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



NAME
       mremap - re-map a virtual memory address

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

       void *mremap(void *old_address, size_t old_size,
                    size_t new_size, int 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.

       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 seg-
       ments will also cause a segmentation violation.

       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:

       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 relocated, then absolute pointers into the old mapping loca-
              tion 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.

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 resource limit.

       EFAULT "Segmentation  fault."  Some address in the range old_address to old_address+old_size is an invalid vir-
              tual 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.

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), 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.  Tan-
       nenbaum, Inside Linux by Randolf Bentson, The Design of the UNIX Operating System by Maurice J. Bach.)

COLOPHON
       This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2005-09-13                         MREMAP(2)