#include <assert.h>
+#include <stdalign.h>
#include <stddef.h>
typedef struct CRBNode CRBNode;
@@ -58,7 +59,10 @@ typedef struct CRBTree CRBTree;
* C_RBNODE_INIT.
*/
struct CRBNode {
- unsigned long __parent_and_flags;
+ union {
+ unsigned long __parent_and_flags;
+ alignas(4) char __dmmy_for_struct_alignment;
+ };
CRBNode *left;
CRBNode *right;
};