Issue #236: Add -Wcast-qual and fix casts to retain constness.

To better distinguish between entry->k and entry->v being const within linkhash, but non-const outside, add lh_entry_v() and lh_entry_k() accessors.
Make lh_entry->k const.

configure.ac

@@ -103,7 +103,7 @@ AS_IF([test "x$enable_Bsymbolic" = "xcheck"],
 AS_IF([test "x$enable_Bsymbolic" = "xyes"], [JSON_BSYMBOLIC_LDFLAGS=-Wl[,]-Bsymbolic-functions])
 AC_SUBST(JSON_BSYMBOLIC_LDFLAGS)
 
-AX_APPEND_COMPILE_FLAGS([-Wall -Werror -Wno-error=deprecated-declarations])
+AX_APPEND_COMPILE_FLAGS([-Wall -Werror -Wcast-qual -Wno-error=deprecated-declarations])
 AX_APPEND_COMPILE_FLAGS([-Wextra -Wwrite-string -Wno-unused-parameter])
 AX_APPEND_COMPILE_FLAGS([-D_GNU_SOURCE -D_REENTRANT])
 

json_object.c

@@ -84,7 +84,7 @@ static void json_object_fini(void)
 			   json_object_table->count);
 			lh_foreach(json_object_table, ent)
 			{
-				struct json_object* obj = (struct json_object*)ent->v;
+				struct json_object* obj = (struct json_object*)lh_entry_v(ent);
 				MC_DEBUG("\t%s:%p\n", json_type_to_name(obj->o_type), obj);
 			}
 		}
@@ -385,8 +385,8 @@ static int json_object_object_to_json_string(struct json_object* jso,
 static void json_object_lh_entry_free(struct lh_entry *ent)
 {
 	if (!ent->k_is_constant)
-		free(ent->k);
-	json_object_put((struct json_object*)ent->v);
+		free(lh_entry_k(ent));
+	json_object_put((struct json_object*)lh_entry_v(ent));
 }
 
 static void json_object_object_delete(struct json_object* jso)
@@ -435,17 +435,17 @@ void json_object_object_add_ex(struct json_object* jso,
 	// and re-adding it, so the existing key remains valid.
 	json_object *existing_value = NULL;
 	struct lh_entry *existing_entry;
-	const unsigned long hash = lh_get_hash(jso->o.c_object, (void*)key);
+	const unsigned long hash = lh_get_hash(jso->o.c_object, (const void *)key);
 	existing_entry = (opts & JSON_C_OBJECT_ADD_KEY_IS_NEW) ? NULL : 
-			      lh_table_lookup_entry_w_hash(jso->o.c_object, (void*)key, hash);
+			      lh_table_lookup_entry_w_hash(jso->o.c_object, (const void *)key, hash);
 	if (!existing_entry)
 	{
-		void *const k = (opts & JSON_C_OBJECT_KEY_IS_CONSTANT) ?
-					(void*)key : strdup(key);
+		const void *const k = (opts & JSON_C_OBJECT_KEY_IS_CONSTANT) ?
+					(const void *)key : strdup(key);
 		lh_table_insert_w_hash(jso->o.c_object, k, val, hash, opts);
 		return;
 	}
-	existing_value = (json_object *)existing_entry->v;
+	existing_value = (json_object *)lh_entry_v(existing_entry);
 	if (existing_value)
 		json_object_put(existing_value);
 	existing_entry->v = val;
@@ -458,8 +458,8 @@ int json_object_object_add(struct json_object* jso, const char *key,
 	// and re-adding it, so the existing key remains valid.
 	json_object *existing_value = NULL;
 	struct lh_entry *existing_entry;
-	const unsigned long hash = lh_get_hash(jso->o.c_object, (void*)key);
-	existing_entry = lh_table_lookup_entry_w_hash(jso->o.c_object, (void*)key, hash);
+	const unsigned long hash = lh_get_hash(jso->o.c_object, (const void *)key);
+	existing_entry = lh_table_lookup_entry_w_hash(jso->o.c_object, (const void *)key, hash);
 	if (!existing_entry)
 	{
 		char *keydup = strdup(key);
@@ -468,7 +468,7 @@ int json_object_object_add(struct json_object* jso, const char *key,
 
