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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON |
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MREMAP(2) Linux Programmer's Manual MREMAP(2)
mremap - remap a virtual memory address
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <sys/mman.h>
void *mremap(void *old_address, size_t old_size,
size_t new_size, int flags, ... /* void *new_address */);
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. An optional fifth argument, new_address, may be provided;
see the description of MREMAP_FIXED below.
If the value of old_size is zero, and old_address refers to a
shareable mapping (see mmap(2) MAP_SHARED), then mremap() will create
a new mapping of the same pages. new_size will be the size of the
new mapping and the location of the new mapping may be specified with
new_address; see the description of MREMAP_FIXED below. If a new
mapping is requested via this method, then the MREMAP_MAYMOVE flag
must also be specified.
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_MAYMOVE
By 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.
EAGAIN The caller tried to expand a memory segment that is locked,
but this was not possible without exceeding the RLIMIT_MEMLOCK
resource limit.
EFAULT "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.
EINVAL 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;
* the new address range specified by new_address and new_size
overlapped the old address range specified by old_address
and old_size;
* MREMAP_FIXED was specified without also specifying
MREMAP_MAYMOVE;
* old_size was zero and old_address does not refer to a
shareable mapping (but see BUGS);
* old_size was zero and the MREMAP_MAYMOVE flag was not
specified.
ENOMEM 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.
If mremap() is used to move or expand an area locked with mlock(2) or
equivalent, the mremap() call will make a best effort to populate the
new area but will not fail with ENOMEM if the area cannot be
populated.
Before Linux 4.14, if old_size was zero and the mapping referred to
by old_address was a private mapping (mmap(2) MAP_PRIVATE), mremap()
created a new private mapping unrelated to the original mapping.
This behavior was unintended and probably unexpected in user-space
applications (since the intention of mremap() is to create a new
mapping based on the original mapping). Since Linux 4.14, mremap()
fails with the error EINVAL in this scenario.
brk(2), getpagesize(2), getrlimit(2), mlock(2), mmap(2), sbrk(2),
malloc(3), realloc(3)
Your favorite text book on operating systems for more information on
paged memory (e.g., Modern Operating Systems by Andrew S. Tanenbaum,
Inside Linux by Randolf Bentson, The Design of the UNIX Operating
System by Maurice J. Bach)
This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-25 MREMAP(2)
Pages that refer to this page: memusage(1), getrlimit(2), ioctl_userfaultfd(2), mmap2(2), mmap(2), remap_file_pages(2), syscalls(2), userfaultfd(2)
Copyright and license for this manual page
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