--- textscreen/txt_inputbox.c
+++ textscreen/txt_inputbox.c
@@ -26,6 +26,16 @@
 #include "txt_utf8.h"
 #include "txt_window.h"
 
+#ifndef HAVE_STRNLEN
+size_t strnlen(const char *s, size_t maxlen) {
+    size_t len = 0;
+    while (len < maxlen && s[len] != '\0') {
+        len++;
+    }
+    return len;
+}
+#endif
+
 txt_widget_class_t txt_inputbox_class;
 txt_widget_class_t txt_int_inputbox_class;