mremap

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



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.

       If the value of old_size is zero, and old_address refers to a shareable
       mapping (see mmap(2) MAP_SHARED), then mremap() will create a new
       mapping of the same pages.  new_size will be the size of the new
       mapping and the location of the new mapping may be specified with
       new_address; see the description of MREMAP_FIXED below.  If a new
       mapping is requested via this method, then the MREMAP_MAYMOVE flag must
       also be specified.

       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;

              *  the new address range specified by new_address and new_size
                 overlapped the old address range specified by old_address and
                 old_size;

              *  MREMAP_FIXED was specified without also specifying
                 MREMAP_MAYMOVE;

              *  old_size was zero and old_address does not refer to a
                 shareable mapping (but see BUGS);

              *  old_size was zero and the MREMAP_MAYMOVE flag was not
                 specified.

       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.

       If mremap() is used to move or expand an area locked with mlock(2) or
       equivalent, the mremap() call will make a best effort to populate the
       new area but will not fail with ENOMEM if the area cannot be populated.

BUGS
       Before Linux 4.14, if old_size was zero and the mapping referred to by
       old_address was a private mapping (mmap(2) MAP_PRIVATE), mremap()
       created a new private mapping unrelated to the original mapping.  This
       behavior was unintended and probably unexpected in user-space
       applications (since the intention of mremap() is to create a new
       mapping based on the original mapping).  Since Linux 4.14, mremap()
       fails with the error EINVAL in this scenario.

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 5.03 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.



Linux                             2019-03-06                         MREMAP(2)