utils: Add a Container_of() macro.

Author: lorinder

src/csnip/util.h

@@ -13,6 +13,7 @@
  *  one.
  */
 
+#include <stddef.h>
 #include <string.h>
 #include <stdint.h>
 
@@ -69,6 +70,20 @@
 /** Length of a C static array. */
 #define csnip_Static_len(a) (sizeof(a) / sizeof(*(a)))
 
+/** Find the containing struct of a pointer to a member.
+ *
+ *  @param	ptr
+ *		pointer to a member variable.
+ *
+ *  @param	type
+ *		The type of the container.
+ *
+ *  @param	member
+ *		The name of the container member.
+ */
+#define csnip_Container_of(ptr, type, member) \
+	((type*)(((char*)ptr) - offsetof(type, member)))
+
 /** Compute the next power of 2 of a number. */
 inline size_t csnip_next_pow_of_2(size_t a)
 {
@@ -149,6 +164,7 @@ inline size_t csnip_next_pow_of_2(size_t a)
 #define Max			csnip_Max
 #define Clamp			csnip_Clamp
 #define Static_len		csnip_Static_len
+#define Container_of		csnip_Container_of
 #define next_pow_of_2		csnip_next_pow_of_2
 #define Fill_n			csnip_Fill_n
 #define Fill			csnip_Fill

test/util_test0.c

@@ -39,10 +39,23 @@ void test_clamp(void)
 	CHECK(Clamp(-3, 5, 3) == 3);
 }
 
+struct X {
+	int u;
+	double v;
+};
+
+void test_container_of(void)
+{
+	struct X c;
+	CHECK(Container_of(&c.v, struct X, v) == &c);
+	CHECK(Container_of(&c.u, struct X, u) == &c);
+}
+
 int main(void)
 {
 	test_min();
 	test_max();
 	test_clamp();
+	test_container_of();
 	return 0;
 }