diff --git a/man2/sigaction.2 b/man2/sigaction.2
index b516b0d9f..1ef0320c4 100644
--- a/man2/sigaction.2
+++ b/man2/sigaction.2
@@ -838,6 +838,16 @@ for a list of the async-signal-safe functions that can be
 safely called inside from inside a signal handler.
 .\"
 .SS C library/kernel ABI differences
+The glibc wrapper function for
+.BR sigaction ()
+gives an error
+.RB ( EINVAL )
+on attempts to change the disposition of the two real-time signals
+used internally by the NPTL threading implementation.
+See
+.BR nptl (7)
+for details.
+
 The original Linux system call was named
 .BR sigaction ().
 However, with the addition of real-time signals in Linux 2.2,