tcon: Encode integer values into "type" canaries

Add the ability to encode, as well as types, integer values (must be
positive, compile time constant and in the range of size_t) into TCON
constructed "type" canaries.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

ccan/tcon/tcon.h

@@ -166,6 +166,28 @@
  */
 #define tcon_sizeof(x, canary) sizeof((x)->_tcon[0].canary)
 
+/**
+ * TCON_VALUE - encode an integer value in a type canary
+ * @canary: name of the value canary
+ * @val: positive integer compile time constant value
+ *
+ * This macro can be included inside the declarations in a TCON() or
+ * TCON_WRAP(), constructing a special "type" canary which encodes the
+ * integer value @val (which must be a compile time constant, and a
+ * positive integer in the range of size_t).
+ */
+#define TCON_VALUE(canary, val)	char _value_##canary[val]
+
+/**
+ * tcon_value - retrieve the value of a TCON_VALUE canary
+ * @x: the structure containing the TCON
+ * @canary: name of the value canary
+ *
+ * This macros expands to the value previously encoded into a TCON
+ * using TCON_VALUE().
+ */
+#define tcon_value(x, canary)	tcon_sizeof(x, _value_##canary)
+
 /**
  * tcon_ptr_type - pointer to the type within a container (or void *)
  * @x: the structure containing the TCON.

ccan/tcon/test/compile_ok-value.c

+#include <ccan/tcon/tcon.h>
+#include <ccan/build_assert/build_assert.h>
+#include <stdlib.h>
+#include <stddef.h>
+
+struct container {
+	void *p;
+};
+
+struct val_container {
+	struct container raw;
+	TCON(TCON_VALUE(fixed_val, 17));
+};
+
+struct other_struct {
+	char junk1;
+	int x1;
+	long junk2;
+	char *x2;
+	short junk3;
+};
+
+struct offs_container {
+	struct container raw;
+	TCON(TCON_VALUE(off1, offsetof(struct other_struct, x1));
+	     TCON_VALUE(off2, offsetof(struct other_struct, x2)));
+};
+
+int main(int argc, char *argv[])
+{
+	struct val_container valcon;
+	struct offs_container offscon;
+	TCON_WRAP(struct container, TCON_VALUE(fixed_val, 17)) valconw;
+	TCON_WRAP(struct container,
+		  TCON_VALUE(off1, offsetof(struct other_struct, x1));
+		  TCON_VALUE(off2, offsetof(struct other_struct, x2))) offsconw;
+
+	BUILD_ASSERT(tcon_value(&valcon, fixed_val) == 17);
+	BUILD_ASSERT(tcon_value(&valconw, fixed_val) == 17);
+
+	BUILD_ASSERT(tcon_value(&offscon, off1)
+		     == offsetof(struct other_struct, x1));
+	BUILD_ASSERT(tcon_value(&offscon, off2)
+		     == offsetof(struct other_struct, x2));
+	BUILD_ASSERT(tcon_value(&offsconw, off1)
+		     == offsetof(struct other_struct, x1));
+	BUILD_ASSERT(tcon_value(&offsconw, off2)
+		     == offsetof(struct other_struct, x2));
+
+	return 0;
+}