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

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Add a comment mentioning the limitation even though the _ref_count value is hanled atomically.
This commit is contained in:
Eric Haszlakiewicz
2017-09-02 14:48:17 -04:00
parent ab0d4dbc69
commit 5b11e9adff
2 changed files with 32 additions and 2 deletions
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24
configure.ac
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@@ -66,6 +66,30 @@ AC_CHECK_DECLS([isnan], [], [], [[#include <math.h>]])
AC_CHECK_DECLS([isinf], [], [], [[#include <math.h>]]) AC_CHECK_DECLS([isinf], [], [], [[#include <math.h>]])
AC_CHECK_DECLS([_isnan], [], [], [[#include <float.h>]]) AC_CHECK_DECLS([_isnan], [], [], [[#include <float.h>]])
AC_CHECK_DECLS([_finite], [], [], [[#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 "${host_os}" in case "${host_os}" in
linux*) linux*)

10
json_object.c
View File

@@ -165,7 +165,7 @@ extern struct json_object* json_object_get(struct json_object *jso)
{ {
if (!jso) return jso; if (!jso) return jso;
#if defined __GNUC__ #ifdef HAVE_ATOMIC_BUILTINS
__sync_add_and_fetch(&jso->_ref_count, 1); __sync_add_and_fetch(&jso->_ref_count, 1);
#else #else
++jso->_ref_count; ++jso->_ref_count;
@@ -178,7 +178,13 @@ int json_object_put(struct json_object *jso)
{ {
if(!jso) return 0; 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; if (__sync_sub_and_fetch(&jso->_ref_count, 1) > 0) return 0;
#else #else
if (--jso->_ref_count > 0) return 0; if (--jso->_ref_count > 0) return 0;