mremap — re-map a virtual memory address
#define _GNU_SOURCE #include <sys/mman.h>
void
*mremap( |
void * | old_address, |
| size_t | old_size, | |
| size_t | new_size, | |
| int | flags); |
mremap() expands (or
shrinks) an existing memory mapping, potentially moving it at
the same time (controlled by the flags argument and the
available virtual address space).
old_address is the
old address of the virtual memory block that you want to
expand (or shrink). Note that old_address has to be page
aligned. old_size is
the old size of the virtual memory block. new_size is the requested size
of the virtual memory block after the resize.
In Linux the memory is divided into pages. A user process has (one or) several linear virtual memory segments. Each virtual memory segment has one or more mappings to real memory pages (in the page table). Each virtual memory segment has its own protection (access rights), which may cause a segmentation violation if the memory is accessed incorrectly (e.g., writing to a read-only segment). Accessing virtual memory outside of the segments will also cause a segmentation violation.
mremap() uses the Linux page
table scheme. mremap() changes
the mapping between virtual addresses and memory pages. This
can be used to implement a very efficient realloc(3).
The flags bit-mask
argument may be 0, or include the following flag:
MREMAP_MAYMOVEBy default, if there is not sufficient space to
expand a mapping at its current location, then
mremap() fails. If this
flag is specified, then the kernel is permitted to
relocate the mapping to a new virtual address, if
necessary. If the mapping is relocated, then absolute
pointers into the old mapping location become invalid
(offsets relative to the starting address of the
mapping should be employed).
MREMAP_FIXED (since Linux
2.3.31)This flag serves a similar purpose to the
MAP_FIXED flag of
mmap(2). If this flag
is specified, then mremap() accepts a fifth argument,
void
*new_address, which specifies a
page-aligned address to which the mapping must be
moved. Any previous mapping at the address range
specified by new_address and
new_size is
unmapped. If MREMAP_FIXED
is specified, then MREMAP_MAYMOVE must also be
specified.
If the memory segment specified by old_address and old_size is locked (using
mlock(2) or similar), then
this lock is maintained when the segment is resized and/or
relocated. As a consequence, the amount of memory locked by
the process may change.
On success mremap() returns
a pointer to the new virtual memory area. On error, the value
MAP_FAILED (that is,
(void *) −1) is
returned, and errno is set
appropriately.
The caller tried to expand a memory segment that is
locked, but this was not possible without exceeding the
RLIMIT_MEMLOCK resource
limit.
"Segmentation fault." Some address in the range
old_address to
old_address+old_size is an invalid
virtual memory address for this process. You can also
get EFAULT even if there
exist mappings that cover the whole address space
requested, but those mappings are of different
types.
An invalid argument was given. Possible causes are:
old_address was
not page aligned; a value other than MREMAP_MAYMOVE or MREMAP_FIXED was specified in
flags;
new_size was
zero; new_size
or new_address was
invalid; or the new address range specified by
new_address
and new_size
overlapped the old address range specified by
old_address and
old_size; or
MREMAP_FIXED was
specified without also specifying MREMAP_MAYMOVE.
The memory area cannot be expanded at the current
virtual address, and the MREMAP_MAYMOVE flag is not set in
flags. Or,
there is not enough (virtual) memory available.
This call is Linux-specific, and should not be used in programs intended to be portable.
Prior to version 2.4, glibc did not expose the definition
of MREMAP_FIXED, and the
prototype for mremap() did not
allow for the new_address argument.
brk(2), getpagesize(2), getrlimit(2), mlock(2), mmap(2), sbrk(2), malloc(3), realloc(3), feature_test_macros(7)
Your favorite OS text book for more information on paged memory. (Modern Operating Systems by Andrew S. Tannenbaum, Inside Linux by Randolf Bentson, The Design of the UNIX Operating System by Maurice J. Bach.)
This page is part of release 2.79 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man-pages/.
|
Copyright (c) 1996 Tom Bjorkholm <tombmydata.se> This is free documentation; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. The GNU General Public License's references to "object code" and "executables" are to be interpreted as the output of any document formatting or typesetting system, including intermediate and printed output. This manual is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this manual; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111, USA. 1996-04-11 Tom Bjorkholm <tombmydata.se> First version written (1.3.86) 1996-04-12 Tom Bjorkholm <tombmydata.se> Update for Linux 1.3.87 and later 2005-10-11 mtk: Added NOTES for MREMAP_FIXED; revised EINVAL text. |