diff --git a/libc-test/src/functionalext/common/sigchain_util.h b/libc-test/src/functionalext/common/sigchain_util.h
index ed509101a6e0df4ce0703b9f124567151508ab55..c9674e5185da706a3a4ec5c54fcc6580454873ee 100644
--- a/libc-test/src/functionalext/common/sigchain_util.h
+++ b/libc-test/src/functionalext/common/sigchain_util.h
@@ -13,6 +13,10 @@
  * limitations under the License.
  */
 
+/* 
+ * SIG_BLOCK: Mask the signal according to the mask word provided by parameter set. 
+ * And save the original signal mask to oldset
+ */
 #define SIGCHAIN_TEST_SET_MASK(set, fun, signo, num)  do{ \
     int result = sigemptyset(&set);                       \
     if (result != 0) {                                    \