Differential D26908 Diff 78829 head/lib/libc/sys/mmap.2

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head/lib/libc/sys/mmap.2

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	.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT			.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
	.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY			.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
	.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF			.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
	.\" SUCH DAMAGE.			.\" SUCH DAMAGE.
	.\"			.\"
	.\" @(#)mmap.2 8.4 (Berkeley) 5/11/95			.\" @(#)mmap.2 8.4 (Berkeley) 5/11/95
	.\" $FreeBSD$			.\" $FreeBSD$
	.\"			.\"
	.Dd October 21, 2020			.Dd October 27, 2020
	.Dt MMAP 2			.Dt MMAP 2
	.Os			.Os
	.Sh NAME			.Sh NAME
	.Nm mmap			.Nm mmap
	.Nd allocate memory, or map files or devices into memory			.Nd allocate memory, or map files or devices into memory
	.Sh LIBRARY			.Sh LIBRARY
	.Lb libc			.Lb libc
	.Sh SYNOPSIS			.Sh SYNOPSIS
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	.Dv PROT_WRITE ,			.Dv PROT_WRITE ,
	it does not eliminate soft faults on the initial write accesses to the			it does not eliminate soft faults on the initial write accesses to the
	region.			region.
	.It Dv MAP_PRIVATE			.It Dv MAP_PRIVATE
	Modifications are private.			Modifications are private.
	.It Dv MAP_SHARED			.It Dv MAP_SHARED
	Modifications are shared.			Modifications are shared.
	.It Dv MAP_STACK			.It Dv MAP_STACK
				Creates both a mapped region that grows downward on demand and an
				adjoining guard that both reserves address space for the mapped region
				to grow into and limits the mapped region's growth.
				Together, the mapped region and the guard occupy
				.Fa len
				bytes of the address space.
				The guard starts at the returned address, and the mapped region ends at
				the returned address plus
				.Fa len
				bytes.
				Upon access to the guard, the mapped region automatically grows in size,
				and the guard shrinks by an equal amount.
				Essentially, the boundary between the guard and the mapped region moves
				downward so that the access falls within the enlarged mapped region.
				However, the guard will never shrink to less than the number of pages
				specified by the sysctl
				.Dv security.bsd.stack_guard_page ,
				thereby ensuring that a gap for detecting stack overflow always exists
				between the downward growing mapped region and the closest mapped region
				beneath it.
				.Pp
	.Dv MAP_STACK			.Dv MAP_STACK
	implies			implies
	.Dv MAP_ANON ,			.Dv MAP_ANON
	and			and
	.Fa offset			.Fa offset
	of 0.			of 0.
	The			The
	.Fa fd			.Fa fd
	argument			argument
	must be -1 and			must be -1 and
	.Fa prot			.Fa prot
	must include at least			must include at least
	.Dv PROT_READ			.Dv PROT_READ
	and			and
	.Dv PROT_WRITE .			.Dv PROT_WRITE .
	.Pp
	This option creates
	a memory region that grows to at most
	.Fa len
	bytes in size, starting from the stack top and growing down.
	The
	stack top is the starting address returned by the call, plus
	.Fa len
	bytes.
	The bottom of the stack at maximum growth is the starting
	address returned by the call.
	.Pp
	Stacks created with
	.Dv MAP_STACK
	automatically grow.
	Guards prevent inadvertent use of the regions into which those
	stacks can grow without requiring mapping the whole stack in advance.
	The size of the guard, in pages, is specified by sysctl			The size of the guard, in pages, is specified by sysctl
	.Dv security.bsd.stack_guard_page .			.Dv security.bsd.stack_guard_page .
	.El			.El
	.Pp			.Pp
	The			The
	.Xr close 2			.Xr close 2
	system call does not unmap pages, see			system call does not unmap pages, see
	.Xr munmap 2			.Xr munmap 2
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