diff --git a/lib/stub/sigaction.c b/lib/stub/sigaction.c
index 304392e5..e0843b5f 100644
--- a/lib/stub/sigaction.c
+++ b/lib/stub/sigaction.c
@@ -1,6 +1,6 @@
 /* -*-comment-start: "//";comment-end:""-*-
  * GNU Mes --- Maxwell Equations of Software
- * Copyright © 2017,2018 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
+ * Copyright © 2017,2018,2022 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
  *
  * This file is part of GNU Mes.
  *
@@ -24,5 +24,10 @@
 int
 sigaction (int signum, struct sigaction const *act, struct sigaction *oldact)
 {
+  static int stub = 0;
+  if (__mes_debug () && !stub)
+    eputs ("sigaction stub\n");
+  stub = 1;
+  errno = 0;
   return 0;
 }
diff --git a/lib/stub/sigaddset.c b/lib/stub/sigaddset.c
index 94e8e7d0..e468a6be 100644
--- a/lib/stub/sigaddset.c
+++ b/lib/stub/sigaddset.c
@@ -1,6 +1,6 @@
 /* -*-comment-start: "//";comment-end:""-*-
  * GNU Mes --- Maxwell Equations of Software
- * Copyright © 2018 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
+ * Copyright © 2018,2022 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
  *
  * This file is part of GNU Mes.
  *
@@ -22,7 +22,12 @@
 #include <signal.h>
 
 int
-sigaddset (sigset_t * set, int signum)
+sigaddset (sigset_t *set, int signum)
 {
+  static int stub = 0;
+  if (__mes_debug () && !stub)
+    eputs ("sigaddset stub\n");
+  stub = 1;
+  errno = 0;
   return 0;
 }
diff --git a/lib/stub/sigblock.c b/lib/stub/sigblock.c
index 241331af..f80f3652 100644
--- a/lib/stub/sigblock.c
+++ b/lib/stub/sigblock.c
@@ -1,6 +1,6 @@
 /* -*-comment-start: "//";comment-end:""-*-
  * GNU Mes --- Maxwell Equations of Software
- * Copyright © 2018 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
+ * Copyright © 2018,2022 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
  *
  * This file is part of GNU Mes.
  *
@@ -24,5 +24,10 @@
 int
 sigblock (int mask)
 {
+  static int stub = 0;
+  if (__mes_debug () && !stub)
+    eputs ("sigblock stub\n");
+  stub = 1;
+  errno = 0;
   return 0;
 }
diff --git a/lib/stub/sigdelset.c b/lib/stub/sigdelset.c
index a5f3b076..ee4f8b4e 100644
--- a/lib/stub/sigdelset.c
+++ b/lib/stub/sigdelset.c
@@ -1,6 +1,6 @@
 /* -*-comment-start: "//";comment-end:""-*-
  * GNU Mes --- Maxwell Equations of Software
- * Copyright © 2018 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
+ * Copyright © 2018,2022 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
  *
  * This file is part of GNU Mes.
  *
@@ -22,7 +22,12 @@
 #include <signal.h>
 
 int
-sigdelset (sigset_t * set, int signum)
+sigdelset (sigset_t *set, int signum)
 {
+  static int stub = 0;
+  if (__mes_debug () && !stub)
+    eputs ("sigdelset stub\n");
+  stub = 1;
+  errno = 0;
   return 0;
 }
diff --git a/lib/stub/sigemptyset.c b/lib/stub/sigemptyset.c
index a16ca7de..cc0c5eec 100644
--- a/lib/stub/sigemptyset.c
+++ b/lib/stub/sigemptyset.c
@@ -1,6 +1,6 @@
 /* -*-comment-start: "//";comment-end:""-*-
  * GNU Mes --- Maxwell Equations of Software
- * Copyright © 2017,2018 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
+ * Copyright © 2017,2018,2022 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
  *
  * This file is part of GNU Mes.
  *
@@ -22,7 +22,12 @@
 #include <signal.h>
 
 int
-sigemptyset (sigset_t * set)
+sigemptyset (sigset_t *set)
 {
+  static int stub = 0;
+  if (__mes_debug () && !stub)
+    eputs ("sigemptyset stub\n");
+  stub = 1;
+  errno = 0;
   return 0;
 }