diff --git a/crate/sync/src/main.rs b/crate/sync/src/main.rs
index 49c86fa..0837505 100644
--- a/crate/sync/src/main.rs
+++ b/crate/sync/src/main.rs
@@ -1,3 +1,5 @@
+//! entrance to test the communication in processes with solving five philosophers problem
+
 mod mutex;
 mod monitor;
 
diff --git a/crate/sync/src/monitor.rs b/crate/sync/src/monitor.rs
index 2c7cb8d..b984fbc 100644
--- a/crate/sync/src/monitor.rs
+++ b/crate/sync/src/monitor.rs
@@ -1,3 +1,5 @@
+//! solve the five philosophers problem with monitor
+
 use std::thread;
 use std::sync::{Mutex, Condvar, Arc};
 use std::time::Duration;
@@ -51,6 +53,7 @@ struct Table {
     fork_condvar: Vec<Condvar>,
 }
 
+// the main function to test
 pub fn main() {
     let table = Arc::new(Table {
         fork_status: Mutex::new(vec![false; 5]),
diff --git a/crate/sync/src/mutex.rs b/crate/sync/src/mutex.rs
index 06b3223..eeef0f4 100644
--- a/crate/sync/src/mutex.rs
+++ b/crate/sync/src/mutex.rs
@@ -1,3 +1,5 @@
+//! solve the five philosophers problem with mutex
+
 use std::thread;
 use std::sync::{Mutex, Arc};
 use std::time::Duration;
@@ -35,6 +37,7 @@ struct Table {
     forks: Vec<Mutex<()>>,
 }
 
+// the main function to test
 pub fn main() {
     let table = Arc::new(Table {
         forks: vec![