diff --git a/pipi/combine/mean.c b/pipi/combine/mean.c
index 7bd09e2..eccdc92 100644
--- a/pipi/combine/mean.c
+++ b/pipi/combine/mean.c
@@ -18,6 +18,8 @@
 
 #include "config.h"
 
+#include <stdlib.h>
+
 #include "pipi.h"
 #include "pipi_internals.h"
 
diff --git a/pipi/combine/minmax.c b/pipi/combine/minmax.c
index 874ad9f..f1f6f64 100644
--- a/pipi/combine/minmax.c
+++ b/pipi/combine/minmax.c
@@ -18,6 +18,8 @@
 
 #include "config.h"
 
+#include <stdlib.h>
+
 #include "pipi.h"
 #include "pipi_internals.h"
 
diff --git a/pipi/combine/mulscreen.c b/pipi/combine/mulscreen.c
index f53300c..d9fd3c3 100644
--- a/pipi/combine/mulscreen.c
+++ b/pipi/combine/mulscreen.c
@@ -18,6 +18,8 @@
 
 #include "config.h"
 
+#include <stdlib.h>
+
 #include "pipi.h"
 #include "pipi_internals.h"
 
diff --git a/pipi/combine/rgb.c b/pipi/combine/rgb.c
index 509c086..fc9afbe 100644
--- a/pipi/combine/rgb.c
+++ b/pipi/combine/rgb.c
@@ -18,6 +18,8 @@
 
 #include "config.h"
 
+#include <stdlib.h>
+
 #include "pipi.h"
 #include "pipi_internals.h"
 
diff --git a/pipi/combine/subadd.c b/pipi/combine/subadd.c
index 88fd95e..14c6b4a 100644
--- a/pipi/combine/subadd.c
+++ b/pipi/combine/subadd.c
@@ -18,6 +18,7 @@
 
 #include "config.h"
 
+#include <stdlib.h>
 #include <math.h>
 
 #include "pipi.h"