path: root/git-compat-util.h
diff options
authorEric Wong <>2019-10-06 23:30:41 (GMT)
committerJunio C Hamano <>2019-10-07 01:20:11 (GMT)
commit23dee69f53cf5024ca79e0b707dcb03c63f33bef (patch)
treee99f2fd2a4e1fe622451eeaafa8c37b40413ec10 /git-compat-util.h
parentc8e424c9c94d97b18cd335be17f32a8ce94a5b7f (diff)
OFFSETOF_VAR macro to simplify hashmap iterators
While we cannot rely on a `__typeof__' operator being portable to use with `offsetof'; we can calculate the pointer offset using an existing pointer and the address of a member using pointer arithmetic for compilers without `__typeof__'. This allows us to simplify usage of hashmap iterator macros by not having to specify a type when a pointer of that type is already given. In the future, list iterator macros (e.g. list_for_each_entry) may also be implemented using OFFSETOF_VAR to save hackers the trouble of using container_of/list_entry macros and without relying on non-portable `__typeof__'. v3: use `__typeof__' to avoid clang warnings Signed-off-by: Eric Wong <> Reviewed-by: Derrick Stolee <> Signed-off-by: Junio C Hamano <>
Diffstat (limited to 'git-compat-util.h')
1 files changed, 13 insertions, 0 deletions
diff --git a/git-compat-util.h b/git-compat-util.h
index 4a23b90..8605cb4 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -1337,4 +1337,17 @@ static inline void *container_of_or_null_offset(void *ptr, size_t offset)
#define container_of_or_null(ptr, type, member) \
(type *)container_of_or_null_offset(ptr, offsetof(type, member))
+ * like offsetof(), but takes a pointer to a a variable of type which
+ * contains @member, instead of a specified type.
+ * @ptr is subject to multiple evaluation since we can't rely on __typeof__
+ * everywhere.
+ */
+#if defined(__GNUC__) /* clang sets this, too */
+#define OFFSETOF_VAR(ptr, member) offsetof(__typeof__(*ptr), member)
+#else /* !__GNUC__ */
+#define OFFSETOF_VAR(ptr, member) \
+ ((uintptr_t)&(ptr)->member - (uintptr_t)(ptr))
+#endif /* !__GNUC__ */