diff --git a/lib/libc/sys/open.2 b/lib/libc/sys/open.2
index c3fc4d831a78..1fb294543e99 100644
--- a/lib/libc/sys/open.2
+++ b/lib/libc/sys/open.2
@@ -245,6 +245,9 @@ operation was interrupted by a signal.
 or
 .Dv O_EXLOCK
 is specified but the underlying file system does not support locking.
+.It Bq Er EOPNOTSUPP
+The named file is a special file mounted through a filesystem that
+does not support access to it (e.g. NFS).
 .It Bq Er EWOULDBLOCK
 .Dv O_NONBLOCK
 and one of
@@ -316,7 +319,8 @@ and
 .Xr lseek 2 ,
 .Xr read 2 ,
 .Xr umask 2 ,
-.Xr write 2
+.Xr write 2 ,
+.Xr fopen 3
 .Sh HISTORY
 The
 .Fn open
diff --git a/lib/libc/sys/read.2 b/lib/libc/sys/read.2
index badf49f16612..3cb0223d8b25 100644
--- a/lib/libc/sys/read.2
+++ b/lib/libc/sys/read.2
@@ -163,9 +163,13 @@ was negative.
 .It Bq Er EAGAIN
 The file was marked for non-blocking I/O,
 and no data were ready to be read.
-.It Bq Er EPERM
-The file descriptor is associated with a non regular file
-(e.g. a directory) mounted over NFS.
+.It Bq Er EISDIR
+The file descriptor is associated with a directory residing
+on a filesystem that does not allow regular read operations on
+directories (e.g. NFS).
+.It Bq Er EOPNOTSUPP
+The file descriptor is associated with a filesystem and file type that
+do not allow regular read operations on it.
 .El
 .Pp
 In addition,
@@ -207,11 +211,14 @@ The file descriptor is associated with a pipe, socket, or FIFO.
 .Sh SEE ALSO
 .Xr dup 2 ,
 .Xr fcntl 2 ,
+.Xr getdirentries 2 ,
 .Xr open 2 ,
 .Xr pipe 2 ,
 .Xr select 2 ,
 .Xr socket 2 ,
-.Xr socketpair 2
+.Xr socketpair 2 ,
+.Xr fread 3 ,
+.Xr readdir 3
 .Sh STANDARDS
 The
 .Fn read