Provided by: manpages-dev_4.15-1_all 
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.
If the value of old_size is zero, and old_address refers to a shareable mapping (see
mmap(2) MAP_SHARED), 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.
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;
* 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 was specified without also specifying MREMAP_MAYMOVE;
* old_size was zero and old_address does not refer to a shareable mapping (but see
BUGS);
* old_size was zero and the MREMAP_MAYMOVE flag was not specified.
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.
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.
BUGS
Before Linux 4.14, if old_size was zero and the mapping referred to by old_address was a
private mapping (mmap(2) MAP_PRIVATE), 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
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
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