diff --git a/lib/libc/sys/msync.2 b/lib/libc/sys/msync.2
index 2583c158d328..d976e304599e 100644
--- a/lib/libc/sys/msync.2
+++ b/lib/libc/sys/msync.2
@@ -1,73 +1,91 @@
 .\" Copyright (c) 1991, 1993
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 .\"	California, Berkeley and its contributors.
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 .\"	@(#)msync.2	8.1 (Berkeley) 6/9/93
 .\"
 .Dd June 9, 1993
 .Dt MSYNC 2
 .Os
 .Sh NAME
 .Nm msync
 .Nd synchronize a mapped region
 .Sh SYNOPSIS
-.Fn msync "caddr_t addr" "int len"
+.Fn msync "caddr_t addr" "int len" "int flags"
 .Sh DESCRIPTION
 The
 .Fn msync
 system call
 writes any modified pages back to the filesystem and updates
 the file modification time.
 If
 .Fa len
 is 0, all modified pages within the region containing
 .Fa addr
 will be flushed;
 if
 .Fa len
 is non-zero, only the pages containing
 .Fa addr
 and
 .Fa len
 succeeding locations will be examined.
-Any required synchronization of memory caches
-will also take place at this time.
-Filesystem operations on a file that is mapped for shared modifications
-are unpredictable except after an
-.Fn msync .
+The
+.Fa flags
+argument may be specified as follows:
+.Bd -literal
+MS_ASYNC	Return immediately (not currently implemented)
+MS_INVALIDATE	Invalidate all cached data
+.Ed
+.Sh RETURN VALUES
+If any errors occur, -1 is returned and errno is set to indicate the
+error. Otherwise, a 0 value is returned.
+.Sh ERRORS
+.Fn msync
+will fail if:
+.Bl -tag -width Er
+.It Bq Er EINVAL
+.Fa addr
+is not a multiple of the hardware page size.
+.It Bq Er EINVAL
+.Fa len
+is too large or negative.
+.It Bq Er EINVAL
+.Fa flags
+was both MS_ASYNC and MS_INVALIDATE. Only one of these flags is allowed.
 .Sh SEE ALSO
 .Xr madvise 2 ,
 .Xr munmap 2 ,
 .Xr mprotect 2 ,
 .Xr mincore 2
 .Sh HISTORY
 The
 .Fn msync
 function first appeared in 4.4BSD.