 		return lh_table_insert_w_hash(jso->o.c_object, keydup, val, hash, 0);
 	}
-	existing_value = (json_object  *)existing_entry->v;
+	existing_value = (json_object *)lh_entry_v(existing_entry);
 	if (existing_value)
 		json_object_put(existing_value);
 	existing_entry->v = val;
@@ -500,7 +500,7 @@ json_bool json_object_object_get_ex(const struct json_object* jso, const char *k
 	switch(jso->o_type)
 	{
 	case json_type_object:
-		return lh_table_lookup_ex(jso->o.c_object, (void*)key, (void**)value);
+		return lh_table_lookup_ex(jso->o.c_object, (const void *)key, (void**)value);
 	default:
 		if (value != NULL)
 			*value = NULL;
@@ -865,7 +865,7 @@ struct json_object* json_object_new_string_len(const char *s, int len)
 		}
 		dstbuf = jso->o.c_string.str.ptr;
 	}
-	memcpy(dstbuf, (void *)s, len);
+	memcpy(dstbuf, (const void *)s, len);
 	dstbuf[len] = '\0';
 	jso->o.c_string.len = len;
 	return jso;

json_object.h

@@ -455,8 +455,8 @@ extern void json_object_object_del(struct json_object* obj, const char *key);
 	struct json_object *val __attribute__((__unused__)) = NULL; \
 	for(struct lh_entry *entry ## key = json_object_get_object(obj)->head, *entry_next ## key = NULL; \
 		({ if(entry ## key) { \
-			key = (char*)entry ## key->k; \
-			val = (struct json_object*)entry ## key->v; \
+			key = (char*)lh_entry_k(entry ## key); \
+			val = (struct json_object*)lh_entry_v(entry ## key); \
 			entry_next ## key = entry ## key->next; \
 		} ; entry ## key; }); \
 		entry ## key = entry_next ## key )
@@ -470,8 +470,8 @@ extern void json_object_object_del(struct json_object* obj, const char *key);
 	struct lh_entry *entry_next ## key = NULL; \
 	for(entry ## key = json_object_get_object(obj)->head; \
 		(entry ## key ? ( \
-			key = (char*)entry ## key->k, \
-			val = (struct json_object*)entry ## key->v, \
+			key = (char*)lh_entry_k(entry ## key), \
+			val = (struct json_object*)lh_entry_v(entry ## key), \
 			entry_next ## key = entry ## key->next, \
 			entry ## key) : 0); \
 		entry ## key = entry_next ## key)
@@ -483,7 +483,9 @@ extern void json_object_object_del(struct json_object* obj, const char *key);
  * @param iter the object iterator
  */
 #define json_object_object_foreachC(obj,iter) \
- for(iter.entry = json_object_get_object(obj)->head; (iter.entry ? (iter.key = (char*)iter.entry->k, iter.val = (struct json_object*)iter.entry->v, iter.entry) : 0); iter.entry = iter.entry->next)
+ for(iter.entry = json_object_get_object(obj)->head; \
+     (iter.entry ? (iter.key = (char*)lh_entry_k(iter.entry), iter.val = (struct json_object*)lh_entry_v(iter.entry), iter.entry) : 0); \
+     iter.entry = iter.entry->next)
 
 /* Array type methods */
 

json_object_iterator.c

@@ -105,7 +105,7 @@ json_object_iter_next(struct json_object_iterator* iter)
     JASSERT(NULL != iter);
     JASSERT(kObjectEndIterValue != iter->opaque_);
 
-    iter->opaque_ = ((struct lh_entry *)iter->opaque_)->next;
+    iter->opaque_ = ((const struct lh_entry *)iter->opaque_)->next;
 }
 
 
@@ -118,7 +118,7 @@ json_object_iter_peek_name(const struct json_object_iterator* iter)
     JASSERT(NULL != iter);
     JASSERT(kObjectEndIterValue != iter->opaque_);
 
-    return (const char*)(((struct lh_entry *)iter->opaque_)->k);
+    return (const char*)(((const struct lh_entry *)iter->opaque_)->k);
 }
 
 
@@ -131,7 +131,7 @@ json_object_iter_peek_value(const struct json_object_iterator* iter)
     JASSERT(NULL != iter);
     JASSERT(kObjectEndIterValue != iter->opaque_);
 
-    return (struct json_object*)(((struct lh_entry *)iter->opaque_)->v);
+    return (struct json_object*)lh_entry_v((const struct lh_entry *)iter->opaque_);
 }
 
 

linkhash.c

@@ -441,7 +441,7 @@ static uint32_t hashlittle( const void *key, size_t length, uint32_t initval)
  */
 static unsigned long lh_perllike_str_hash(const void *k) 
 {
-    const char *rkey = (char*) k;
+    const char *rkey = (const char *)k;
     unsigned hashval = 1;
 
     while (*rkey)
@@ -572,7 +572,7 @@ void lh_table_free(struct lh_table *t)
 }
 
 
-int lh_table_insert_w_hash(struct lh_table *t, void *k, const void *v, const unsigned long h, const unsigned opts)
+int lh_table_insert_w_hash(struct lh_table *t, const void *k, const void *v, const unsigned long h, const unsigned opts)
 {
 	unsigned long n;
 
