trusty (2) mremap.2.gz

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.

       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 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 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.

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
              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.

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)

       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 System
       by Maurice J. Bach)

COLOPHON

       This page is part of release 3.54 of the Linux man-pages project.  A  description  of  the  project,  and
       information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.