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 3.14 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/.
