diff --git a/man2/sigprocmask.2 b/man2/sigprocmask.2
index d9c99be6d..c2babf5e5 100644
--- a/man2/sigprocmask.2
+++ b/man2/sigprocmask.2
@@ -143,6 +143,14 @@ See
 for details on manipulating signal sets.
 .\"
 .SS C library/kernel ABI differences
+The glibc wrapper function for
+.BR sigprocmask ()
+silently ignores attempts to block the two real-time signals that
+are used internally by the NPTL threading implementation.
+See
+.BR nptl (7)
+for details.
+
 The original Linux system call was named
 .BR sigprocmask ().
 However, with the addition of real-time signals in Linux 2.2,