mremap


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  viola-
       tion.

       mremap()  uses  the Linux page table scheme.  mremap() changes the map-
       ping 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 relo-
              cated,  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  previ-
              ous  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
              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  map-
              pings  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 spec-
              ified 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), mal-
       loc(3), realloc(3)

       Your favorite OS text book for more information on paged memory.  (Mod-
       ern  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 3.35 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://man7.org/linux/man-pages/.



Linux                             2010-06-10                         MREMAP(2)
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