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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

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