NAME¶
mremap - remap a virtual memory address
SYNOPSIS¶
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <sys/mman.h>
void *mremap(void *old_address, 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
optional fifth argument,
new_address, may be provided; see the
description of
MREMAP_FIXED below.
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.
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)
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
System by Maurice J. Bach)
COLOPHON¶
This page is part of release 3.44 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/.