getsockopt.2: clarify that SO_TIMESTAMP is not 100% reliable

When SO_TIMESTAMP is set, the kernel will attempt to attach a timestamp as
ancillary data to each IP datagram that is received on the socket. However,
it may fail, for example due to insufficient memory. In that case the
packet will still be received but not timestamp will be attached.

Reviewed by:	kib
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D21607

diff --git a/lib/libc/sys/getsockopt.2 b/lib/libc/sys/getsockopt.2
index 942346cdd728197a8f8bda215ac5c23c5ccacad1..3f786e67793fc06e3d21e2aef507abb5202d0abe 100644 (file)
--- a/lib/libc/sys/getsockopt.2
+++ b/lib/libc/sys/getsockopt.2
@@ -28,7 +28,7 @@
 .\"     @(#)getsockopt.2       8.4 (Berkeley) 5/2/95
 .\" $FreeBSD$
 .\"
-.Dd February 10, 2019
+.Dd September 11, 2019
 .Dt GETSOCKOPT 2
 .Os
 .Sh NAME
@@ -431,7 +431,8 @@ option is enabled on a
 .Dv SOCK_DGRAM
 socket, the
 .Xr recvmsg 2
-call will return a timestamp corresponding to when the datagram was received.
+call may return a timestamp corresponding to when the datagram was received.
+However, it may not, for example due to a resource shortage.
 The
 .Va msg_control
 field in the