Explicitly check for GCC's atomic functions instead of depending on the __GNUC__ define.

Add a comment mentioning the limitation even though the _ref_count value is hanled atomically.

configure.ac

@@ -66,6 +66,30 @@ AC_CHECK_DECLS([isnan], [], [], [[#include <math.h>]])
 AC_CHECK_DECLS([isinf], [], [], [[#include <math.h>]])
 AC_CHECK_DECLS([_isnan], [], [], [[#include <float.h>]])
 AC_CHECK_DECLS([_finite], [], [], [[#include <float.h>]])
+AC_MSG_CHECKING(for GCC atomic builtins)
+AC_LINK_IFELSE(
+[
+ AC_LANG_SOURCE([[
+   int main() {
+      volatile unsigned int val = 1;
+      /* Note: __sync_val_compare_and_swap isn't checked here
+       * because it's protected by __GCC_HAVE_SYNC_COMPARE_AND_SWAP_<n>,
+       * which is automatically defined by gcc.
+       */
+      __sync_add_and_fetch(&val, 1);
+      __sync_sub_and_fetch(&val, 1);
+      return 0;
+   }
+ ]])
+],
+[
+ AC_MSG_RESULT([yes])
+ AC_DEFINE([HAVE_ATOMIC_BUILTINS],[1],[Has atomic builtins])
+],
+[
+ AC_MSG_RESULT([no])
+ AC_MSG_WARN([json-c will be built without atomic refcounts because atomic builtins are missing])
+])
 
 case "${host_os}" in
 	linux*)

json_object.c

@@ -165,7 +165,7 @@ extern struct json_object* json_object_get(struct json_object *jso)
 {
 	if (!jso) return jso;
 
-#if defined __GNUC__
+#ifdef HAVE_ATOMIC_BUILTINS
 	__sync_add_and_fetch(&jso->_ref_count, 1);
 #else
 	++jso->_ref_count;
@@ -178,7 +178,13 @@ int json_object_put(struct json_object *jso)
 {
 	if(!jso) return 0;
 
-#if defined __GNUC__
+#ifdef HAVE_ATOMIC_BUILTINS
+	/* Note: this only allow the refcount to remain correct
+	 * when multiple threads are adjusting it.  It is still an error 
+	 * for a thread to decrement the refcount if it doesn't "own" it,
+	 * as that can result in the thread that loses the race to 0
+	 * operating on an already-freed object.
+	 */
 	if (__sync_sub_and_fetch(&jso->_ref_count, 1) > 0) return 0;
 #else
 	if (--jso->_ref_count > 0) return 0;