Index: lib/libc/sys/mq_open.2
===================================================================
--- lib/libc/sys/mq_open.2
+++ lib/libc/sys/mq_open.2
@@ -37,7 +37,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd September 28, 2019
+.Dd October 15, 2021
 .Dt MQ_OPEN 2
 .Os
 .Sh NAME
@@ -272,6 +272,30 @@
 or
 .Va mq_msgsize
 was less than or equal to zero.
+.It Bq Er EINVAL
+If
+.Dv O_CREAT
+is set, successfully opening the named message queue would cause
+.Va kern.mqueue.maxmq
+to be exceeded.
+See
+.Xr mqueuefs 5
+for information on this sysctl.
+.It Bq Er EINVAL
+If
+.Dv O_CREAT
+is set, and
+.Fa attr
+is not
+.Dv NULL ,
+successfully opening the named message queue would cause
+.Va kern.mqueue.maxmsgsize
+or
+.Va kern.mqueue.maxmsg
+to be exceeded.
+See
+.Xr mqueuefs 5
+for information on these sysctls.
 .It Bq Er EMFILE
 Too many message queue descriptors or file descriptors are currently in use
 by this process.
@@ -300,7 +324,8 @@
 .Xr mq_unlink 2 ,
 .Xr mq_timedreceive 3 ,
 .Xr mq_timedsend 3 ,
-.Xr mqueuefs 5
+.Xr mqueuefs 5 ,
+.Xr sysctl 8
 .Sh STANDARDS
 The
 .Fn mq_open
Index: share/man/man5/mqueuefs.5
===================================================================
--- share/man/man5/mqueuefs.5
+++ share/man/man5/mqueuefs.5
@@ -27,7 +27,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd November 30, 2005
+.Dd October 16, 2021
 .Dt MQUEUEFS 5
 .Os
 .Sh NAME
@@ -113,11 +113,26 @@
 To see the queue's attributes, just read the file:
 .Pp
 .Dl "cat /mnt/mqueue/myqueue"
+.Sh SYSCTL VARIABLES
+.Bl -tag -width "kern.mqueue.maxmsgsize"
+.It Va kern.mqueue.maxmq
+The maximum number of message queues.
+The default is 100.
+.It Va kern.mqueue.maxmsgsize
+The maximum size of a message, in bytes.
+The default is 16384.
+.It Va kern.mqueue.maxmsg
+The maximum number of messages in a queue.
+The default is 100.
+.It Va kern.mqueue.curmq
+The current number of message queues.
+.El
 .Sh SEE ALSO
 .Xr mq_open 2 ,
 .Xr nmount 2 ,
 .Xr unmount 2 ,
 .Xr mount 8 ,
+.Xr sysctl 8 ,
 .Xr umount 8
 .Sh AUTHORS
 This manual page was written by