summaryrefslogtreecommitdiffstats
path: root/third_party/libwebrtc/rtc_base/ref_counter.h
diff options
context:
space:
mode:
Diffstat (limited to 'third_party/libwebrtc/rtc_base/ref_counter.h')
-rw-r--r--third_party/libwebrtc/rtc_base/ref_counter.h75
1 files changed, 75 insertions, 0 deletions
diff --git a/third_party/libwebrtc/rtc_base/ref_counter.h b/third_party/libwebrtc/rtc_base/ref_counter.h
new file mode 100644
index 0000000000..6ffeda8ced
--- /dev/null
+++ b/third_party/libwebrtc/rtc_base/ref_counter.h
@@ -0,0 +1,75 @@
+/*
+ * Copyright 2017 The WebRTC Project Authors. All rights reserved.
+ *
+ * Use of this source code is governed by a BSD-style license
+ * that can be found in the LICENSE file in the root of the source
+ * tree. An additional intellectual property rights grant can be found
+ * in the file PATENTS. All contributing project authors may
+ * be found in the AUTHORS file in the root of the source tree.
+ */
+#ifndef RTC_BASE_REF_COUNTER_H_
+#define RTC_BASE_REF_COUNTER_H_
+
+#include <atomic>
+
+#include "rtc_base/ref_count.h"
+
+namespace webrtc {
+namespace webrtc_impl {
+
+class RefCounter {
+ public:
+ explicit RefCounter(int ref_count) : ref_count_(ref_count) {}
+ RefCounter() = delete;
+
+ void IncRef() {
+ // Relaxed memory order: The current thread is allowed to act on the
+ // resource protected by the reference counter both before and after the
+ // atomic op, so this function doesn't prevent memory access reordering.
+ ref_count_.fetch_add(1, std::memory_order_relaxed);
+ }
+
+ // Returns kDroppedLastRef if this call dropped the last reference; the caller
+ // should therefore free the resource protected by the reference counter.
+ // Otherwise, returns kOtherRefsRemained (note that in case of multithreading,
+ // some other caller may have dropped the last reference by the time this call
+ // returns; all we know is that we didn't do it).
+ rtc::RefCountReleaseStatus DecRef() {
+ // Use release-acquire barrier to ensure all actions on the protected
+ // resource are finished before the resource can be freed.
+ // When ref_count_after_subtract > 0, this function require
+ // std::memory_order_release part of the barrier.
+ // When ref_count_after_subtract == 0, this function require
+ // std::memory_order_acquire part of the barrier.
+ // In addition std::memory_order_release is used for synchronization with
+ // the HasOneRef function to make sure all actions on the protected resource
+ // are finished before the resource is assumed to have exclusive access.
+ int ref_count_after_subtract =
+ ref_count_.fetch_sub(1, std::memory_order_acq_rel) - 1;
+ return ref_count_after_subtract == 0
+ ? rtc::RefCountReleaseStatus::kDroppedLastRef
+ : rtc::RefCountReleaseStatus::kOtherRefsRemained;
+ }
+
+ // Return whether the reference count is one. If the reference count is used
+ // in the conventional way, a reference count of 1 implies that the current
+ // thread owns the reference and no other thread shares it. This call performs
+ // the test for a reference count of one, and performs the memory barrier
+ // needed for the owning thread to act on the resource protected by the
+ // reference counter, knowing that it has exclusive access.
+ bool HasOneRef() const {
+ // To ensure resource protected by the reference counter has exclusive
+ // access, all changes to the resource before it was released by other
+ // threads must be visible by current thread. That is provided by release
+ // (in DecRef) and acquire (in this function) ordering.
+ return ref_count_.load(std::memory_order_acquire) == 1;
+ }
+
+ private:
+ std::atomic<int> ref_count_;
+};
+
+} // namespace webrtc_impl
+} // namespace webrtc
+
+#endif // RTC_BASE_REF_COUNTER_H_