363 lines
12 KiB
Perl
363 lines
12 KiB
Perl
.\" $NetBSD: 1.2.t,v 1.2 1998/01/09 06:54:41 perry Exp $
|
|
.\"
|
|
.\" Copyright (c) 1983, 1993, 1994
|
|
.\" The Regents of the University of California. All rights reserved.
|
|
.\"
|
|
.\" Redistribution and use in source and binary forms, with or without
|
|
.\" modification, are permitted provided that the following conditions
|
|
.\" are met:
|
|
.\" 1. Redistributions of source code must retain the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer.
|
|
.\" 2. Redistributions in binary form must reproduce the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer in the
|
|
.\" documentation and/or other materials provided with the distribution.
|
|
.\" 3. All advertising materials mentioning features or use of this software
|
|
.\" must display the following acknowledgement:
|
|
.\" This product includes software developed by the University of
|
|
.\" California, Berkeley and its contributors.
|
|
.\" 4. Neither the name of the University nor the names of its contributors
|
|
.\" may be used to endorse or promote products derived from this software
|
|
.\" without specific prior written permission.
|
|
.\"
|
|
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
|
|
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
|
|
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
|
|
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
|
|
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
|
|
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
|
|
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
|
|
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
|
|
.\" 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
|
|
.\" SUCH DAMAGE.
|
|
.\"
|
|
.\" @(#)1.2.t 8.9 (Berkeley) 5/29/94
|
|
.\"
|
|
.Sh 2 "Memory management
|
|
.Sh 3 "Text, data, and stack
|
|
.PP
|
|
Each process begins execution with three logical areas of memory
|
|
called text, data, and stack.
|
|
The text area is read-only and shared,
|
|
while the data and stack areas are writable and private to the process.
|
|
Both the data and stack areas may be extended and contracted on program
|
|
request.
|
|
The call:
|
|
.DS
|
|
.Fd brk 1 "set data section size
|
|
brk(addr);
|
|
caddr_t addr;
|
|
.DE
|
|
sets the end of the data segment to the specified address.
|
|
More conveniently, the end can be extended by \fIincr\fP bytes,
|
|
and the base of the new area returned with the call:
|
|
.DS
|
|
.Fd sbrk 1 "change data section size
|
|
addr = sbrk(incr);
|
|
result caddr_t addr; int incr;
|
|
.DE
|
|
Application programs normally use the library routines
|
|
.Fn malloc
|
|
and
|
|
.Fn free ,
|
|
which provide a more convenient interface than
|
|
.Fn brk
|
|
and
|
|
.Fn sbrk .
|
|
.LP
|
|
There is no call for extending the stack,
|
|
as it is automatically extended as needed.
|
|
.Sh 3 "Mapping pages
|
|
.PP
|
|
The system supports sharing of data between processes
|
|
by allowing pages to be mapped into memory. These mapped
|
|
pages may be \fIshared\fP with other processes or \fIprivate\fP
|
|
to the process.
|
|
Protection and sharing options are defined in \fI<sys/mman.h>\fP as:
|
|
.DS
|
|
.TS
|
|
l s
|
|
l l.
|
|
Protections are chosen from these bits, or-ed together:
|
|
|
|
PROT_READ /* pages can be read */
|
|
PROT_WRITE /* pages can be written */
|
|
PROT_EXEC /* pages can be executed */
|
|
.TE
|
|
.DE
|
|
.DS
|
|
.TS
|
|
l s
|
|
l l.
|
|
Flags contain sharing type and options. Sharing options, choose one:
|
|
|
|
MAP_SHARED /* share changes */
|
|
MAP_PRIVATE /* changes are private */
|
|
.TE
|
|
.DE
|
|
.DS
|
|
.TS
|
|
l s
|
|
l l.
|
|
Option flags\(dg:
|
|
|
|
MAP_ANON /* allocated from virtual memory; \fIfd\fP ignored */
|
|
MAP_FIXED /* map addr must be exactly as requested */
|
|
MAP_NORESERVE /* don't reserve needed swap area */
|
|
MAP_INHERIT /* region is retained after exec */
|
|
MAP_HASSEMAPHORE /* region may contain semaphores */
|
|
.TE
|
|
.DE
|
|
.FS
|
|
\(dg In 4.4BSD, only MAP_ANON and MAP_FIXED are implemented.
|
|
.FE
|
|
The size of a page is cpu-dependent, and is returned by the
|
|
.Fn sysctl
|
|
interface described in section
|
|
.Xr 1.7.1 .
|
|
The
|
|
.Fn getpagesize
|
|
library routine is provided for convenience and backward compatibility:
|
|
.DS
|
|
.Fd getpagesize 0 "get system page size
|
|
pagesize = getpagesize();
|
|
result int pagesize;
|
|
.DE
|
|
.LP
|
|
The call:
|
|
.DS
|
|
.Fd mmap 6 "map files or devices into memory
|
|
maddr = mmap(addr, len, prot, flags, fd, pos);
|
|
result caddr_t maddr; caddr_t addr; size_t len; int prot, flags, fd; off_t pos;
|
|
.DE
|
|
causes the pages starting at \fIaddr\fP and continuing
|
|
for at most \fIlen\fP bytes to be mapped from the object represented by
|
|
descriptor \fIfd\fP, starting at byte offset \fIpos\fP.
