mremap(2) System Calls Manual mremap(2)
NAME
mremap - remap a virtual memory address
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <sys/mman.h>
void *mremap(void old_address[.old_size], size_t old_size,
size_t new_size, int flags, ... /* void *new_address */);
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. An op-
tional 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 relo-
cated, then absolute pointers into the old mapping location be-
come 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 un-
mapped.
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 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_ad-
dress 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 map-
ping while leaving the source mapped. See NOTES for some possi-
ble 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 seg-
ment is resized and/or relocated. As a consequence, the amount of mem-
ory 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 to indicate the error.
ERRORS
EAGAIN The caller tried to expand a memory segment that is locked, but
this was not possible without exceeding the RLIMIT_MEMLOCK re-
source 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 ad-
dress 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 share-
able mapping (but see BUGS);
• old_size was zero and the MREMAP_MAYMOVE flag was not speci-
fied.
ENOMEM Not enough memory was available to complete the operation. Pos-
sible causes are:
• The memory area cannot be expanded at the current virtual ad-
dress, 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
Linux.
HISTORY
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 argu-
ment.
NOTES
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 out-
side 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.
BUGS
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 origi-
nal 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
brk(2), getpagesize(2), getrlimit(2), mlock(2), mmap(2), sbrk(2), mal-
loc(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, In-
side Linux by Randolph Bentson, The Design of the UNIX Operating System
by Maurice J. Bach)
Linux man-pages 6.9.1 2024-05-02 mremap(2)
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