tcon: Add tcon_sizeof() helper

Add a new tcon_sizeof() helper macro which returns the size of a type for
which a canary has been constructed with TCON or TCON_WRAP.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

ccan/tcon/tcon.h

@@ -159,6 +159,13 @@
 #define tcon_type(x, canary) void *
 #endif
 
+/**
+ * tcon_sizeof - the size of type within a container
+ * @x: the structure containing the TCON.
+ * @canary: which canary to check against.
+ */
+#define tcon_sizeof(x, canary) sizeof((x)->_tcon[0].canary)
+
 /**
  * tcon_ptr_type - pointer to the type within a container (or void *)
  * @x: the structure containing the TCON.

ccan/tcon/test/compile_ok-sizeof.c

+#include <ccan/tcon/tcon.h>
+#include <ccan/build_assert/build_assert.h>
+#include <stdlib.h>
+
+struct container {
+	void *p;
+};
+
+struct int_container {
+	struct container raw;
+	TCON(int tc);
+};
+
+struct charp_and_int_container {
+	struct container raw;
+	TCON(int tc1; char *tc2);
+};
+
+int main(int argc, char *argv[])
+{
+	struct int_container icon;
+	struct charp_and_int_container cicon;
+	TCON_WRAP(struct container, int tc) iconw;
+	TCON_WRAP(struct container, int tc1; char *tc2) ciconw;
+
+	BUILD_ASSERT(tcon_sizeof(&icon, tc) == sizeof(int));
+	BUILD_ASSERT(tcon_sizeof(&cicon, tc1) == sizeof(int));
+	BUILD_ASSERT(tcon_sizeof(&cicon, tc2) == sizeof(char *));
+
+	BUILD_ASSERT(tcon_sizeof(&iconw, tc) == sizeof(int));
+	BUILD_ASSERT(tcon_sizeof(&ciconw, tc1) == sizeof(int));
+	BUILD_ASSERT(tcon_sizeof(&ciconw, tc2) == sizeof(char *));
+
+	return 0;
+}