|
|
If \fIaddr\fP is NULL, the system picks an unused address for the region.
|
|
The starting address of the region is returned;
|
|
for the convenience of the system,
|
|
it may differ from that supplied
|
|
unless the MAP_FIXED flag is given,
|
|
in which case the exact address will be used or the call will fail.
|
|
The \fIaddr\fP parameter
|
|
must be a multiple of the pagesize (if MAP_FIXED is given).
|
|
If \fIpos\fP and \fIlen\fP are not a multiple of pagesize,
|
|
they will be rounded (down and up respectively)
|
|
to a page boundary by the system;
|
|
the rounding will cause the mapped region to extend past the specified range.
|
|
A successful
|
|
.Fn mmap
|
|
will delete any previous mapping
|
|
in the allocated address range.
|
|
The parameter \fIprot\fP specifies the accessibility
|
|
of the mapped pages.
|
|
The parameter \fIflags\fP specifies
|
|
the type of object to be mapped,
|
|
mapping options, and
|
|
whether modifications made to
|
|
this mapped copy of the page
|
|
are to be kept \fIprivate\fP, or are to be \fIshared\fP with
|
|
other references.
|
|
Possible types include MAP_SHARED or MAP_PRIVATE that
|
|
map a regular file or character-special device memory,
|
|
and MAP_ANON, which maps memory not associated with any specific file.
|
|
The file descriptor used when creating MAP_ANON regions is not used
|
|
and should be \-1.
|
|
The MAP_INHERIT flag allows a region to be inherited after an
|
|
.Fn execve .
|
|
The MAP_HASSEMAPHORE flag allows special handling for
|
|
regions that may contain semaphores.
|
|
The MAP_NORESERVE flag allows processes to allocate regions whose
|
|
virtual address space, if fully allocated,
|
|
would exceed the available memory plus swap resources.
|
|
Such regions may get a SIGSEGV signal if they page fault and resources
|
|
are not available to service their request;
|
|
typically they would free up some resources via
|
|
.Fn munmap
|
|
so that when they return from the signal the page
|
|
fault could be completed successfully.
|
|
.LP
|
|
A facility is provided to synchronize a mapped region with the file
|
|
it maps; the call:
|
|
.DS
|
|
.Fd msync 2 "synchronize a mapped region
|
|
msync(addr, len);
|
|
caddr_t addr; size_t len;
|
|
.DE
|
|
causes any modified pages in the specified region to be synchronized
|
|
with their source and other mappings.
|
|
If necessary, it writes any modified pages back to the filesystem, and updates
|
|
the file modification time.
|
|
If \fIlen\fP is 0, all modified pages within the region containing \fIaddr\fP
|
|
will be flushed;
|
|
this usage is provisional, and may be withdrawn.
|
|
If \fIlen\fP is non-zero, only the pages containing \fIaddr\fP and \fIlen\fP
|
|
succeeding locations will be examined.
|
|
Any required synchronization of memory caches
|
|
will also take place at this time.
|
|
.LP
|
|
Filesystem operations on a file that is mapped for shared modifications
|
|
are currently unpredictable except after an
|
|
.Fn msync .
|
|
.LP
|
|
A mapping can be removed by the call
|
|
.DS
|
|
.Fd munmap 2 "remove a mapping
|
|
munmap(addr, len);
|
|
caddr_t addr; size_t len;
|
|
.DE
|
|
This call deletes the mappings for the specified address range,
|
|
and causes further references to addresses within the range
|
|
to generate invalid memory references.
|
|
.Sh 3 "Page protection control
|
|
.LP
|
|
A process can control the protection of pages using the call:
|
|
.DS
|
|
.Fd mprotect 3 "control the protection of pages
|
|
mprotect(addr, len, prot);
|
|
caddr_t addr; size_t len; int prot;
|
|
.DE
|
|
This call changes the specified pages to have protection \fIprot\fP\|.
|
|
Not all implementations will guarantee protection on a page basis;
|
|
the granularity of protection changes may be as large as an entire region.
|
|
.Sh 3 "Giving and getting advice
|
|
.LP
|
|
A process that has knowledge of its memory behavior may
|
|
use the
|
|
.Fn madvise \(dg
|
|
call:
|
|
.FS
|
|
\(dg The entry point for this system call is defined,
|
|
but is not implemented,
|
|
so currently always returns with the error ``Operation not supported.''
|
|
.FE
|
|
.DS
|
|
.Fd madvise 3 "give advise about use of memory
|
|
madvise(addr, len, behav);
|
|
caddr_t addr; size_t len; int behav;
|
|
.DE
|
|
\fIBehav\fP describes expected behavior, as given
|
|
in \fI<sys/mman.h>\fP:
|
|
.DS
|
|
.TS
|
|
l l.
