diff --git a/vlib/sync/sync_default.c.v b/vlib/sync/sync_default.c.v
index efaafdbaad..be15deada9 100644
--- a/vlib/sync/sync_default.c.v
+++ b/vlib/sync/sync_default.c.v
@@ -21,10 +21,12 @@ fn C.pthread_mutex_destroy(voidptr) int
 fn C.pthread_rwlockattr_init(voidptr) int
 fn C.pthread_rwlockattr_setkind_np(voidptr, int) int
 fn C.pthread_rwlockattr_setpshared(voidptr, int) int
+fn C.pthread_rwlockattr_destroy(voidptr) int
 fn C.pthread_rwlock_init(voidptr, voidptr) int
 fn C.pthread_rwlock_rdlock(voidptr) int
 fn C.pthread_rwlock_wrlock(voidptr) int
 fn C.pthread_rwlock_unlock(voidptr) int
+fn C.pthread_rwlock_destroy(voidptr) int
 fn C.sem_init(voidptr, int, u32) int
 fn C.sem_post(voidptr) int
 fn C.sem_wait(voidptr) int
@@ -64,22 +66,26 @@ pub struct Semaphore {
 	sem C.sem_t
 }
 
+// new_mutex create and init a new mutex object. You should not call `init` again.
 pub fn new_mutex() &Mutex {
 	mut m := &Mutex{}
 	m.init()
 	return m
 }
 
+// init the mutex object.
 pub fn (mut m Mutex) init() {
 	C.pthread_mutex_init(&m.mutex, C.NULL)
 }
 
+// new_rwmutex create and init a new rwmutex object. You should not call `init` again.
 pub fn new_rwmutex() &RwMutex {
 	mut m := &RwMutex{}
 	m.init()
 	return m
 }
 
+// init the rwmutex object.
 pub fn (mut m RwMutex) init() {
 	a := RwMutexAttr{}
 	C.pthread_rwlockattr_init(&a.attr)
@@ -87,55 +93,82 @@ pub fn (mut m RwMutex) init() {
 	C.pthread_rwlockattr_setkind_np(&a.attr, C.PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP)
 	C.pthread_rwlockattr_setpshared(&a.attr, C.PTHREAD_PROCESS_PRIVATE)
 	C.pthread_rwlock_init(&m.mutex, &a.attr)
+	C.pthread_rwlockattr_destroy(&a.attr) // destroy the attr when done
 }
 
-// @lock(), for *manual* mutex handling, since `lock` is a keyword
+// @lock the mutex, wait and return after got the mutex lock.
 pub fn (mut m Mutex) @lock() {
 	C.pthread_mutex_lock(&m.mutex)
 }
 
+// unlock the mutex. The mutex is released.
 pub fn (mut m Mutex) unlock() {
 	C.pthread_mutex_unlock(&m.mutex)
 }
 
-// RwMutex has separate read- and write locks
+// destroy the mutex object.
+pub fn (mut m Mutex) destroy() {
+	res := C.pthread_mutex_destroy(&m.mutex)
+	if res == 0 {
+		return
+	}
+	panic(unsafe { tos_clone(&u8(C.strerror(res))) })
+}
+
+// @rlock read-lock the rwmutex, wait and return after got read access.
 pub fn (mut m RwMutex) @rlock() {
 	C.pthread_rwlock_rdlock(&m.mutex)
 }
 
+// @lock read & write-lock the rwmutex, wait and return after got read & write access.
 pub fn (mut m RwMutex) @lock() {
 	C.pthread_rwlock_wrlock(&m.mutex)
 }
 
+// destroy the rwmutex object.
+pub fn (mut m RwMutex) destroy() {
+	res := C.pthread_rwlock_destroy(&m.mutex)
+	if res == 0 {
+		return
+	}
+	panic(unsafe { tos_clone(&u8(C.strerror(res))) })
+}
+
 // Windows SRWLocks have different function to unlock
 // So provide two functions here, too, to have a common interface
 pub fn (mut m RwMutex) runlock() {
 	C.pthread_rwlock_unlock(&m.mutex)
 }
 
+// unlock the rwmutex object. The rwmutex is released.
 pub fn (mut m RwMutex) unlock() {
 	C.pthread_rwlock_unlock(&m.mutex)
 }
 
+// new_semaphore create a new semaphore, with zero initial value.
 [inline]
 pub fn new_semaphore() &Semaphore {
 	return new_semaphore_init(0)
 }
 
+// new_semaphore_init create a semaphore, with `n` initial value.
 pub fn new_semaphore_init(n u32) &Semaphore {
 	mut sem := &Semaphore{}
 	sem.init(n)
 	return sem
 }
 
+// init the semaphore, with `n` initial value.
 pub fn (mut sem Semaphore) init(n u32) {
 	C.sem_init(&sem.sem, 0, n)
 }
 
+// post increase the semaphore's value by 1.
 pub fn (mut sem Semaphore) post() {
 	C.sem_post(&sem.sem)
 }
 
+// wait decrease the semaphore's value by 1. If semaphore's original value is zero, then wait.
 pub fn (mut sem Semaphore) wait() {
 	for {
 		if C.sem_wait(&sem.sem) == 0 {
@@ -153,7 +186,7 @@ pub fn (mut sem Semaphore) wait() {
 	}
 }
 
-// `try_wait()` should return as fast as possible so error handling is only
+// try_wait should return as fast as possible so error handling is only
 // done when debugging
 pub fn (mut sem Semaphore) try_wait() bool {
 	$if !debug {
@@ -174,6 +207,8 @@ pub fn (mut sem Semaphore) try_wait() bool {
 	}
 }
 
+// timed_wait decrease the semaphore's value by 1. If semaphore's original
+// value is zero, then wait. If `timeout` return false.
 pub fn (mut sem Semaphore) timed_wait(timeout time.Duration) bool {
 	$if macos {
 		time.sleep(timeout)
@@ -202,7 +237,8 @@ pub fn (mut sem Semaphore) timed_wait(timeout time.Duration) bool {
 	return false
 }
 
-pub fn (sem Semaphore) destroy() {
+// destroy the semaphore object.
+pub fn (mut sem Semaphore) destroy() {
 	res := C.sem_destroy(&sem.sem)
 	if res == 0 {
 		return
diff --git a/vlib/v/slow_tests/valgrind/sync.v b/vlib/v/slow_tests/valgrind/sync.v
new file mode 100644
index 0000000000..9bc45407cd
--- /dev/null
+++ b/vlib/v/slow_tests/valgrind/sync.v
@@ -0,0 +1,20 @@
+import sync
+
+fn main() {
+	mut mutex := sync.new_mutex()
+	mutex.@lock()
+	mutex.unlock()
+	mutex.destroy()
+
+	mut rwmutex := sync.new_rwmutex()
+	rwmutex.@rlock()
+	rwmutex.unlock()
+	rwmutex.@lock()
+	rwmutex.unlock()
+	rwmutex.destroy()
+
+	mut sem := sync.new_semaphore()
+	sem.post()
+	sem.wait()
+	sem.destroy()
+}