mremap

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



NAME
       mremap - re-map a virtual memory address

SYNOPSIS
       #include <unistd.h>
       #include <sys/mman.h>

       void  *  mremap(void  *old_address,  size_t old_size , size_t new_size,
       unsigned long flags);

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.

       The flags argument is a bitmap of flags.

       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 mapping
       between virtual addresses and memory pages.  This can be used to imple-
       ment a very efficient realloc.


FLAGS
       MREMAP_MAYMOVE
              indicates  if  the  operation should fail, or change the virtual
              address if the resize cannot be  done  at  the  current  virtual
              address.


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
       EINVAL An  invalid  argument was given. Most likely old_address was not
              page aligned.

       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.

       EAGAIN The memory segment is locked and cannot be re-mapped.

       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.

NOTES
       With current  glibc  includes,  in  order  to  get  the  definition  of
       MREMAP_MAYMOVE,   you  need  to  define  _GNU_SOURCE  before  including
       <sys/mman.h>.

CONFORMING TO
       This call is  Linux-specific,  and  should  not  be  used  in  programs
       intended  to  be  portable.   4.2BSD had a (never actually implemented)
       mremap(2) call with completely different semantics.

SEE ALSO
       getpagesize(2), realloc(3), malloc(3), brk(2), sbrk(2), mmap(2)

       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.)




Linux 1.3.87                      1996-04-12                         MREMAP(2)