MREMAP(2) Linux Programmer's Manual MREMAP(2)
mremap - remap a virtual memory address
#define _GNU_SOURCE /* See feature_test_macros(7) */
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 map-
ping 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
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 viola-
mremap() uses the Linux page table scheme. mremap() changes the map-
ping 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:
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 relo-
cated, 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 previ-
ous 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 seg-
ment is resized and/or relocated. As a consequence, the amount of mem-
ory 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
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 map-
pings 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 speci-
fied 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
* MREMAP_FIXED was specified without also specifying
* old_size was zero and old_address does not refer to a share-
able mapping (but see BUGS);
* old_size was zero and the MREMAP_MAYMOVE flag was not speci-
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
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() cre-
ated a new private mapping unrelated to the original mapping. This
behavior was unintended and probably unexpected in user-space applica-
tions (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), mal-
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 Sys-
tem 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
Linux 2017-09-25 MREMAP(2)
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