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       mremap - remap a virtual memory address


       #define _GNU_SOURCE
       #include <sys/mman.h>

       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

       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

       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:

              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.


       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.


       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.


       This  call  is  Linux-specific,  and  should  not  be  used in programs
       intended to be portable.


       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.


       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.


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