diff --git a/src/LocalMapping.cc b/src/LocalMapping.cc
index 6c87a6e55a..b3a9d83948 100644
--- a/src/LocalMapping.cc
+++ b/src/LocalMapping.cc
@@ -23,6 +23,8 @@
 #include "ORBmatcher.h"
 #include "Optimizer.h"
 
+#include <unistd.h>
+
 #include<mutex>
 
 namespace ORB_SLAM2
diff --git a/src/LoopClosing.cc b/src/LoopClosing.cc
index 5e317dd420..a128bbefaa 100644
--- a/src/LoopClosing.cc
+++ b/src/LoopClosing.cc
@@ -28,6 +28,8 @@
 
 #include "ORBmatcher.h"
 
+#include <unistd.h>
+
 #include<mutex>
 #include<thread>
 
diff --git a/src/System.cc b/src/System.cc
index 8df4157095..0d7a2a5c7a 100644
--- a/src/System.cc
+++ b/src/System.cc
@@ -22,6 +22,7 @@
 
 #include "System.h"
 #include "Converter.h"
+#include <unistd.h>
 #include <thread>
 #include <pangolin/pangolin.h>
 #include <iomanip>
diff --git a/src/Tracking.cc b/src/Tracking.cc
index 2273b2ce48..147f6b7dad 100644
--- a/src/Tracking.cc
+++ b/src/Tracking.cc
@@ -33,6 +33,8 @@
 #include"Optimizer.h"
 #include"PnPsolver.h"
 
+#include <unistd.h>
+
 #include<iostream>
 
 #include<mutex>
diff --git a/src/Viewer.cc b/src/Viewer.cc
index dec3204f53..63d66bc75c 100644
--- a/src/Viewer.cc
+++ b/src/Viewer.cc
@@ -21,6 +21,8 @@
 #include "Viewer.h"
 #include <pangolin/pangolin.h>
 
+#include <unistd.h>
+
 #include <mutex>
 
 namespace ORB_SLAM2