diff --git a/include/assert.h b/include/assert.h
index dbd33497..fa6c86f1 100644
--- a/include/assert.h
+++ b/include/assert.h
@@ -1,6 +1,6 @@
 /* -*-comment-start: "//";comment-end:""-*-
  * GNU Mes --- Maxwell Equations of Software
- * Copyright © 2017 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
+ * Copyright © 2017,2022 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
  *
  * This file is part of GNU Mes.
  *
@@ -27,8 +27,9 @@
 #undef __MES_ASSERT_H
 #include_next <assert.h>
 #else // ! SYSTEM_LIBC
-#define assert(x) ((x) ? (void)0 : __assert_fail (#x))
-void __assert_fail (char *s);
+#define assert(x) ((x) ? (void)0 : __assert_fail (#x, 0, 0, 0))
+void __assert_fail (char const *s, char const *file, unsigned line,
+                    char const *function);
 #endif // ! SYSTEM_LIBC
 
 #endif // __MES_ASSERT_H
diff --git a/include/mes/lib.h b/include/mes/lib.h
index 549cca1d..ceb2fb48 100644
--- a/include/mes/lib.h
+++ b/include/mes/lib.h
@@ -67,7 +67,8 @@ extern void (*__call_at_exit) (void);
 #define __FILEDES_MAX 512
 
 #if !SYSTEM_LIBC
-void __assert_fail (char *s);
+void __assert_fail (char const *s, char const *file, unsigned line,
+                    char const *function);
 ssize_t __buffered_read (int filedes, void *buffer, size_t size);
 size_t __buffered_read_clear (int filedes);
 void _exit (int code);
diff --git a/lib/mes/__assert_fail.c b/lib/mes/__assert_fail.c
index c4dda5c7..eb5d0c85 100644
--- a/lib/mes/__assert_fail.c
+++ b/lib/mes/__assert_fail.c
@@ -1,6 +1,6 @@
 /* -*-comment-start: "//";comment-end:""-*-
  * GNU Mes --- Maxwell Equations of Software
- * Copyright © 2016,2017,2018,2019 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
+ * Copyright © 2016,2017,2018,2019,2022 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
  *
  * This file is part of GNU Mes.
  *
@@ -19,14 +19,30 @@
  */
 
 #include <mes/lib.h>
+#include <assert.h>
 
 void
-__assert_fail (char *s)
+__assert_fail (char const *msg, char const *file, unsigned line,
+               char const *function)
 {
+  if (file && file[0])
+    {
+      eputs (file);
+      eputs (":");
+    }
+  if (line)
+    {
+      eputs (itoa (line));
+      eputs (":");
+    }
+  if (function && function[0])
+    {
+      eputs (function);
+      eputs (":");
+    }
   eputs ("assert fail: ");
-  eputs (s);
+  eputs (msg);
   eputs ("\n");
-  char *fail = s;
-  fail = 0;
+  char *fail = 0;
   fail[0] = 0;
 }
diff --git a/lib/mes/assert_msg.c b/lib/mes/assert_msg.c
index 191aa87c..93000be6 100644
--- a/lib/mes/assert_msg.c
+++ b/lib/mes/assert_msg.c
@@ -1,7 +1,7 @@
 /* -*-comment-start: "//";comment-end:""-*-
  * GNU Mes --- Maxwell Equations of Software
  * Copyright © 2019 Jeremiah Orians <jeremiah@pdp10.guru>
- * Copyright © 2019 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
+ * Copyright © 2019,2022 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
  *
  * This file is part of GNU Mes.
  *
@@ -20,11 +20,12 @@
  */
 
 #include <mes/lib.h>
+#include <assert.h>
 #include <stdlib.h>
 
 void
 assert_msg (int bool, char *msg)
 {
   if (bool == 0)
-    __assert_fail (msg);
+    __assert_fail (msg, 0, 0, 0);
 }