@@ -604,7 +604,7 @@ int lh_table_insert_w_hash(struct lh_table *t, void *k, const void *v, const uns
 
 	return 0;
 }
-int lh_table_insert(struct lh_table *t, void *k, const void *v)
+int lh_table_insert(struct lh_table *t, const void *k, const void *v)
 {
 	return lh_table_insert_w_hash(t, k, v, lh_get_hash(t, k), 0);
 }
@@ -641,11 +641,11 @@ json_bool lh_table_lookup_ex(struct lh_table* t, const void* k, void **v)
 {
 	struct lh_entry *e = lh_table_lookup_entry(t, k);
 	if (e != NULL) {
-		if (v != NULL) *v = (void *)e->v;
+		if (v != NULL) *v = lh_entry_v(e);
 		return TRUE; /* key found */
 	}
 	if (v != NULL) *v = NULL;
-	return FALSE; /* key not found */
+		return FALSE; /* key not found */
 }
 
 int lh_table_delete_entry(struct lh_table *t, struct lh_entry *e)

linkhash.h

@@ -78,12 +78,16 @@ typedef int (lh_equal_fn) (const void *k1, const void *k2);
  */
 struct lh_entry {
 	/**
-	 * The key.
+	 * The key.  Use lh_entry_k() instead of accessing this directly.
+	 */
+	const void *k;
+	/**
+	 * A flag for users of linkhash to know whether or not they
+	 * need to free k.
 	 */
-	void *k;
 	int k_is_constant;
 	/**
-	 * The value.
+	 * The value.  Use lh_entry_v() instead of accessing this directly.
 	 */
 	const void *v;
 	/**
@@ -211,7 +215,7 @@ extern void lh_table_free(struct lh_table *t);
  * @return On success, <code>0</code> is returned.
  * 	On error, a negative value is returned.
  */
-extern int lh_table_insert(struct lh_table *t, void *k, const void *v);
+extern int lh_table_insert(struct lh_table *t, const void *k, const void *v);
 
 
 /**
@@ -226,7 +230,7 @@ extern int lh_table_insert(struct lh_table *t, void *k, const void *v);
  * @param h hash value of the key to insert
  * @param opts opts, a subset of JSON_OBJECT_ADD_* flags is supported
  */
-extern int lh_table_insert_w_hash(struct lh_table *t, void *k, const void *v, const unsigned long h, const unsigned opts);
+extern int lh_table_insert_w_hash(struct lh_table *t, const void *k, const void *v, const unsigned long h, const unsigned opts);
 
 
 /**
@@ -335,6 +339,28 @@ static inline unsigned long lh_get_hash(const struct lh_table *t, const void *k)
 	return t->hash_fn(k);
 }
 
+/* Don't use this outside of linkhash.h: */
+#ifdef __UNCONST
+#define _LH_UNCONST(a) __UNCONST(a)
+#else
+#define _LH_UNCONST(a) ((void *)(unsigned long)(const void *)(a))
+#endif
+
+/**
+ * Return a non-const version of lh_entry->k.
+ * k is const to indicate and help ensure that linkhash itself doesn't modify
+ * it, but callers are allowed to do what they want with it.
+ * See also lh_entry->k_is_constant
+ */
+#define lh_entry_k(entry) _LH_UNCONST((entry)->k)
+
+/**
+ * Return a non-const version of lh_entry->v.
+ * v is const to indicate and help ensure that linkhash itself doesn't modify
+ * it, but callers are allowed to do what they want with it.
+ */
+#define lh_entry_v(entry) _LH_UNCONST((entry)->v)
+
 #ifdef __cplusplus
 }
 #endif

tests/test4.c

@@ -28,7 +28,7 @@ int main()
 {
 	const char *input = "\"\\ud840\\udd26,\\ud840\\udd27,\\ud800\\udd26,\\ud800\\udd27\"";
 	const char *expected = "\xF0\xA0\x84\xA6,\xF0\xA0\x84\xA7,\xF0\x90\x84\xA6,\xF0\x90\x84\xA7";
-	struct json_object *parse_result = json_tokener_parse((char*)input);
+	struct json_object *parse_result = json_tokener_parse(input);
 	const char *unjson = json_object_get_string(parse_result);
 
 	printf("input: %s\n", input);