NAME
mremap - remap a virtual memory address
SYNOPSIS
#define _GNU_SOURCE
#include <sys/mman.h>
void *mremap(void *old_address, size_t old_size,
size_t new_size, int flags);
DESCRIPTION
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_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.
RETURN VALUE
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.
ERRORS
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; 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.
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.
CONFORMING TO
This call is Linux-specific, and should not be used in programs
intended to be portable.
NOTES
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.
SEE ALSO
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.)
COLOPHON
This page is part of release 3.24 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/.