This patch documents SHM_R and SHM_W.
It is made in a similar ways to how the mode flags are described in open.2.
--- shmget.2 2019-03-06 17:18:27.000000000 +0100
+++ shmget.2.new 2019-04-10 12:16:35.855032188 +0200
@@ -172,6 +172,17 @@
argument of
.BR open (2).
Presently, execute permissions are not used by the system.
+
+The following symbolic constants are provided for mode.
+Theese can be
+.BR OR 'ed
+together, shifting them right by 3 set the mode for the group, and by 6
set the mode for others.
+.TP
+.B SHM_R
+0400 user has read permission
+.TP
+.B SHM_W
+0200 user has write permission
.PP
When a new shared memory segment is created,
its contents are initialized to zero values, and
