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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | BUGS | SEE ALSO | COLOPHON |
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mremap(2) System Calls Manual mremap(2)
mremap - remap a virtual memory address
Standard C library (libc, -lc)
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <sys/mman.h>
void *mremap(size_t old_size;
void old_address[old_size], 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 the description of MAP_SHARED in mmap(2)),
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.
The flags bit-mask argument may be 0, or include the following
flags:
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.
MREMAP_DONTUNMAP (since Linux 5.7)
This flag, which must be used in conjunction with
MREMAP_MAYMOVE, remaps a mapping to a new address but does
not unmap the mapping at old_address.
The MREMAP_DONTUNMAP flag can be used only with mappings
that are not VM_DONTEXPAND or VM_MIXEDMAP. Before Linux
5.13, the MREMAP_DONTUNMAP flag could be used only with
private anonymous mappings (see the description of
MAP_PRIVATE and MAP_ANONYMOUS in mmap(2)).
After completion, any access to the range specified by
old_address and old_size will result in a page fault. The
page fault will be handled by a userfaultfd(2) handler if
the address is in a range previously registered with
userfaultfd(2). Otherwise, the kernel allocates a zero-
filled page to handle the fault.
The MREMAP_DONTUNMAP flag may be used to atomically move a
mapping while leaving the source mapped. See NOTES for
some possible applications of MREMAP_DONTUNMAP.
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 to indicate the error.
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 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 or
MREMAP_DONTUNMAP 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 or MREMAP_DONTUNMAP was specified without
also specifying MREMAP_MAYMOVE;
• MREMAP_DONTUNMAP was specified, but one or more pages in
the range specified by old_address and old_size were not
private anonymous;
• MREMAP_DONTUNMAP was specified and old_size was not
equal to new_size;
• 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 Not enough memory was available to complete the operation.
Possible causes are:
• 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.
• MREMAP_DONTUNMAP was used causing a new mapping to be
created that would exceed the (virtual) memory
available. Or, it would exceed the maximum number of
allowed mappings.
Linux.
Prior to glibc 2.4, glibc did not expose the definition of
MREMAP_FIXED, and the prototype for mremap() did not allow for the
new_address argument.
mremap() changes the mapping between virtual addresses and memory
pages. This can be used to implement a very efficient realloc(3).
In Linux, memory is divided into pages. A 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
(SIGSEGV) 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.
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.
MREMAP_DONTUNMAP use cases
Possible applications for MREMAP_DONTUNMAP include:
• Non-cooperative userfaultfd(2): an application can yank out a
virtual address range using MREMAP_DONTUNMAP and then employ a
userfaultfd(2) handler to handle the page faults that
subsequently occur as other threads in the process touch pages
in the yanked range.
• Garbage collection: MREMAP_DONTUNMAP can be used in conjunction
with userfaultfd(2) to implement garbage collection algorithms
(e.g., in a Java virtual machine). Such an implementation can
be cheaper (and simpler) than conventional garbage collection
techniques that involve marking pages with protection PROT_NONE
in conjunction with the use of a SIGSEGV handler to catch
accesses to those pages.
Before Linux 4.14, if old_size was zero and the mapping referred
to by old_address was a private mapping (see the description of
MAP_PRIVATE in mmap(2)), 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 Randolph Bentson, The Design of the
UNIX Operating System by Maurice J. Bach)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-06-28 mremap(2)
Pages that refer to this page: memusage(1), getrlimit(2), madvise(2), mmap2(2), mmap(2), PR_SET_TAGGED_ADDR_CTRL(2const), remap_file_pages(2), syscalls(2), UFFDIO_API(2const), userfaultfd(2)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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