mremap

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

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 map-
       ping 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  re-
       quested  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  in-
       correctly  (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 be-
              come 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
       (using mlock(2) or similar), then this lock is maintained when the seg-
       ment is resized and/or relocated.  As a consequence, the amount of mem-
       ory 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 re-
              source 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  speci-
                 fied 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  share-
                 able mapping (but see BUGS);

              *  old_size  was zero and the MREMAP_MAYMOVE flag was not speci-
                 fied.

       ENOMEM The memory area cannot be expanded at the  current  virtual  ad-
              dress,  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  in-
       tended 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()  cre-
       ated a new private mapping unrelated to the original mapping.  This be-
       havior was unintended and probably unexpected  in  user-space  applica-
       tions (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), mal-
       loc(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  Sys-
       tem by Maurice J. Bach)

COLOPHON
       This  page  is  part of release 5.05 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.

Linux                             2019-03-06                         MREMAP(2)