MREMAP(2)                                           Linux Programmer's Manual                                          MREMAP(2)



NAME
       mremap - remap a virtual memory address

SYNOPSIS
       #define _GNU_SOURCE
       #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 neces-
              sary.  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 3.25 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/.



Linux                                                      2010-06-10                                                  MREMAP(2)