libutils: Clarify Condition::signal wakes up exactly one thread
Contrary to POSIX pthread_cond_signal guarantees, bionic will wake up
either zero or one threads.
Change-Id: I628fcd4d5bd8d5290104506f86caa82c54142230
diff --git a/include/utils/Condition.h b/include/utils/Condition.h
index e63ba7e..1c99d1a 100644
--- a/include/utils/Condition.h
+++ b/include/utils/Condition.h
@@ -60,7 +60,7 @@
status_t wait(Mutex& mutex);
// same with relative timeout
status_t waitRelative(Mutex& mutex, nsecs_t reltime);
- // Signal the condition variable, allowing one thread to continue.
+ // Signal the condition variable, allowing exactly one thread to continue.
void signal();
// Signal the condition variable, allowing one or all threads to continue.
void signal(WakeUpType type) {
@@ -132,6 +132,17 @@
#endif // HAVE_PTHREAD_COND_TIMEDWAIT_RELATIVE
}
inline void Condition::signal() {
+ /*
+ * POSIX says pthread_cond_signal wakes up "one or more" waiting threads.
+ * However bionic follows the glibc guarantee which wakes up "exactly one"
+ * waiting thread.
+ *
+ * man 3 pthread_cond_signal
+ * pthread_cond_signal restarts one of the threads that are waiting on
+ * the condition variable cond. If no threads are waiting on cond,
+ * nothing happens. If several threads are waiting on cond, exactly one
+ * is restarted, but it is not specified which.
+ */
pthread_cond_signal(&mCond);
}
inline void Condition::broadcast() {