mremap() - Unix, Linux System Call
mremap - re-map a virtual memory address
void * mremap(void *old_address, size_t old_size , size_t new_size, int flags);
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().
The flags bit-mask argument may be 0, or include the following flag:
If the memory segment specified by
old_size is locked (using
mlock() 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.
By default, if there is not sufficient space to expand a mapping
at its current location, then
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
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
Any previous mapping at the address range specified by
new_size is unmapped.
MREMAP_FIXED is specified, then
MREMAP_MAYMOVE must also be specified.
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.
The caller tried to expand a memory segment that is locked,
but this was not possible without exceeding the
RLIMIT_MEMLOCK resource limit.
"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.
An invalid argument was given.
Possible causes are: old_address was not
page aligned; a value other than
MREMAP_FIXED was specified in
flags; new_size was zero;
new_address was invalid;
or the new address range specified by
new_size overlapped the old address range specified by
MREMAP_FIXED was specified without also specifying
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.
Prior to version 2.4, glibc did not expose the definition of
MREMAP_FIXED, and the prototype for
mremap() did not allow for the
This call is Linux-specific, and should not be used in programs
intended to be portable. 4.2BSD had a (never actually implemented)
call with completely different semantics.
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.)