[vlc-devel] [PATCH v2 01/10] core: add atomic refcounter helper

Romain Vimont rom1v at videolabs.io
Mon Jul 2 16:49:33 CEST 2018


Implement an atomic refcounter with a weak but correct (1) memory order,
and expose a simple API.

(1) See for example "Using weakly ordered C++ atomics correctly" by Hans
    Boehm at CppCon 2016 (the refcounting part also applies to C11):
    <https://www.youtube.com/watch?v=M15UKpNlpeM&t=45m15s>
---
 include/vlc_atomic.h | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/include/vlc_atomic.h b/include/vlc_atomic.h
index 868918fe8e6..2cb5e23f489 100644
--- a/include/vlc_atomic.h
+++ b/include/vlc_atomic.h
@@ -30,6 +30,7 @@
  * Atomic operations do not require locking, but they are not very powerful.
  */
 
+# include <assert.h>
 # include <stdatomic.h>
 
 typedef atomic_uint_least32_t vlc_atomic_float;
@@ -57,4 +58,28 @@ static inline void vlc_atomic_store_float(vlc_atomic_float *atom, float f)
     atomic_store(atom, u.i);
 }
 
+typedef atomic_uint vlc_atomic_rc_t;
+
+/** Init the RC to 1 */
+static inline void vlc_atomic_rc_init(vlc_atomic_rc_t *var)
+{
+    atomic_init(var, 1);
+}
+
+/** Increment the RC */
+static inline void vlc_atomic_rc_inc(vlc_atomic_rc_t *var)
+{
+    unsigned prev = atomic_fetch_add_explicit(var, 1, memory_order_relaxed);
+    assert(prev);
+    VLC_UNUSED(prev);
+}
+
+/** Decrement the RC and return true if it reaches 0 */
+static inline bool vlc_atomic_rc_dec(vlc_atomic_rc_t *var)
+{
+    unsigned prev = atomic_fetch_sub_explicit(var, 1, memory_order_acq_rel);
+    assert(prev);
+    return prev == 1;
+}
+
 #endif
-- 
2.18.0



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