diff --git a/src/io.c b/src/io.c
index f3a10b25..99d312b9 100644
--- a/src/io.c
+++ b/src/io.c
@@ -26,6 +26,7 @@
 // little-endian.
 
 #include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
 #include <errno.h>