--- a/src/plplot/modules/TestForHighBitCharacters.c	2026-04-11 22:28:05.397270022 +0200
+++ b/src/plplot/modules/TestForHighBitCharacters.c	2026-04-11 22:28:35.084933584 +0200
@@ -1,4 +1,5 @@
 #include <ctype.h>
+#include <stdlib.h>
 #if ((' ' & 0x0FF) == 0x020)
 # define ISLOWER(c) ('a' <= (c) && (c) <= 'z')
 # define TOUPPER(c) (ISLOWER(c) ? 'A' + ((c) - 'a') : (c))