diff --git a/lockfile.c b/lockfile.c
index 00dfd60..e127194 100644
--- a/lockfile.c
+++ b/lockfile.c
@@ -22,6 +22,7 @@
  */
 #include <stdio.h>
 #include <string.h>
+#include <stdlib.h>
 #include <unistd.h>
 #include <signal.h>
 #include <sys/types.h>