OFFSETOF_VAR macro to simplify hashmap iterators

[thirdparty/git.git] / git-compat-util.h

diff --git a/git-compat-util.h b/git-compat-util.h
index 4a23b9090b455740b40fd937faddfdd6f007f641..8605cb42029ac8faef3760137b564005ed0ac8f3 100644 (file)
--- 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