diff --git a/lib/sig2str.h b/lib/sig2str.h
index 1abdb140e5a..03d50c94317 100644
--- a/lib/sig2str.h
+++ b/lib/sig2str.h
@@ -19,6 +19,21 @@
 
 #include <signal.h>
 
+/* Maximum size of a signal name returned by sig2str(), including the
+   terminating NUL byte.  */
+#ifndef SIG2STR_MAX
+/* The longest one: "RTMAX", then "+" or "-", then up to 10 digits, then NUL.
+   Add + 2 as a reserve for the future.  */
+# define SIG2STR_MAX (5 + 1 + 10 + 1 + 2)
+#endif
+
+#ifndef HAVE_SIG2STR
+int sig2str (int, char *);
+#endif
+#ifndef HAVE_STR2SIG
+int str2sig (char const *, int *);
+#endif
+
 /* An upper bound on signal numbers allowed by the system.  */
 
 #if defined _sys_nsig