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    Command:

    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.
    
    
           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 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)
    
    
    

    Linux 2010-06-10 MREMAP(2)

    
    
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