diff --git a/include/mes/lib.h b/include/mes/lib.h
index 788f70a6..759a1ff2 100644
--- a/include/mes/lib.h
+++ b/include/mes/lib.h
@@ -73,5 +73,6 @@ unsigned long __mesabi_udiv (unsigned long a, unsigned long b);
 
 void *__memcpy (void *dest, void const *src, size_t n);
 void *__memmove (void *dest, void const *src, size_t n);
+void *__memset (void *s, int c, size_t n);
 
 #endif //__MES_LIB_H
diff --git a/lib/string/__memset.c b/lib/string/__memset.c
new file mode 100644
index 00000000..79892d2d
--- /dev/null
+++ b/lib/string/__memset.c
@@ -0,0 +1,30 @@
+/* -*-comment-start: "//";comment-end:""-*-
+ * GNU Mes --- Maxwell Equations of Software
+ * Copyright © 2017,2018,2020 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
+ *
+ * This file is part of GNU Mes.
+ *
+ * GNU Mes is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or (at
+ * your option) any later version.
+ *
+ * GNU Mes is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with GNU Mes.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <mes/lib.h>
+
+void *
+__memset (void *s, int c, size_t n)
+{
+  char *p = s;
+  while (n--)
+    *p++ = c;
+  return s;
+}