structeq: fix case where we mark padding as unknown.

[ccan] / ccan / structeq / structeq.h

diff --git a/ccan/structeq/structeq.h b/ccan/structeq/structeq.h
index 6b90754c73a92fe3d203644ba9b075c8b0f5fcbd..81799539c51e45f9c2f1b9c23a1b3949b47564c1 100644 (file)
--- a/ccan/structeq/structeq.h
+++ b/ccan/structeq/structeq.h
@@ -9,14 +9,15 @@
 /**
  * STRUCTEQ_DEF - define an ..._eq function to compare two structures.
  * @sname: name of the structure, and function (<sname>_eq) to define.
- * @padbytes: number of bytes of expected padding, or -1 if unknown.
+ * @padbytes: number of bytes of expected padding, or negative "max".
  * @...: name of every member of the structure.
  *
  * This generates a single memcmp() call in the common case where the
  * structure contains no padding.  Since it can't tell the difference between
  * padding and a missing member, @padbytes can be used to assert that
- * there isn't any, or how many we expect.  -1 means "expect some", since
- * it can be platform dependent.
+ * there isn't any, or how many we expect.  A negative number means
+ * "up to or equal to that amount of padding", as padding can be
+ * platform dependent.
  */
 #define STRUCTEQ_DEF(sname, padbytes, ...)                             \
 static inline bool CPPMAGIC_GLUE2(sname, _eq)(const struct sname *_a, \
@@ -25,7 +26,7 @@ static inline bool CPPMAGIC_GLUE2(sname, _eq)(const struct sname *_a, \
        BUILD_ASSERT(((padbytes) < 0 &&                                 \
                      CPPMAGIC_JOIN(+, CPPMAGIC_MAP(STRUCTEQ_MEMBER_SIZE_, \
                                                    __VA_ARGS__))       \
-                     > sizeof(*_a))                                    \
+                     - (padbytes) >= sizeof(*_a))                      \
                     || CPPMAGIC_JOIN(+, CPPMAGIC_MAP(STRUCTEQ_MEMBER_SIZE_, \
                                                      __VA_ARGS__))     \
                     + (padbytes) == sizeof(*_a));                      \