thodg/libgit2

2019-04-16T13:21:03

util: introduce GIT_CONTAINER_OF macro In some parts of our code, we make rather heavy use of casting structures to their respective specialized implementation. One example is the configuration code with the general `git_config_backend` and the specialized `diskfile_header` structures. At some occasions, it can get confusing though with regards to the correct inheritance structure, which led to the recent bug fixed in 2424e64c4 (config: harden our use of the backend objects a bit, 2018-02-28). Object-oriented programming in C is hard, but we can at least try to have some checks when it comes to casting around stuff. Thus, this commit introduces a `GIT_CONTAINER_OF` macro, which accepts as parameters the pointer that is to be casted, the pointer it should be cast to as well as the member inside of the target structure that is the containing structure. This macro then tries hard to detect mis-casts: - It checks whether the source and target pointers are of the same type. This requires support by the compiler, as it makes use of the builtin `__builtin_types_compatible_p`. - It checks whether the embedded member of the target structure is the first member. In order to make this a compile-time constant, the compiler-provided `__builtin_offsetof` is being used for this. - It ties these two checks together by the compiler-builtin `__builtin_choose_expr`. Based on whether the previous two checks evaluate to `true`, the compiler will either compile in the correct cast, or it will output `(void)0`. The second case results in a compiler error, resulting in a compile-time check for wrong casts. The only downside to this is that it relies heavily on compiler-specific extensions. As both GCC and Clang support these features, only define this macro like explained above in case `__GNUC__` is set (Clang also defines `__GNUC__`). If the compiler is not Clang or GCC, just go with a simple cast without any additional checks.

diff --git a/src/util.h b/src/util.h
index d4fe6ee..b9ab652 100644
--- a/src/util.h
+++ b/src/util.h
@@ -30,6 +30,17 @@
 # define max(a,b) ((a) > (b) ? (a) : (b))
 #endif
 
+#if defined(__GNUC__)
+# define GIT_CONTAINER_OF(ptr, type, member) \
+	__builtin_choose_expr( \
+	    __builtin_offsetof(type, member) == 0 && \
+	    __builtin_types_compatible_p(typeof(&((type *) 0)->member), typeof(ptr)), \
+		((type *) (ptr)), \
+		(void)0)
+#else
+# define GIT_CONTAINER_OF(ptr, type, member) (type *)(ptr)
+#endif
+
 #define GIT_DATE_RFC2822_SZ  32
 
 /**