|
|
MADV_NORMAL /* no further special treatment */
|
|
MADV_RANDOM /* expect random page references */
|
|
MADV_SEQUENTIAL /* expect sequential references */
|
|
MADV_WILLNEED /* will need these pages */
|
|
MADV_DONTNEED /* don't need these pages */
|
|
.TE
|
|
.DE
|
|
The
|
|
.Fn mincore \(dg
|
|
function allows a process to obtain information
|
|
about whether pages are memory resident:
|
|
.DS
|
|
.Fd mincore 3 "get advise about use of memory
|
|
mincore(addr, len, vec);
|
|
caddr_t addr; size_t len; result char *vec;
|
|
.DE
|
|
Here the current memory residency of the pages is returned
|
|
in the character array \fIvec\fP, with a value of 1 meaning
|
|
that the page is in-memory.
|
|
.Fn Mincore
|
|
provides only transient information about page residency.
|
|
Real-time processes that need guaranteed residence over time
|
|
can use the call:
|
|
.DS
|
|
.Fd mlock 2 "lock physical pages in memory
|
|
mlock(addr, len);
|
|
caddr_t addr; size_t len;
|
|
.DE
|
|
This call locks the pages for the specified address range into memory
|
|
(paging them in if necessary)
|
|
ensuring that further references to addresses within the range
|
|
will never generate page faults.
|
|
The amount of memory that may be locked is controlled by a resource limit,
|
|
see section
|
|
.Xr 1.6.3 .
|
|
When the memory is no longer critical it can be unlocked using:
|
|
.DS
|
|
.Fd munlock 2 "unlock physical pages in memory
|
|
munlock(addr, len);
|
|
caddr_t addr; size_t len;
|
|
.DE
|
|
After the
|
|
.Fn munlock
|
|
call, the pages in the specified address range are still accessible
|
|
but may be paged out if memory is needed and they are not accessed.
|
|
.Sh 3 "Synchronization primitives
|
|
Primitives are provided for synchronization using semaphores
|
|
in shared memory.\(dd
|
|
.FS
|
|
\(dd All currently unimplemented, no entry points exists.
|
|
.FE
|
|
These primitives are expected to be superseded by the semaphore
|
|
interface being specified by the POSIX 1003 Pthread standard.
|
|
They are provided as an efficient interim solution.
|
|
Application programmers are encouraged to use the Pthread interface
|
|
when it becomes available.
|
|
.PP
|
|
Semaphores must lie within a MAP_SHARED region with at least modes
|
|
PROT_READ and PROT_WRITE.
|
|
The MAP_HASSEMAPHORE flag must have been specified when the region was created.
|
|
To acquire a lock a process calls:
|
|
.DS
|
|
.Fd mset 2 "acquire and set a semaphore
|
|
value = mset(sem, wait);
|
|
result int value; semaphore *sem; int wait;
|
|
.DE
|
|
.Fn Mset
|
|
indivisibly tests and sets the semaphore \fIsem\fP.
|
|
If the previous value is zero, the process has acquired the lock and
|
|
.Fn mset
|
|
returns true immediately.
|
|
Otherwise, if the \fIwait\fP flag is zero,
|
|
failure is returned.
|
|
If \fIwait\fP is true and the previous value is non-zero,
|
|
.Fn mset
|
|
relinquishes the processor until notified that it should retry.
|
|
.LP
|
|
To release a lock a process calls:
|
|
.DS
|
|
.Fd mclear 2 "release a semaphore and awaken waiting processes
|
|
mclear(sem);
|
|
semaphore *sem;
|
|
.DE
|
|
.Fn Mclear
|
|
indivisibly tests and clears the semaphore \fIsem\fP.
|
|
If the ``WANT'' flag is zero in the previous value,
|
|
.Fn mclear
|
|
returns immediately.
|
|
If the ``WANT'' flag is non-zero in the previous value,
|
|
.Fn mclear
|
|
arranges for waiting processes to retry before returning.
|
|
.PP
|
|
Two routines provide services analogous to the kernel
|
|
.Fn sleep
|
|
and
|
|
.Fn wakeup
|
|
functions interpreted in the domain of shared memory.
|
|
A process may relinquish the processor by calling
|
|
.Fn msleep
|
|
with a set semaphore:
|
|
.DS
|
|
.Fd msleep 1 "wait for a semaphore
|
|
msleep(sem);
|
|
semaphore *sem;
|
|
.DE
|
|
If the semaphore is still set when it is checked by the kernel,
|
|
the process will be put in a sleeping state
|
|
until some other process issues an
|
|
.Fn mwakeup
|
|
for the same semaphore within the region using the call:
|
|
.DS
|
|
.Fd mwakeup 1 "awaken process(es) sleeping on a semaphore
|
|
mwakeup(sem);
|
|
semaphore *sem;
|
|
.DE
|
|
An
|
|
.Fn mwakeup
|
|
may awaken all sleepers on the semaphore,
|
|
or may awaken only the next sleeper on a queue.
|