mremap(2) - Linux manual page (original) (raw)
mremap(2) System Calls Manual mremap(2)
NAME top
mremap - remap a virtual memory addressLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#define _GNU_SOURCE** /* See feature_test_macros(7) */
**#include <sys/mman.h>**
**void *mremap(void** _oldaddress_**[.**_oldsize_**], size_t** _oldsize_**,**
**size_t** _newsize_**, int** _flags_**, ... /* void ***_newaddress_ ***/);**DESCRIPTION top
**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).
_oldaddress_ is the old address of the virtual memory block that
you want to expand (or shrink). Note that _oldaddress_ has to be
page aligned. _oldsize_ is the old size of the virtual memory
block. _newsize_ is the requested size of the virtual memory block
after the resize. An optional fifth argument, _newaddress_, may be
provided; see the description of **MREMAP_FIXED** below.
If the value of _oldsize_ is zero, and _oldaddress_ refers to a
shareable mapping (see the description of **MAP_SHARED** in [mmap(2)](../man2/mmap.2.html)),
then **mremap**() will create a new mapping of the same pages.
_newsize_ will be the size of the new mapping and the location of
the new mapping may be specified with _newaddress_; 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)](../man2/mmap.2.html). If this flag is specified, then **mremap**() accepts
a fifth argument, _void *newaddress_, which specifies a
page-aligned address to which the mapping must be moved.
Any previous mapping at the address range specified by
_newaddress_ and _newsize_ 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 _oldaddress_.
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)](../man2/mmap.2.html)).
After completion, any access to the range specified by
_oldaddress_ and _oldsize_ will result in a page fault. The
page fault will be handled by a [userfaultfd(2)](../man2/userfaultfd.2.html) handler if
the address is in a range previously registered with
[userfaultfd(2)](../man2/userfaultfd.2.html). 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 _oldaddress_ and _oldsize_ is
locked (using [mlock(2)](../man2/mlock.2.html) 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 top
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](../man3/errno.3.html)_ is set to indicate the error.ERRORS top
**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 _oldaddress_ to
_oldaddress_+_oldsize_ 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:
• _oldaddress_ was not page aligned;
• a value other than **MREMAP_MAYMOVE** or **MREMAP_FIXED** or
**MREMAP_DONTUNMAP** was specified in _flags_;
• _newsize_ was zero;
• _newsize_ or _newaddress_ was invalid;
• the new address range specified by _newaddress_ and
_newsize_ overlapped the old address range specified by
_oldaddress_ and _oldsize_;
• **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 _oldaddress_ and _oldsize_ were not
private anonymous;
• **MREMAP_DONTUNMAP** was specified and _oldsize_ was not
equal to _newsize_;
• _oldsize_ was zero and _oldaddress_ does not refer to a
shareable mapping (but see BUGS);
• _oldsize_ 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.STANDARDS top
Linux.HISTORY top
Prior to glibc 2.4, glibc did not expose the definition of
**MREMAP_FIXED**, and the prototype for **mremap**() did not allow for the
_newaddress_ argument.NOTES top
**mremap**() changes the mapping between virtual addresses and memory
pages. This can be used to implement a very efficient [realloc(3)](../man3/realloc.3.html).
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)](../man2/mlock.2.html)
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)](../man2/userfaultfd.2.html): an application can yank out a
virtual address range using **MREMAP_DONTUNMAP** and then employ a
[userfaultfd(2)](../man2/userfaultfd.2.html) 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)](../man2/userfaultfd.2.html) 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.BUGS top
Before Linux 4.14, if _oldsize_ was zero and the mapping referred
to by _oldaddress_ was a private mapping (see the description of
**MAP_PRIVATE** in [mmap(2)](../man2/mmap.2.html)), **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.SEE ALSO top
[brk(2)](../man2/brk.2.html), [getpagesize(2)](../man2/getpagesize.2.html), [getrlimit(2)](../man2/getrlimit.2.html), [mlock(2)](../man2/mlock.2.html), [mmap(2)](../man2/mmap.2.html), [sbrk(2)](../man2/sbrk.2.html),
[malloc(3)](../man3/malloc.3.html), [realloc(3)](../man3/realloc.3.html)
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)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-11-13 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)