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 <e@80x24.org>
Reviewed-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/git-compat-util.h b/git-compat-util.h
index 4a23b9090b..8605cb4202 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__ */
+
 #endif