libthr.3: describe SIGTHR

Explain how SIGTHR is used and that it should be not touched by user
code.  Note about SIGLIBRT.

Reviewed by:	emaste
Sponsored by:	The FreeBSD Foundation
MFC after:	3 days
Differential revision:	https://reviews.freebsd.org/D56384

1 files changed, 19 insertions, 0 deletions

diff --git a/lib/libthr/libthr.3 b/lib/libthr/libthr.3
index 8c7b84490547..5d9c5ec6706b 100644
--- a/lib/libthr/libthr.3
+++ b/lib/libthr/libthr.3
@@ -269,6 +269,25 @@ the critical section.
 This should be taken into account when interpreting
 .Xr ktrace 1
 logs.
+.Pp
+The
+.Nm
+library uses the
+.Va SIGTHR
+signal for internal operations, in particular,
+for cancellation requests.
+This signal's masking and disposition is controlled by the library,
+and user programs should not try to modify them.
+The library interposes functions controlling signals to prevent
+inadvertent modifications and to guard portable code against
+exposure to
+.Va SIGTHR .
+.Pp
+Note: similarly, the
+.Va SIGLIBRT
+signal is reserved for use by
+.Lb librt ,
+and should be not modified by users.
 .Sh PROCESS-SHARED SYNCHRONIZATION OBJECTS
 In the
 .Li libthr