diff --git a/_demo/syncdebug/syncdebug.go b/_demo/syncdebug/syncdebug.go
new file mode 100644
index 00000000..54bcedb1
--- /dev/null
+++ b/_demo/syncdebug/syncdebug.go
@@ -0,0 +1,31 @@
+package main
+
+import (
+	"io"
+	"os"
+	"sync"
+	"unsafe"
+
+	llsync "github.com/goplus/llgo/c/pthread/sync"
+)
+
+type L struct {
+	mu sync.Mutex
+	s  string
+	i  int
+	w  io.Writer
+}
+
+func main() {
+	l := &L{s: "hello", i: 123, w: os.Stdout}
+	println("sizeof(L):", unsafe.Sizeof(L{}))
+	println("sizeof(sync.Mutex):", unsafe.Sizeof(sync.Mutex{}))
+	println("sizeof(llsync.Mutex):", unsafe.Sizeof(llsync.Mutex{}))
+	println("l:", l, "l.s:", l.s, "l.i:", l.i, "l.w:", l.w)
+	l.mu.Lock()
+	println("locked")
+	println("l:", l, "l.s:", l.s, "l.i:", l.i, "l.w:", l.w)
+	l.w.Write([]byte(l.s))
+	l.w.Write([]byte("\n"))
+	l.mu.Unlock()
+}