Index: lib/libc/sys/mmap.2
===================================================================
--- lib/libc/sys/mmap.2
+++ lib/libc/sys/mmap.2
@@ -28,7 +28,7 @@
 .\"	@(#)mmap.2	8.4 (Berkeley) 5/11/95
 .\" $FreeBSD$
 .\"
-.Dd February 4, 2017
+.Dd June 22, 2017
 .Dt MMAP 2
 .Os
 .Sh NAME
@@ -199,6 +199,22 @@
 .Dv MAP_EXCL
 is specified, the request will fail if a mapping
 already exists within the range.
+.It Dv MAP_GUARD
+Instead of the mapping, create a guard of the specified size.
+The guard prevents any other mappings at its address range,
+until it is unmapped or another mapping is created there with
+the
+.Dv MAP_FIXED
+flag.
+Any memory access to the guarded range results in the
+.Dv SIGSEGV
+signal delivered to the thread.
+.Pp
+Guards allow to create reservations in the address space, which
+can be replaced by the actual mappings later.
+For instance, system uses guards to prevent inadvertent use of grow
+areas for auto-expanded stacks created with
+.Dv MAP_STACK .
 .It Dv MAP_HASSEMAPHORE
 Notify the kernel that the region may contain semaphores and that special
 handling may be necessary.
Index: lib/libc/sys/munmap.2
===================================================================
--- lib/libc/sys/munmap.2
+++ lib/libc/sys/munmap.2
@@ -28,7 +28,7 @@
 .\"	@(#)munmap.2	8.3 (Berkeley) 5/27/94
 .\" $FreeBSD$
 .\"
-.Dd May 27, 1994
+.Dd June 22, 2017
 .Dt MUNMAP 2
 .Os
 .Sh NAME
@@ -44,7 +44,7 @@
 The
 .Fn munmap
 system call
-deletes the mappings for the specified address range,
+deletes the mappings and guards for the specified address range,
 and causes further references to addresses within the range
 to generate invalid memory references.
 .Sh RETURN VALUES